mesh-host/internal/apply/apply.go

// Package apply makes a machine match a declaration.
//
// Three properties, each following a recorded decision, and each of them the difference
// between this and a script that writes files:
//
//   - A failed step fails the apply (novox/hq ADR 0010). Not "logs and continues": a partial
//     apply that reports success is the mesh's most expensive shape.
//   - Every applier READS BACK. Setting a value is not evidence the value took.
//   - What was applied is recorded after it works, never before (ADR 0018). A failed apply
//     leaves the machine in whatever state it reached, and nothing must claim otherwise.
package apply

import (
	"context"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"

	"github.com/novox/mesh-host/internal/declaration"
	"github.com/novox/mesh-host/internal/store"
	"github.com/novox/mesh-host/internal/system"
)

// Runner executes a command. The real one is used everywhere outside unit tests; behaviour
// against a real system is tested alongside rather than mocked (novox/hq ADR 0017).
type Runner = system.Runner

// Outcome is what happened to one resource.
type Outcome struct {
	ID     string `json:"id"`
	Type   string `json:"type"`
	Target string `json:"target"`
	// Action is created · updated · unchanged · corrected · removed.
	//
	// "corrected" is its own answer and not a kind of "updated": it means the machine had drifted
	// from what this host last wrote, so somebody changed it by hand. The mesh converging is
	// right either way; being unable to say which happened is not.
	Action string `json:"action"`
	Detail string `json:"detail,omitempty"`

	// wrote is a digest of what this apply put there, kept so the next one can tell a machine
	// that drifted from one the mesh changed its mind about. Not reported: it is bookkeeping.
	wrote string
	// into is what a file written into held before the mesh's keys (novox/hq ADR 0102).
	into *store.Into
	// kept is where this apply kept the original of a file it wrote over (novox/hq ADR 0100).
	kept string
	// stateless is a service whose unit's lifecycle is the machine's (novox/hq ADR 0117).
	stateless bool
	// found is, for a service, its unit as the host first found it (novox/hq ADR 0118).
	found *store.FoundUnit
	// reads is, for a container, the digest of each file it was created reading, by path — so
	// the next apply can say which one changed (novox/hq 04-ISSUES/103).
	reads map[string]string
}

// Report is what an apply did, in the order it did it.
type Report struct {
	Outcomes []Outcome `json:"outcomes"`
	// Tunnel is what this apply says about the tunnel the private network took over, when the
	// declaration names one (novox/hq ADR 0105).
	Tunnel *TakenTunnel `json:"tunnel,omitempty"`
}

// Changed reports whether anything about the machine actually moved. An apply that changed
// nothing is the ordinary steady state, and saying so is not the same as saying it failed.
func (r Report) Changed() bool {
	for _, o := range r.Outcomes {
		// Holding is keeping the machine as it was found, which is not moving it.
		if o.Action != "unchanged" && o.Action != "held" {
			return true
		}
	}
	return false
}

// Error is a failure part-way through, carrying what had already been done.
//
// The outcomes matter as much as the message: the machine is in whatever state the apply
// reached, and the only honest thing to hand back is the list of what did happen.
type Error struct {
	Resource string
	Err      error
	Done     Report
	// Others is how many more resources also failed. Named rather than folded into the message,
	// because "one thing failed" and "eleven things failed" are different machines and the first
	// line is what somebody reads.
	Others int

	// Gated is true when what failed was an action, so nothing after it was attempted. A person
	// reading a report needs to know the difference between "these things failed" and "these
	// things failed and the rest was never tried".
	Gated bool
}

func (e *Error) Error() string {
	also := ""
	if e.Others == 1 {
		also = ", and one other resource also failed"
	} else if e.Others > 1 {
		also = fmt.Sprintf(", and %d other resources also failed", e.Others)
	}
	rest := "everything was attempted, so what is not listed as failed was done."
	if e.Gated {
		// An action is a gate: it exists to make something true before the next thing needs it.
		rest = "this is an action, so nothing after it was attempted — the machine is in " +
			"whatever state that left it."
	}
	return fmt.Sprintf("applying %q: %v%s\n\n%d resource(s) were applied and remain; %s",
		e.Resource, e.Err, also, len(e.Done.Outcomes), rest)
}

func (e *Error) Unwrap() error { return e.Err }

// Apply makes the machine match the declaration, and returns what it did.
//
// Removal happens FIRST, and the order is not arbitrary. A resource that leaves a declaration
// while another arrives at the same path is an ordinary rename: removing afterwards would
// delete the file that had just been written. Removing first risks losing the old state if the
// apply then fails — a recovery concern, where the other is a correctness one. The one exception
// is what protects an adopted node, the openings and the guard: that goes last, and only when
// everything else applied (novox/hq ADR 0103).
func Apply(
	ctx context.Context,
	sys system.System,
	d *declaration.Declaration,
	known store.State,
	origin string,
	run Runner,
	log func(string),
	unseal Unseal,
) (Report, store.State, error) {
	return ApplyKeeping(ctx, sys, d, known, origin, run, log, unseal, nil)
}

// ApplyKeeping is Apply on a node that may be adopted: keep is where the original of a file found
// there is recorded before anything else happens to it (novox/hq ADR 0100). Nil is a caller that
// can never be handed an adopted declaration — the carried bundle, which may not say it.
func ApplyKeeping(
	ctx context.Context,
	sys system.System,
	d *declaration.Declaration,
	known store.State,
	origin string,
	run Runner,
	log func(string),
	unseal Unseal,
	keep Keep,
) (Report, store.State, error) {
	if log == nil {
		log = func(string) {}
	}
	report := Report{}

	declared := map[string]bool{}
	for _, r := range d.Resources {
		declared[r.Identity()] = true
	}
	if svc := takesOver(d); svc != nil {
		// The found tunnel's configuration is held under an id of its own, declared for as long
		// as the service that took it over is (novox/hq ADR 0105).
		declared[takeOverID(svc)] = true
	}

	// Which firewall is found here, before anything else, since an unsupported one refuses the
	// whole declaration (novox/hq ADR 0100). Nothing for a converged node.
	fw, err := foundFirewall(ctx, d, &known, run, log)
	if err != nil {
		return report, known, &Error{Resource: "the firewall found on this machine", Err: err, Done: report}
	}

	// What an adopted node's untaken modules find on the machine, looked at before anything in
	// this apply — a removal included — could change it or its records (novox/hq ADR 0103).
	before := lookBefore(ctx, sys, d, known, run)

	removeOrphan := func(orphan store.Applied) error {
		var action, detail string
		var err error
		if declaration.Type(orphan.Type) == declaration.TypeOpening {
			action, detail, err = removeOpening(ctx, orphan, run, known.Firewall)
		} else {
			action, detail, err = remove(ctx, sys, orphan, run)
		}
		if err != nil {
			return &Error{Resource: orphan.ID, Err: err, Done: report}
		}
		known.Forget(orphan.ID)
		report.Outcomes = append(report.Outcomes, Outcome{
			ID: orphan.ID, Type: orphan.Type, Target: orphan.Target,
			Action: action, Detail: detail,
		})
		log(fmt.Sprintf("  %s %s (%s)", action, orphan.ID, orphan.Target))
		return nil
	}

	// **What protects an adopted node goes last on the flip, and first on the way back** (novox/hq
	// ADR 0103). The openings and the guard are what keep the mesh reachable through the found
	// firewall and the store unreachable from outside.
	//
	// When a node is converged they leave the declaration, and removing them first would leave the
	// store open from the moment the guard stops until the derived filter loads — and for ever, if
	// the filter then fails. So on a converged declaration they are removed only once everything
	// else applied and the found firewall is retired; if anything failed, they stay, recorded, for
	// the next try.
	//
	// Returned to adopted, it is the mirror image: removing the derived filter first would leave
	// the store open until the guard loads. So the guard's own resources are applied before any
	// orphan is removed, and if a removal then fails the guard is already up. A stale opening on an
	// adopted node is removed as any orphan is.
	var protecting, orphans []store.Applied
	for _, orphan := range known.Orphans(declared, origin) {
		if d.Adoption == nil && strings.HasPrefix(orphan.ID, declaration.AdoptionPrefix) {
			protecting = append(protecting, orphan)
			continue
		}
		orphans = append(orphans, orphan)
	}
	ordered := d.Resources
	guardFirst := 0
	if d.Adoption != nil {
		ordered = nil
		for _, r := range d.Resources {
			if strings.HasPrefix(r.Identity(), guardPrefix) {
				ordered = append(ordered, r)
			}
		}
		guardFirst = len(ordered)
		for _, r := range d.Resources {
			if !strings.HasPrefix(r.Identity(), guardPrefix) {
				ordered = append(ordered, r)
			}
		}
	}
	orphansRemoved := false
	removeOrphans := func() error {
		orphansRemoved = true
		for _, orphan := range orphans {
			if err := removeOrphan(orphan); err != nil {
				return err
			}
		}
		return nil
	}

	// **A hold whose resource is no longer declared is let go, and nothing on disk is touched.**
	// What was found stays as it was; only the host's note that it holds it for a module goes, so
	// the node stops reporting a hold for a module no longer assigned. Should the module come back,
	// what is there is present with no record and is found, and held, again — its first kept
	// original is never overwritten (novox/hq ADR 0100). Only a declaration from the mesh says
	// what is assigned: a carried bundle's silence is not an unassignment.
	if origin == store.OriginDeclared {
		for _, h := range append([]store.Held{}, known.Held...) {
			if declared[h.ID] {
				continue
			}
			known.Release(h.ID)
			report.Outcomes = append(report.Outcomes, Outcome{ID: h.ID, Type: h.Kind, Target: h.Target,
				Action: "forgotten", Detail: "no longer declared; left as found"})
			log(fmt.Sprintf("  forgotten %s (%s): no longer declared; left as found", h.ID, h.Target))
		}
	}

	// What moved in this apply, so a service that must reflect a file can be told the file
	// moved. Only within one apply: a change from an earlier one has already been reflected, and
	// restarting for it every time would make a steady machine restart its services for ever.
	changed := map[string]bool{}

	// **What each resource currently says, so a container can be identified by its inputs.**
	//
	// `changed` above is edge-triggered and only within one apply, which is right for "restart it
	// because this just moved" and wrong for "is this container running the file that is there
	// now". A file written in an earlier apply, or written before the container declared it as a
	// dependency, leaves a container holding values nothing will ever re-read: it is up, the
	// machine reports success, and what is inside is using a credential the mesh has replaced
	// (novox/hq 04-ISSUES/045). Folding these into the container's spec makes the comparison a
	// standing one instead.
	//
	// And what each file a container reads at creation holds — its env-files and what is mounted
	// into it — by the digest this host recorded when it wrote the file, read from `known` as it
	// stands when the container is reached, so a file rewritten earlier in this same apply is
	// already the new one (novox/hq 04-ISSUES/103). That needs the file applied before the
	// container, which is the declared order; a container declared ahead of its file sees the
	// change one apply late, and never misses it.
	in := inputs{declares: map[string]string{}, known: &known}
	for _, resource := range d.Resources {
		in.declares[resource.Identity()] = declaredDigest(resource)
	}

	// Everything is attempted, and every failure is reported.
	//
	// **It used to stop at the first one**, and that made one broken resource hold the whole
	// machine hostage: a module declaring a package that does not exist meant every module
	// ordered after it was never applied, for ever, and the mesh reported "failed" without
	// saying that the rest had not been tried. A machine with one bad module and nine good ones
	// ran none of the nine.
	//
	// The argument for stopping was that a resource may depend on an earlier one. It still may —
	// and it will then fail its own check and be reported, which is more information than not
	// attempting it. A service started against a file that was never written does not verify, and
	// this host reads back after every write precisely so that is caught rather than assumed.
	//
	// What does not change: **a declaration that cannot be parsed is still refused whole.** That
	// is a different thing — one is "this machine could not do it", the other is "this was never
	// a declaration", and they are fixed in different places.
	var failures []*Error
	for i, resource := range ordered {
		if !orphansRemoved && i == guardFirst {
			// **Only a guard that is up may let the filter go.** Removing the derived filter's
			// resources stops its unit, whose stop deletes the mesh's table; if a guard resource
			// failed, doing that would leave the node with neither, and the store open until some
			// later reconcile gets the guard up (novox/hq ADR 0103).
			if len(failures) > 0 {
				first := failures[0]
				first.Done = report
				first.Others = len(failures) - 1
				log("  kept " + guardPrefix + "*: the guard is not up, so what it replaces was left in force")
				return report, known, first
			}
			if err := removeOrphans(); err != nil {
				return report, known, err
			}
		}
		// **On an adopted node, what is found is kept until its module is taken** (novox/hq ADR
		// 0100, ADR 0103). Before anything is applied: whatever of a module not yet taken is
		// present with no record of this host making it — or would reach what is — is held as it
		// is and reported. Once held it stays held until its module is taken, and it is never
		// recorded as applied, so it is never removed as an orphan either.
		if d.Adoption != nil {
			isHeld, news, outcome, err := holdOnAdopted(ctx, sys, resource, d, &known, before, run, keep,
				changed, time.Now().UTC())
			if err != nil {
				failures = append(failures, &Error{Resource: resource.Identity(), Err: err, Done: report})
				log(fmt.Sprintf("  failed %s (%s): %v", resource.Identity(), resource.Target(), err))
				continue
			}
			if isHeld {
				report.Outcomes = append(report.Outcomes, outcome)
				if news {
					log(fmt.Sprintf("  held %s (%s): %s", outcome.ID, outcome.Target, outcome.Detail))
				}
				continue
			}
		}

		// The private network takes over the tunnel it found, ahead of the service that replaces
		// it (novox/hq ADR 0105): its configuration kept, its unit stopped and disabled, never
		// flushed. A failure here fails the service too — the mesh's interface is not started on a
		// port the found one still holds.
		stoppedFound := false
		if svc, ok := resource.(*declaration.Service); ok && svc.TakesOver != nil {
			var outcome Outcome
			var facts TakenTunnel
			var err error
			if d.Adoption == nil {
				err = errNotAdopted
				facts = TakenTunnel{Interface: svc.TakesOver.Interface, State: NotTaken}
			} else {
				outcome, facts, stoppedFound, err = takeOver(ctx, sys, svc, d, &known, run, keep, time.Now().UTC())
			}
			// Always an account, failure included: the last account standing must never be an
			// older "taken" over a machine whose takeover has since gone wrong.
			report.Tunnel = &facts
			if err != nil {
				report.Tunnel.Note = err.Error()
				if stoppedFound {
					// The found unit is down and the mesh's not up: the one state where the
					// peers reach nothing. Started again, and said.
					restoreFound(ctx, sys, svc.TakesOver.Unit, run, report.Tunnel)
				}
				failures = append(failures, &Error{Resource: svc.Identity(), Err: err, Done: report})
				log(fmt.Sprintf("  failed %s (%s): %v", svc.Identity(), svc.Unit, err))
				continue
			}
			report.Outcomes = append(report.Outcomes, outcome)
			log(fmt.Sprintf("  held %s (%s): %s", outcome.ID, outcome.Target, outcome.Detail))
		}

		was, _ := known.Find(resource.Identity())
		var outcome Outcome
		var err error
		if o, isOpening := resource.(*declaration.Opening); isOpening {
			outcome, err = applyOpening(ctx, o, run, fw)
		} else {
			// A file this host has no record of, under any id, is the machine's until the mesh
			// writes over it — on any node, adopted or not: its original is kept first.
			var keepFound Keep
			if f, isFile := resource.(*declaration.File); isFile && was.ID == "" &&
				!known.Recorded(string(declaration.TypeFile), f.Path) {
				keepFound = keep
			}
			outcome, err = applyOne(ctx, sys, resource, run, changed, in, was, unseal, keepFound)
		}
		if err != nil {
			failed := &Error{Resource: resource.Identity(), Err: err, Done: report}
			failures = append(failures, failed)
			log(fmt.Sprintf("  failed %s (%s): %v", resource.Identity(), outcome.Target, err))
			if svc, ok := resource.(*declaration.Service); ok && svc.TakesOver != nil && report.Tunnel != nil {
				// The mesh's interface did not come up after the found one was stopped: no
				// tunnel at all. The found unit is started again — the machine goes back to
				// what it had — and the account says so (novox/hq ADR 0105).
				report.Tunnel.Note = "the mesh's interface did not come up: " + err.Error()
				if stoppedFound {
					restoreFound(ctx, sys, svc.TakesOver.Unit, run, report.Tunnel)
				} else {
					report.Tunnel.State = tunnelState(ctx, sys, svc.TakesOver.Unit, svc.Unit, run)
				}
			}

			// **A failed action stops what follows. Nothing else does.**
			//
			// An action is the only shape whose purpose is to make something true *before* the
			// next thing needs it — which is why it is the only one with a `verify`. The
			// bootstrap is a row of them: the store answers, then its databases exist, then their
			// schemas, then the broker. Carrying on past one that did not happen means starting
			// things against a machine that is not ready, and on a small machine that is how a
			// database still initialising gets its memory taken away and shuts down. Observed,
			// in the lab, caused by an earlier version of this loop.
			//
			// Everything else is independent state. A package that will not install has nothing
			// to do with a file on the other side of the declaration, and stopping there is what
			// made one broken module hold a whole machine hostage
			// (novox/hq 04-ISSUES/011).
			// A run-once container is a step with the same purpose as an action: to make
			// something true *before* the next thing needs it (novox/hq ADR 0052). A broker whose
			// dynsec store was not seeded must not be started, so a run-once step that did not
			// complete gates what follows exactly as a failed action does — that is the whole of
			// how "before that container starts" is enforced, since the step is declared before it.
			gates := resource.Kind() == declaration.TypeAction
			if c, ok := resource.(*declaration.Container); ok && c.RunOnce {
				gates = true
			}
			if gates {
				failed.Done = report
				failed.Others = len(failures) - 1
				failed.Gated = true
				return report, known, failed
			}
			continue
		}

		// Where the original of what this file replaced was kept, carried for as long as the
		// resource is recorded: kept by this apply, by a hold its module's cutover ends, or before.
		held, wasHeld := known.HeldAt(resource.Identity())
		kept := outcome.kept
		if kept == "" && wasHeld {
			kept = held.Kept
		}
		if kept == "" {
			kept = was.Kept
		}
		// Only now. The record follows the fact, never leads it.
		known.Record(store.Applied{
			Origin: origin,
			ID:     resource.Identity(), Type: string(resource.Kind()),
			Target: outcome.Target, AppliedAt: time.Now().UTC(),
			Wrote:     outcome.wrote,
			Into:      outcome.into,
			Kept:      kept,
			Reads:     outcome.reads,
			Stateless: outcome.stateless,
			Found:     outcome.found,
			Holds:     holds(resource),
		})
		// Its module has been taken, and what was held for it is now the mesh's. A file written
		// into, or a service whose lifecycle is the machine's, replaced nothing that was found, so
		// its outcome says what the apply did, not that a cutover happened; a hold from when it was
		// declared otherwise goes all the same — here, after the apply worked, so a failed one
		// keeps the hold and where its original is.
		if wasHeld {
			known.Release(held.ID)
			if !replacesNothing(resource) {
				outcome.Detail = takenDetail(held)
			}
		}
		if svc, ok := resource.(*declaration.Service); ok && svc.TakesOver != nil && report.Tunnel != nil {
			// The found interface is down and the mesh's is up in its place: the tunnel changed
			// hands (novox/hq ADR 0105). Read from the machine, not assumed.
			report.Tunnel.State = tunnelState(ctx, sys, svc.TakesOver.Unit, svc.Unit, run)
		}
		report.Outcomes = append(report.Outcomes, outcome)
		if outcome.Action != "unchanged" {
			changed[resource.Identity()] = true
			// With the detail, when there is one: "updated app" says a container was replaced;
			// which file made that happen is what somebody reading the log at the time needs
			// (novox/hq 04-ISSUES/103).
			line := fmt.Sprintf("  %s %s (%s)", outcome.Action, outcome.ID, outcome.Target)
			if outcome.Detail != "" {
				line += ": " + outcome.Detail
			}
			log(line)
		}
	}

	if !orphansRemoved {
		if err := removeOrphans(); err != nil {
			return report, known, err
		}
	}

	// A converged node whose found firewall was in force retires it only now, once everything —
	// the mesh's derived filter among it — applied cleanly (novox/hq ADR 0100).
	if len(failures) == 0 {
		if err := retireFirewall(ctx, d, origin, &known, run, log); err != nil {
			return report, known, &Error{Resource: "the firewall found on this machine", Err: err, Done: report}
		}
		for _, orphan := range protecting {
			if err := removeOrphan(orphan); err != nil {
				return report, known, err
			}
		}
	}

	if len(failures) > 0 {
		// The first, carrying everything that did happen. One error is what the caller reports
		// and what a person reads first; the rest are in the report, which is what the mesh
		// keeps.
		first := failures[0]
		first.Done = report
		first.Others = len(failures) - 1
		return report, known, first
	}
	return report, known, nil
}

// guardPrefix is the ids of the mesh's guard on an adopted node: its package, table, unit and
// service (novox/hq ADR 0100).
const guardPrefix = declaration.AdoptionPrefix + "guard"

// Unseal opens a value the mesh sealed to this node. Nil when the node has no sealing key, which
// makes every sealed file an error rather than a silently skipped one.
type Unseal func(sealed string) ([]byte, error)

func applyOne(ctx context.Context, sys system.System, r declaration.Resource, run Runner,
	changed map[string]bool, in inputs, previous store.Applied,
	unseal Unseal, keepFound Keep) (Outcome, error) {
	switch res := r.(type) {
	case *declaration.Directory:
		return applyDirectory(res)
	case *declaration.File:
		return applyFile(res, previous, unseal, keepFound)
	case *declaration.Service:
		return applyService(ctx, sys, res, run, changed, previous)
	case *declaration.Package:
		return applyPackage(ctx, sys, res, run)
	case *declaration.Container:
		return applyContainer(ctx, res, run, changed, in, previous)
	case *declaration.User:
		return applyUser(ctx, sys, res, run)
	case *declaration.Archive:
		return applyArchive(ctx, res, previous)
	case *declaration.Process:
		return applyProcess(ctx, res, run, changed, previous)
	case *declaration.Action:
		return applyAction(ctx, res, run)
	case *declaration.Network:
		return applyNetwork(ctx, res, run)
	case *declaration.Access:
		return applyAccess(res)
	default:
		// Unreachable: the declaration refused this already. Present because "unreachable"
		// stops being true the moment someone adds a kind and forgets this switch.
		return Outcome{}, fmt.Errorf("no applier for type %q", r.Kind())
	}
}

// begin starts an outcome from any resource, so the three facts a report needs are read from
// the resource itself rather than restated by each applier.
func begin(r declaration.Resource) Outcome {
	return Outcome{ID: r.Identity(), Type: string(r.Kind()), Target: r.Target()}
}

func modeOf(spec string, fallback os.FileMode) (os.FileMode, error) {
	if spec == "" {
		return fallback, nil
	}
	parsed, err := strconv.ParseUint(spec, 8, 32)
	if err != nil {
		return 0, fmt.Errorf("mode %q: %w", spec, err)
	}
	return os.FileMode(parsed), nil
}

func applyDirectory(r *declaration.Directory) (Outcome, error) {
	out := begin(r)
	mode, err := modeOf(r.Mode, 0o755)
	if err != nil {
		return out, err
	}

	before, err := os.Stat(r.Path)
	existed := err == nil
	if err != nil && !errors.Is(err, os.ErrNotExist) {
		return out, err
	}
	if existed && !before.IsDir() {
		return out, fmt.Errorf("%s exists and is not a directory", r.Path)
	}

	if !existed {
		if err := os.MkdirAll(r.Path, mode); err != nil {
			return out, err
		}
	}
	// Set explicitly even when it existed: MkdirAll applies the mode only on creation, and a
	// permission set at creation is not a permission maintained — a lesson this repository
	// already paid for once, with world-readable environment files.
	if err := os.Chmod(r.Path, mode); err != nil {
		return out, err
	}

	// Read back.
	after, err := os.Stat(r.Path)
	if err != nil {
		return out, fmt.Errorf("made %s and cannot stat it: %w", r.Path, err)
	}
	if !after.IsDir() {
		return out, fmt.Errorf("%s is not a directory after applying", r.Path)
	}
	if after.Mode().Perm() != mode.Perm() {
		return out, fmt.Errorf("%s is mode %o after setting %o", r.Path, after.Mode().Perm(), mode.Perm())
	}

	ownedAlready, err := ownedBy(r.Path, r.Owner)
	if err != nil {
		return out, err
	}
	if !ownedAlready {
		if err := own(r.Path, r.Owner); err != nil {
			return out, err
		}
	}

	out.Action = "unchanged"
	if !existed {
		out.Action = "created"
	} else if !ownedAlready {
		out.Action = "updated"
	} else if before.Mode().Perm() != mode.Perm() {
		out.Action = "updated"
		out.Detail = fmt.Sprintf("mode %o to %o", before.Mode().Perm(), mode.Perm())
	}
	return out, nil
}

// applyAccess confirms an operator-owned path is present, and owns nothing about it.
//
// **The mirror image of applyDirectory** (novox/hq ADR 0051). A directory the host makes, chmods,
// chowns and removes when empty. An access it does none of: the path is the operator's — a media
// library, a download spool that several modules share — and the host's only job is to be sure it
// is there before anything mounts it.
//
// **Absent is refused, not created.** A bind mount whose source does not exist is made for you by
// the container runtime, as root, with whatever mode it picks — which is exactly the silent
// wrong-ownership 04-ISSUES/026 records. So the host checks first and says plainly that the
// operator must provide the path, rather than conjuring a directory it does not own and cannot
// give the right owner. Nothing is written, so this never reports a change: the machine did not
// move, the host merely confirmed a fact about it.
func applyAccess(r *declaration.Access) (Outcome, error) {
	out := begin(r)
	info, err := os.Stat(r.Path)
	if errors.Is(err, os.ErrNotExist) {
		return out, fmt.Errorf(
			"%s is not there, and the mesh does not own it — the operator must provide it. It is "+
				"shared, pre-existing data (novox/hq ADR 0051): the host mounts it and creates "+
				"nothing, so a missing one is said here rather than made as root by the container "+
				"runtime", r.Path)
	}
	if err != nil {
		return out, err
	}
	if !info.IsDir() {
		return out, fmt.Errorf(
			"%s is not a directory, and an access is a shared directory the operator provides",
			r.Path)
	}
	mode := declaration.AccessRead
	if r.Mode != "" {
		mode = r.Mode
	}
	out.Action = "unchanged"
	out.Detail = "operator-owned; present, " + mode + ", nothing managed"
	return out, nil
}

// keepFound, when not nil, is where the original of a file this host has no record of is kept
// before it is written over (novox/hq ADR 0100): once, never overwritten, and named in the outcome.
func applyFile(r *declaration.File, previous store.Applied, unseal Unseal, keepFound Keep) (Outcome, error) {
	if r.Into == declaration.IntoBlock {
		return applyBlock(r, previous)
	}
	if r.Into != "" {
		return applyInto(r, previous)
	}
	out := begin(r)

	// What actually goes on disk. For a sealed file the mesh never had this, and neither did
	// whatever carried the declaration here.
	content := r.Content
	if r.Bytes != "" {
		// Not text. Decoded here rather than written as base64, because what a declaration says
		// is in a file has to be what ends up in it — a wallpaper stored as its own encoding is
		// a wallpaper nothing can open.
		decoded, err := base64.StdEncoding.DecodeString(r.Bytes)
		if err != nil {
			return out, fmt.Errorf("%s carries bytes that are not base64: %w", r.Path, err)
		}
		content = string(decoded)
	}
	// A secret written world-readable is a secret. The default differs from an ordinary file's
	// for that reason alone; an explicit mode still wins, because a module may need its own user
	// to read it and only the module knows which.
	fallback := os.FileMode(0o644)
	if r.Secret() {
		fallback = 0o600
		if unseal == nil {
			// Refused rather than skipped. A machine that quietly does not apply the one resource
			// carrying a credential is a machine that looks configured and cannot connect.
			return out, fmt.Errorf(
				"%s is sealed to this node and this node has no sealing key", r.Path)
		}
		opened, err := unseal(r.Sealed)
		if err != nil {
			return out, fmt.Errorf("cannot open %s: %w", r.Path, err)
		}
		content = string(opened)
	}
	// Sealed values into the holes the content left for them. **The host is the only thing that
	// ever holds both** — the mesh discarded the value, and the module wrote the document without
	// it (novox/hq ADR 0024).
	if len(r.Secrets) > 0 {
		if unseal == nil {
			return out, fmt.Errorf(
				"%s needs %d sealed value(s) and this node has no sealing key",
				r.Path, len(r.Secrets))
		}
		for _, name := range r.SecretsUsed() {
			opened, err := unseal(r.Secrets[name])
			if err != nil {
				return out, fmt.Errorf("cannot open the secret %q for %s: %w", name, r.Path, err)
			}
			content = strings.ReplaceAll(content, "${secret:"+name+"}", string(opened))
		}
		// A file carrying a credential is not world-readable, whatever else it also carries.
		if r.Mode == "" {
			fallback = 0o600
		}
	}
	out.wrote = digestOf(content)
	mode, err := modeOf(r.Mode, fallback)
	if err != nil {
		return out, err
	}

	existing, readErr := os.ReadFile(r.Path)
	existed := readErr == nil
	if readErr != nil && !errors.Is(readErr, os.ErrNotExist) {
		return out, readErr
	}
	// A seed that is already there is left exactly as it is — whatever has grown in it since is
	// not the mesh's to put back (novox/hq issue 035). What this host records is what it once
	// wrote, so a later declaration that changes the seed is not mistaken for drift either.
	if r.CreateOnce && existed {
		out.wrote = previous.Wrote
		if out.wrote == "" {
			out.wrote = digestOf(string(existing))
		}
		out.Action = "kept"
		out.Detail = "created once, and present; what is in it now is not the mesh's to change"
		return out, nil
	}

	var beforeMode os.FileMode
	if existed {
		if info, err := os.Stat(r.Path); err == nil {
			beforeMode = info.Mode().Perm()
		}
	}

	contentSame := existed && string(existing) == content

	// Whether the machine still holds what this host last put there. When it does not, and the
	// declaration has not changed either, somebody edited it — and saying so is the whole
	// difference between a change that vanishes mysteriously and one that is reported.
	drifted := existed && previous.Wrote != "" && digestOf(string(existing)) != previous.Wrote
	modeSame := existed && beforeMode == mode.Perm()

	kept := ""
	if !contentSame {
		if existed && keepFound != nil {
			// Before anything is written: a keep that fails stops the write, since the
			// original could not be had back otherwise.
			if kept, err = keepFound(r.Path, existing, beforeMode); err != nil {
				return out, fmt.Errorf("keeping the original of %s before writing over it: %w", r.Path, err)
			}
		}
		if err := os.MkdirAll(filepath.Dir(r.Path), 0o755); err != nil {
			return out, err
		}
		if err := writeAtomically(r.Path, []byte(content), mode); err != nil {
			return out, err
		}
	} else if !modeSame {
		if err := os.Chmod(r.Path, mode); err != nil {
			return out, err
		}
	}

	// Read back — the file, not the call that wrote it.
	written, err := os.ReadFile(r.Path)
	if err != nil {
		return out, fmt.Errorf("wrote %s and cannot read it back: %w", r.Path, err)
	}
	if string(written) != content {
		return out, fmt.Errorf("%s does not contain what was declared after writing it", r.Path)
	}
	info, err := os.Stat(r.Path)
	if err != nil {
		return out, err
	}
	if info.Mode().Perm() != mode.Perm() {
		return out, fmt.Errorf("%s is mode %o after setting %o", r.Path, info.Mode().Perm(), mode.Perm())
	}

	// And who it belongs to. Checked before setting, so a file already owned correctly is not
	// reported as changed on every apply — which would make every reconcile look like work.
	ownedAlready, err := ownedBy(r.Path, r.Owner)
	if err != nil {
		return out, err
	}
	if !ownedAlready {
		if err := own(r.Path, r.Owner); err != nil {
			return out, err
		}
		contentSame = false
	}

	switch {
	case !existed:
		out.Action = "created"
	case drifted:
		// Somebody changed this on the machine. The mesh puts it back either way — that is what
		// holding a machine to what it was told means — but a change that vanishes with nothing
		// said is how a person ends up editing the same file every five minutes, believing the
		// machine is broken.
		out.Action = "corrected"
		out.Detail = "it had been changed on the machine since this host last wrote it"
	case !contentSame && !modeSame:
		out.Action = "updated"
		out.Detail = "content and mode"
	case !contentSame:
		out.Action = "updated"
		out.Detail = "content"
	case !modeSame:
		out.Action = "updated"
		out.Detail = fmt.Sprintf("mode %o to %o", beforeMode, mode.Perm())
	default:
		out.Action = "unchanged"
	}
	out.kept = kept
	if kept != "" {
		if out.Detail != "" {
			out.Detail += "; "
		}
		out.Detail += "the file found here, which the mesh had no record of, was kept at " + kept
	}
	return out, nil
}

// writeAtomically writes through a temporary file in the same directory.
//
// A reader of a managed file must never see half of one. The mesh's own configuration is read
// by daemons that reload on change, so a torn write is a service reading a truncated config.
func writeAtomically(path string, content []byte, mode os.FileMode) error {
	tmp, err := os.CreateTemp(filepath.Dir(path), ".mesh-host-*")
	if err != nil {
		return err
	}
	defer os.Remove(tmp.Name())

	if _, err := tmp.Write(content); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Close(); err != nil {
		return err
	}
	if err := os.Chmod(tmp.Name(), mode); err != nil {
		return err
	}
	return os.Rename(tmp.Name(), path)
}

// reflects reports whether anything this service must mirror changed in this apply.
func reflects(r *declaration.Service, changed map[string]bool) bool {
	return len(reflected(r, changed)) > 0
}

// reflected is which of them changed, so the outcome can say why the service was restarted. A
// restart with no reason given is indistinguishable from a service that keeps falling over.
func reflected(r *declaration.Service, changed map[string]bool) []string {
	return restartedBy(r.RestartOn, changed)
}

// serviceReloader is a service manager that can tell a running unit to read its configuration again.
type serviceReloader interface {
	ReloadService(ctx context.Context, run system.Runner, unit string) error
}

// unitReloader is a service manager that caches unit files and must be told to read them again.
type unitReloader interface {
	ReloadUnits(ctx context.Context, run system.Runner) error
}

func applyService(ctx context.Context, sys system.System, r *declaration.Service, run Runner,
	changed map[string]bool, previous store.Applied) (Outcome, error) {
	if r.Stateless() {
		return reflectOnly(ctx, sys, r, run, changed)
	}
	out := begin(r)

	// **What the unit was before the mesh touched it**, read once — the first time this host
	// applies it — and carried in the record from then on (novox/hq ADR 0118). Undeclared, the
	// unit is given back to exactly this: it is the one fact that separates the container runtime,
	// running before the mesh arrived and to be left running, from the mesh's packet filter,
	// stopped until the mesh started it and to be stopped again. A record that predates this
	// field is not read now: the unit's state by then is the mesh's doing, not what was found.
	switch {
	case previous.Found != nil:
		out.found = previous.Found
	case previous.ID == "":
		state, err := sys.ServiceState(ctx, run, r.Unit)
		if err != nil {
			return out, err
		}
		found := &store.FoundUnit{State: state}
		if r.Boot != "" {
			if found.Boot, err = sys.ServiceBoot(ctx, run, r.Unit); err != nil {
				return out, err
			}
		}
		out.found = found
	}
	var changes []string

	// A file the service reflects changed, and it may be the unit's own file or a drop-in: the
	// service manager reads those again only when told to, and a restart without it runs the unit
	// it had already loaded.
	if reflects(r, changed) {
		if u, ok := sys.(unitReloader); ok {
			if err := u.ReloadUnits(ctx, run); err != nil {
				return out, fmt.Errorf("reloading the service manager's units for %s: %w", r.Unit, err)
			}
		}
	}

	// Boot first. A unit asked to be running and enabled should survive this apply failing
	// half way in the more useful direction: enabled-and-stopped comes back at the next boot,
	// where running-and-disabled does not.
	if r.Boot != "" {
		bootBefore, err := sys.ServiceBoot(ctx, run, r.Unit)
		if err != nil {
			return out, err
		}
		if bootBefore != r.Boot {
			if err := sys.SetServiceBoot(ctx, run, r.Unit, r.Boot); err != nil {
				return out, fmt.Errorf("setting %s to %s at boot: %w", r.Unit, r.Boot, err)
			}
			bootAfter, err := sys.ServiceBoot(ctx, run, r.Unit)
			if err != nil {
				return out, err
			}
			if bootAfter != r.Boot {
				return out, fmt.Errorf(
					"%s was asked to be %s at boot and is %s", r.Unit, r.Boot, bootAfter)
			}
			changes = append(changes, "boot "+bootBefore+" to "+bootAfter)
		}
	}

	before, err := sys.ServiceState(ctx, run, r.Unit)
	if err != nil {
		return out, err
	}
	if before != r.State {
		if err := sys.SetServiceState(ctx, run, r.Unit, r.State); err != nil {
			return out, fmt.Errorf("setting %s to %s: %w", r.Unit, r.State, err)
		}

		// Read back. A service manager accepting a command says the transaction was accepted,
		// not that the unit is running — one that starts and immediately dies satisfies it.
		after, err := sys.ServiceState(ctx, run, r.Unit)
		if err != nil {
			return out, err
		}
		if after != r.State {
			return out, fmt.Errorf("%s was asked to be %s and is %s", r.Unit, r.State, after)
		}
		changes = append(changes, before+" to "+after)
	} else if r.State == "running" && reflects(r, changed) {
		// The service is already in the state it was asked for, and something it must reflect
		// changed in this same apply. A running service does not re-read its configuration, so
		// leaving it alone here is how a machine ends up correct on disk and wrong in fact —
		// with every check passing.
		if err := sys.SetServiceState(ctx, run, r.Unit, "stopped"); err != nil {
			return out, fmt.Errorf("restarting %s: stopping it: %w", r.Unit, err)
		}
		if err := sys.SetServiceState(ctx, run, r.Unit, "running"); err != nil {
			return out, fmt.Errorf("restarting %s: starting it again: %w", r.Unit, err)
		}
		// Read back, for the same reason as above: a unit that starts and immediately dies
		// satisfies a service manager and nothing else.
		after, err := sys.ServiceState(ctx, run, r.Unit)
		if err != nil {
			return out, err
		}
		if after != "running" {
			return out, fmt.Errorf(
				"%s was restarted to pick up a change and is %s", r.Unit, after)
		}
		changes = append(changes, "restarted for "+strings.Join(reflected(r, changed), ", "))
	} else if r.State == "running" && len(restartedBy(r.ReloadOn, changed)) > 0 {
		// Told to read its configuration again, not stopped: for a service whose restart would
		// stop what it runs — every container, for the container runtime (novox/hq ADR 0102).
		reloader, ok := sys.(serviceReloader)
		if !ok {
			return out, fmt.Errorf("%s must be reloaded for %s and this machine's service manager "+
				"cannot reload a unit", r.Unit, strings.Join(restartedBy(r.ReloadOn, changed), ", "))
		}
		if err := reloader.ReloadService(ctx, run, r.Unit); err != nil {
			return out, fmt.Errorf("reloading %s: %w", r.Unit, err)
		}
		after, err := sys.ServiceState(ctx, run, r.Unit)
		if err != nil {
			return out, err
		}
		if after != "running" {
			return out, fmt.Errorf("%s was reloaded to pick up a change and is %s", r.Unit, after)
		}
		changes = append(changes, "reloaded for "+strings.Join(restartedBy(r.ReloadOn, changed), ", "))
	}

	if len(changes) == 0 {
		out.Action = "unchanged"
		out.Detail = before
		return out, nil
	}
	out.Action = "updated"
	out.Detail = strings.Join(changes, ", ")
	return out, nil
}

// reflectOnly is a service whose unit's lifecycle is the machine's (novox/hq ADR 0117): nothing is
// started, stopped, enabled or disabled, and a changed trigger is acted on only where the unit is
// already running. An inactive unit is left so — started, it would be a second network manager on
// a machine that uses another — and it reads the change when whatever starts it does.
func reflectOnly(ctx context.Context, sys system.System, r *declaration.Service, run Runner,
	changed map[string]bool) (Outcome, error) {
	out := begin(r)
	out.stateless = true
	restart := reflected(r, changed)
	reload := restartedBy(r.ReloadOn, changed)
	if len(restart) == 0 && len(reload) == 0 {
		out.Action = "unchanged"
		out.Detail = "its lifecycle is the machine's; nothing it reflects changed"
		return out, nil
	}
	state, err := sys.ServiceState(ctx, run, r.Unit)
	if err != nil {
		return out, err
	}
	if state != "running" {
		out.Action = "unchanged"
		out.Detail = "not running; the change applies at its next start"
		return out, nil
	}
	if len(restart) > 0 {
		// The same as a stated service: the unit's own file may be what changed, and the manager
		// reads that again only when told to.
		if u, ok := sys.(unitReloader); ok {
			if err := u.ReloadUnits(ctx, run); err != nil {
				return out, fmt.Errorf("reloading the service manager's units for %s: %w", r.Unit, err)
			}
		}
		if err := sys.SetServiceState(ctx, run, r.Unit, "stopped"); err != nil {
			return out, fmt.Errorf("restarting %s: stopping it: %w", r.Unit, err)
		}
		if err := sys.SetServiceState(ctx, run, r.Unit, "running"); err != nil {
			return out, fmt.Errorf("restarting %s: starting it again: %w", r.Unit, err)
		}
	} else {
		reloader, ok := sys.(serviceReloader)
		if !ok {
			return out, fmt.Errorf("%s must be reloaded for %s and this machine's service manager "+
				"cannot reload a unit", r.Unit, strings.Join(reload, ", "))
		}
		if err := reloader.ReloadService(ctx, run, r.Unit); err != nil {
			return out, fmt.Errorf("reloading %s: %w", r.Unit, err)
		}
	}
	// Read back: it was running, and a restart or reload that left it otherwise is a failure —
	// the machine's network manager down is not a change to report and move past.
	after, err := sys.ServiceState(ctx, run, r.Unit)
	if err != nil {
		return out, err
	}
	out.Action = "updated"
	if len(restart) > 0 {
		out.Detail = "restarted for " + strings.Join(restart, ", ")
	} else {
		out.Detail = "reloaded for " + strings.Join(reload, ", ")
	}
	if after != "running" {
		return out, fmt.Errorf("%s was %s to pick up a change and is %s", r.Unit,
			strings.Fields(out.Detail)[0], after)
	}
	return out, nil
}

// remove undoes one resource the host applied and the declaration no longer names, and reports
// what it actually did.
//
// Only ever called for something in the store, which is what bounds it: the host is
// authoritative over its own footprint and inert everywhere else (novox/hq ADR 0005).
//
// It returns the action rather than assuming "removed", because for half the vocabulary the
// honest word is "forgotten". A host that reported a package removed when it left the package
// installed would be describing an effect it declined to have.
func remove(ctx context.Context, sys system.System, a store.Applied, run Runner) (string, string, error) {
	switch declaration.Type(a.Type) {
	case declaration.TypeDirectory:
		// **A directory with anything left in it is kept, and that is the rule that protects
		// data.** Everything the mesh put inside is itself a declared resource, and orphans are
		// removed in reverse declaration order — so by the time a directory comes to be removed,
		// what the mesh wrote there is already gone. Anything still present is something nobody
		// declared: a database's files, a mail spool, somebody's uploads.
		//
		// Before this, unassigning a module deleted its data directory and everything under it,
		// and the report said "removed". Nothing anywhere said what had been in there.
		//
		// This is the host's own line, applied to the one shape where getting it wrong is not
		// recoverable: it removes what it made and leaves what it merely configured. An empty
		// directory is what it made. A full one is not.
		entries, err := os.ReadDir(a.Target)
		if errors.Is(err, os.ErrNotExist) {
			return "forgotten", "no longer there", nil
		}
		if err != nil {
			return "", "", err
		}
		if len(entries) > 0 {
			return "kept", fmt.Sprintf(
				"no longer declared, and %d item(s) inside that the mesh did not put there — "+
					"remove it by hand once you know what it is", len(entries)), nil
		}
		if err := os.Remove(a.Target); err != nil {
			return "", "", err
		}
		return "removed", "no longer declared, and empty", nil

	case declaration.TypeFile:
		if a.Into != nil && a.Into.Format == declaration.IntoBlock {
			return removeBlock(a)
		}
		if a.Into != nil {
			return removeInto(a)
		}
		if err := os.RemoveAll(a.Target); err != nil {
			return "", "", err
		}
		if _, err := os.Stat(a.Target); !errors.Is(err, os.ErrNotExist) {
			return "", "", fmt.Errorf("%s is still there after removing it", a.Target)
		}
		return "removed", "no longer declared", nil

	case declaration.TypeService:
		return removeService(ctx, sys, a, run)

	case declaration.TypeProcess:
		// The other side of the same line: a process's unit is the host's own — it wrote the unit
		// file and unpacked the bundle — so it goes with its declaration (novox/hq ADR 0118).
		return removeProcess(ctx, a, run)

	case declaration.TypeContainer:
		// The host CREATED this one, so the host removes it. That is the line: it removes what
		// it made and leaves what it merely configured.
		if _, err := run(ctx, "docker", "rm", "-f", a.Target); err != nil {
			// Already gone is the state removal wants. Anything else is a real failure.
			if _, alive := containerState(ctx, a.Target, run); alive == nil {
				return "", "", fmt.Errorf("removing container %s: %w", a.Target, err)
			}
		}
		if _, err := containerState(ctx, a.Target, run); err == nil {
			return "", "", fmt.Errorf("container %s is still there after removing it", a.Target)
		}
		return "removed", "no longer declared", nil

	case declaration.TypePackage:
		// Deliberately not uninstalled, and this is a decision rather than an omission.
		//
		// The host cannot know what else on this machine needs the package. Uninstalling a
		// container runtime because a declaration changed would stop every container on the
		// node, and the machine may have had the package before the mesh ever saw it
		// (novox/hq research 012: adopted, not installed). Undeclaring says "the mesh no
		// longer requires this", which is not the same as "remove it".
		return "forgotten", "left installed; the host does not uninstall what it cannot know is unused", nil

	case declaration.TypeAction:
		// An action has no footprint the host can undo — it ran, and whatever it did belongs
		// to whatever it acted on.
		return "forgotten", "an action leaves nothing the host owns", nil

	case declaration.TypeAccess:
		// The path is the operator's and the host never owned it (novox/hq ADR 0051). Undeclaring
		// it says only that this module no longer reaches it — not that the media library should
		// be touched. So the record is dropped and the path left exactly as it is; removing it
		// would be the data loss ADR 0030 exists to prevent, on a directory the mesh never made.
		return "forgotten", "an operator-owned path is never the host's to remove", nil

	case declaration.TypeNetwork:
		// **The reason this is a shape at all** (novox/hq ADR 0029). Orphans are removed in
		// reverse declaration order, so a network written before the containers that join it is
		// removed after they are gone — and a runtime refusing to remove one still in use is
		// reported rather than swallowed, because that means something the mesh did not declare
		// is holding it.
		cri, err := containerRuntime(ctx, run)
		if err != nil {
			return "", "", fmt.Errorf("%w, so the network %q cannot be removed", err, a.Target)
		}
		if _, err := run(ctx, cri, "network", "inspect", a.Target); err != nil {
			return "forgotten", "no longer there", nil
		}
		if _, err := run(ctx, cri, "network", "rm", a.Target); err != nil {
			return "", "", fmt.Errorf("cannot remove the network %q: %w", a.Target, err)
		}
		return "removed", "no longer declared", nil

	default:
		return "", "", fmt.Errorf("no way to remove a %q", a.Type)
	}
}

// ExecRunner runs a real command, with stdin closed and output captured.
func ExecRunner(ctx context.Context, name string, args ...string) (string, error) {
	cmd := exec.CommandContext(ctx, name, args...)
	cmd.Stdin = nil
	out, err := cmd.Output()
	if err != nil {
		var exit *exec.ExitError
		if errors.As(err, &exit) {
			return string(out), fmt.Errorf("%s exited %d: %s",
				name, exit.ExitCode(), strings.TrimSpace(string(exit.Stderr)))
		}
		return string(out), fmt.Errorf("%s: %w", name, err)
	}
	return string(out), nil
}

// applyPackage installs a package the machine does not have.
//
// It never upgrades and never removes. "Present" is the whole of what a package resource
// asserts, because version is the package manager's business and the mesh does not have a
// second opinion about it (novox/hq ADR 0005 — the host depends on nothing, and that includes
// not becoming a second package manager).
func applyPackage(ctx context.Context, sys system.System, r *declaration.Package, run Runner) (Outcome, error) {
	out := begin(r)

	installed, err := sys.PackageInstalled(ctx, run, r.Package)
	if err != nil {
		return out, err
	}
	if installed {
		out.Action = "unchanged"
		out.Detail = "already installed"
		return out, nil
	}

	if err := sys.InstallPackage(ctx, run, r.Package); err != nil {
		return out, fmt.Errorf("installing %s: %w", r.Package, err)
	}

	// Read back. A package manager exiting zero says the transaction was accepted.
	installed, err = sys.PackageInstalled(ctx, run, r.Package)
	if err != nil {
		return out, err
	}
	if !installed {
		return out, fmt.Errorf(
			"%s was installed without error and the package database does not have it", r.Package)
	}

	out.Action = "created"
	return out, nil
}

// Labels the host puts on every container it creates.
//
// specLabel carries a digest of the declaration that made the container. It is what lets a
// reconcile answer "is this container the one the current declaration describes" without
// comparing every field the runtime reports — which cannot be done reliably, because a runtime
// normalises, defaults and reorders what it is given, and the differences that produces are
// indistinguishable from real drift.
const (
	specLabel = "mesh-host.spec"
	idLabel   = "mesh-host.id"
)

// inputs is what a container takes in when it is created beyond its own declaration: what each
// resource it names under restart-on currently declares, and what the files it reads hold.
type inputs struct {
	// declares is each resource's declared digest, by id (declaredDigest).
	declares map[string]string
	// known is the node's state as it stands when the container is reached — so a file applied
	// earlier in the same pass is already its new self. Nil where nothing was written: a test,
	// or a scheduled fire, which reads no file at creation.
	known *store.State
}

// fileDigest is what a file the container reads holds, by digest.
//
// **What this host wrote when it has a record of writing it, and what is on disk when it has
// not.** The record is preferred because it is what the host means by the file: a seed created
// once digests as the seed, not as whatever the service has grown in it, and a file written into
// digests as the mesh's keys, not the machine's (novox/hq ADR 0102). A file the host never wrote
// — an env-file a predecessor left, the superuser secret genesis writes before any declaration
// names it — is read, so the digest is the same one the host records when it later writes the
// same bytes there, and adopting a running store in place stays a reconcile rather than a
// recreate (bootstrap phase three; genesis writes the value with no line ending for exactly this
// reason). Empty when there is nothing readable there: the runtime refuses an absent env-file
// itself, with a better message than this could give.
func (in inputs) fileDigest(path string) string {
	if in.known != nil {
		if f, recorded := in.known.At(string(declaration.TypeFile), path); recorded {
			return f.Wrote
		}
	}
	info, err := os.Stat(path)
	if err != nil || !info.Mode().IsRegular() {
		return ""
	}
	content, err := os.ReadFile(path)
	if err != nil {
		return ""
	}
	return digestOf(string(content))
}

// reads is every file a running container takes in when it is created, by path and digest
// (novox/hq 04-ISSUES/103).
//
//   - every env-file: the runtime reads it once, at create, and `docker restart` hands the
//     container the same environment it had.
//   - a file bind-mounted into it, DIRECTLY, by its content: a secret, a credential file.
//
// **A directory bind-mounted into it is not looked inside**, not even for the files this host
// wrote there. What a service reads out of a mounted directory, and when, is the service's
// business: the route proxy re-reads its routes file live and would be recreated on every route
// change; a provisioner sidecar polls what it receives every few seconds and would be killed
// mid-reconcile on every grant. A module whose container does read such a file once, at start,
// says so with restart-on — that is what the field is for, and it stays the opt-in.
//
// A step is not here either: a run-once or scheduled container reads its files when it runs, and
// runs fresh each time. Only a container that stays running holds what it read.
func (in inputs) reads(r *declaration.Container) map[string]string {
	if r.RunOnce || r.Schedule != "" {
		return nil
	}
	out := map[string]string{}
	for _, path := range r.EnvFile {
		out[path] = in.fileDigest(path)
	}
	for _, v := range r.Volumes {
		src := mountSource(v)
		if !strings.HasPrefix(src, "/") {
			continue // a named volume: the runtime's, holding data
		}
		// fileDigest is empty for a directory, and for anything else that is not a regular file.
		if digest := in.fileDigest(src); digest != "" {
			out[src] = digest
		}
	}
	if len(out) == 0 {
		return nil
	}
	return out
}

// containerSpec is the identity of a declared container: everything that, if changed, means
// the running container is no longer what was asked for.
func containerSpec(r *declaration.Container, in inputs) string {
	return containerSpecReading(r, in.declares, in.reads(r))
}

// containerSpecReading is containerSpec with what the container reads already read — so an
// applier that also records those digests reads each file once, and the label and the record
// cannot disagree about a file that moved between two reads.
func containerSpecReading(r *declaration.Container, declares, reads map[string]string) string {
	keys := make([]string, 0, len(r.Env))
	for k := range r.Env {
		keys = append(keys, k)
	}
	sort.Strings(keys)

	var b strings.Builder
	b.WriteString(r.Image + "\n" + r.Name + "\n")
	for _, k := range keys {
		b.WriteString("env " + k + "=" + r.Env[k] + "\n")
	}
	for _, p := range r.Ports {
		b.WriteString("port " + p + "\n")
	}
	for _, v := range r.Volumes {
		b.WriteString("volume " + v + "\n")
	}
	for _, a := range r.Args {
		b.WriteString("arg " + a + "\n")
	}
	// The resolver and address are part of what was declared: a container whose dns or ip moved
	// is a different container, or the fields could never reach one that already ran — which is
	// exactly how their first deployment silently changed nothing.
	for _, d := range r.Dns {
		b.WriteString("dns " + d + "\n")
	}
	if r.IP != "" {
		b.WriteString("ip " + r.IP + "\n")
	}
	// The cadence is part of what was declared, so a changed schedule is a changed spec — the marker
	// moves and the install is reported "updated" and re-established. Added only when present, so no
	// ordinary container's or run-once step's digest moves for a field it does not set.
	if r.Schedule != "" {
		b.WriteString("schedule " + r.Schedule + "\n")
	}
	// **What this container reads is part of what it is.**
	//
	// A container takes its environment and its mounted files once, at start, and never looks
	// again. Comparing only the fields above meant a container whose configuration had since been
	// rewritten compared equal and was left alone — running values the machine no longer holds,
	// while every check reported success (novox/hq 04-ISSUES/045). Naming what it depends on here
	// makes that comparison standing rather than a tripwire that fires during one apply and never
	// again. Sorted, so the digest does not move for a reordering nobody made.
	depends := append([]string{}, r.RestartOn...)
	sort.Strings(depends)
	for _, id := range depends {
		b.WriteString("reads " + id + "=" + declares[id] + "\n")
	}
	// And what the files it reads at creation hold — not only their paths, which `volume` and the
	// env-file arguments already name. The spec named the env-file's path and not its content,
	// so the host rewrote two environment files with the store's new port and left both
	// containers running with the old one, healthy-looking, until they answered 502 (novox/hq
	// 04-ISSUES/103). Added only when there is something read, so a container that reads nothing
	// keeps the digest it had.
	for _, path := range sortedKeys(reads) {
		b.WriteString("file " + path + "=" + reads[path] + "\n")
	}
	return fmt.Sprintf("%x", sha256.Sum256([]byte(b.String())))
}

// containerState reports whether a container is running and which spec made it.
// The error means the container does not exist.
//
// `container inspect`, not the bare form: a name is not unique across object kinds (a network and
// its container are routinely named alike), and the bare form can resolve to the wrong kind
// instead of reporting absence — see inspectFound in hold.go for the failure this produces.
func containerState(ctx context.Context, name string, run Runner) (state struct {
	Running bool
	Spec    string
}, err error) {
	out, err := run(ctx, "docker", "container", "inspect", "--format",
		"{{.State.Running}}\t{{index .Config.Labels \""+specLabel+"\"}}", name)
	if err != nil {
		return state, fmt.Errorf("no container named %s", name)
	}
	running, spec, _ := strings.Cut(strings.TrimSpace(out), "\t")
	state.Running = running == "true"
	state.Spec = strings.TrimSpace(spec)
	return state, nil
}

// applyContainer makes the declared container the one that is running.
//
// There is no "update" for a container: a container's configuration is fixed when it is
// created, so any change is a replacement. Saying that plainly is better than a partial
// in-place update that leaves the running thing half-declared.
// applyNetwork creates a named network if the machine does not already have one.
//
// **Existence is the whole of the state.** A network the mesh declared and a network somebody
// made by hand are indistinguishable by name, and that is deliberate: the mesh owns the name, not
// the thing, so it will not tear down and rebuild one that is already there and working. What it
// records is that this resource is now present, which is what lets it be removed later.
//
// Nothing is reconciled beyond presence. A driver or a subnet changed underneath would not be
// noticed — and is not declarable either (novox/hq ADR 0029), so there is nothing to disagree
// with.
func applyNetwork(ctx context.Context, r *declaration.Network, run Runner) (Outcome, error) {
	out := begin(r)

	cri, err := containerRuntime(ctx, run)
	if err != nil {
		return out, fmt.Errorf("%w, so nothing can be said about the network %q", err, r.Name)
	}

	if _, err := run(ctx, cri, "network", "inspect", r.Name); err == nil {
		out.Action = "unchanged"
		out.Detail = "already there"
		return out, nil
	}

	if _, err := run(ctx, cri, "network", "create", r.Name); err != nil {
		return out, fmt.Errorf("cannot create the network %q: %w", r.Name, err)
	}
	// Read back rather than trusting the exit status (novox/hq ADR 0018). A runtime that reports
	// success and made nothing leaves every container that joins it failing to start, with the
	// cause one step away.
	if _, err := run(ctx, cri, "network", "inspect", r.Name); err != nil {
		return out, fmt.Errorf(
			"the network %q was created and is not there afterwards: %w", r.Name, err)
	}
	out.Action = "created"
	out.Detail = "a network for this module's own containers"
	return out, nil
}

func applyContainer(ctx context.Context, r *declaration.Container, run Runner,
	changed map[string]bool, in inputs, previous store.Applied) (Outcome, error) {
	out := begin(r)
	// Read once, so the spec and the record agree on what was read even if a file moves under them.
	reads := in.reads(r)
	want := containerSpecReading(r, in.declares, reads)
	out.reads = reads

	cri, err := containerRuntime(ctx, run)
	if err != nil {
		return out, fmt.Errorf("%w, so nothing can be said about %q", err, r.Name)
	}

	// A scheduled step is state that is present, not a container to start (novox/hq ADR 0053).
	// Installing it records the schedule and reports the node current at once — the deliberate
	// inversion of run-once, which gates. The recurring run is fired by the host's Scheduler off the
	// clock, re-established from this declaration each apply, and NEVER here — so installing does not
	// RUN the container.
	//
	// It does, however, ensure the pinned image is present now. A service or a run-once container
	// gets its image as a side effect of `docker run`; a scheduled step is never run at apply, so
	// without this its image would be absent from the node until the first scheduled fire — which
	// would pay the whole pull latency then, and leave tooling that expects the image present after
	// apply looking at a node that does not have it. Ensuring the image is not running it, so the
	// no-run invariant holds.
	if r.Schedule != "" {
		if err := ensureImage(ctx, cri, r.Image, run); err != nil {
			return out, err
		}
		return applySchedule(r, want, previous)
	}

	if r.RunOnce {
		return applyRunOnce(ctx, r, run, cri, want, previous)
	}

	before, err := containerState(ctx, r.Name, run)
	existed := err == nil

	// A container reads a mounted file once, at start. When one of its restart-on resources changed
	// this pass — a settings-merged config the runtime read, say — the file on disk is new and the
	// running process still holds the old value, and the spec (image, env, volumes) has not moved,
	// so the plain "spec matches, leave it" below would keep the stale process for ever
	// (novox/hq 04-ISSUES/009). Recreating is how a container gets restart-on, which a service
	// already has.
	reasons := restartedBy(r.RestartOn, changed)

	// What this container was created reading, as last recorded. By its declared id first, and by
	// its NAME when that id has no record: the bundle's `store` becomes the postgres module's
	// `postgres.server`, the same container under a new id, and a file change on the day it is
	// adopted is a real change with a real record — under the old id.
	wasReading := previous.Reads
	if previous.ID == "" && in.known != nil {
		if byName, ok := in.known.At(string(declaration.TypeContainer), r.Name); ok {
			wasReading = byName.Reads
		}
	}

	// Which of the files it reads no longer hold what it was created reading. The spec label says
	// only that SOMETHING moved; the record of what was read says what — and that is the line a
	// person needs when a service went stale without a word (novox/hq 04-ISSUES/103).
	var changedFiles []string
	for _, path := range sortedKeys(wasReading) {
		if now, still := reads[path]; still && now != wasReading[path] {
			changedFiles = append(changedFiles, path)
		}
	}

	// **A container labelled before the host folded in what it reads is accepted, not recreated.**
	//
	// Its label is the spec without the file lines. Recreating every such container on the first
	// apply after the host upgraded would be a restart storm across the mesh in declaration order —
	// the store first, under everything that uses it. So a label that matches the spec as it used
	// to be computed is taken as current: what it reads is recorded now, and from the next apply
	// on a changed file is caught by that record; the label itself is rewritten at the next
	// genuine recreate. The trade-off, stated: a container that was ALREADY stale when the host
	// upgraded — created against a file that has since changed — is not caught by this, and could
	// not be by the alternative either, which recreates it without knowing whether it needed to.
	legacy := len(reads) > 0 && before.Spec == containerSpecReading(r, in.declares, nil) &&
		(wasReading == nil || sameReads(wasReading, reads))

	switch {
	case existed && (before.Spec == want || legacy) && before.Running && len(reasons) == 0:
		out.Action = "unchanged"
		return out, nil
	case existed:
		if _, err := run(ctx, cri, "rm", "-f", r.Name); err != nil {
			return out, fmt.Errorf("replacing container %s: %w", r.Name, err)
		}
	}

	args := []string{"run", "--detach", "--name", r.Name, "--restart", "unless-stopped"}
	for _, file := range r.EnvFile {
		args = append(args, "--env-file", file)
	}
	if r.Network != "" {
		args = append(args, "--network", r.Network)
	}
	for _, d := range r.Dns {
		args = append(args, "--dns", d)
	}
	if r.IP != "" {
		args = append(args, "--ip", r.IP)
	}
	args = append(args,
		"--label", specLabel+"="+want, "--label", idLabel+"="+r.ID)
	for _, k := range sortedKeys(r.Env) {
		args = append(args, "--env", k+"="+r.Env[k])
	}
	for _, p := range r.Ports {
		args = append(args, "--publish", p)
	}
	for _, v := range r.Volumes {
		args = append(args, "--volume", v)
	}
	for _, h := range r.Hosts {
		// Written into the container's own hosts file by the runtime. Per container rather than
		// by editing the machine's resolver configuration: that file belongs to something else on
		// most machines, and a host that edited it would be fighting whatever owns it on every
		// boot — the fault this host exists to avoid, in the place it would be hardest to see.
		args = append(args, "--add-host", h)
	}
	args = append(args, r.Image)
	args = append(args, r.Args...)

	if _, err := run(ctx, cri, args...); err != nil {
		return out, fmt.Errorf("starting container %s: %w", r.Name, err)
	}

	// Read back. `docker run --detach` returning an id says the container was created, not
	// that it is still running — a container whose entrypoint exits immediately satisfies the
	// command exactly as one that came up does.
	after, err := containerState(ctx, r.Name, run)
	if err != nil {
		return out, fmt.Errorf("started container %s and it is not there: %w", r.Name, err)
	}
	if !after.Running {
		return out, fmt.Errorf(
			"container %s was started and is not running. It exited; ask the runtime for its "+
				"logs", r.Name)
	}
	if after.Spec != want {
		return out, fmt.Errorf("container %s is not the one that was declared after creating it", r.Name)
	}

	out.Action = "created"
	if existed {
		out.Action = "updated"
		switch {
		case len(changedFiles) > 0:
			out.Detail = "recreated: " + strings.Join(changedFiles, ", ") + " changed"
			if len(reasons) > 0 {
				out.Detail += "; and to pick up " + strings.Join(reasons, ", ")
			}
		case len(reasons) > 0:
			out.Detail = "recreated to pick up " + strings.Join(reasons, ", ")
		default:
			out.Detail = "replaced; a container's configuration is fixed when it is created"
		}
	}
	return out, nil
}

// applyRunOnce runs a container to completion, once, and requires it to exit 0 (novox/hq ADR 0052).
//
// It is a step, not a service: the module's own code seeding a store, migrating a schema or
// gating on health, under the module's own account (ADR 0047), before the container that depends
// on it. Three things make it a step rather than an ordinary container:
//
//   - It is run in the foreground, so the runtime returns the container's exit code. A non-zero
//     exit is an error here, and — because a run-once step gates the apply the way a failed action
//     does — that error stops everything the declaration places after it. That is how "before the
//     broker starts" is enforced: the step is declared first, and the broker is never reached
//     until it has completed.
//   - Its record of having happened is the digest of its declaration, recorded by the caller only
//     after it exits 0 (ADR 0018). The step leaves nothing running to inspect, so the persisted
//     digest — not a live container — is the marker. A re-apply whose declaration digest already
//     matches does nothing; a changed declaration re-runs it.
//   - It writes a seed only if it is absent and never reconciles it, so what a running program
//     grows in that seed afterward is never wiped (04-ISSUES/035). The host's marker keeps the
//     step from re-running; the step's own code keeps it from clobbering on the pass it does run.
func applyRunOnce(ctx context.Context, r *declaration.Container, run Runner, cri, want string, previous store.Applied) (Outcome, error) {
	out := begin(r)

	// Already completed for this exact declaration. The marker is the store, because a run-once
	// step leaves nothing running to ask.
	if previous.Wrote != "" && previous.Wrote == want {
		out.Action = "unchanged"
		out.Detail = "run-once step already completed for this declaration"
		out.wrote = want
		return out, nil
	}

	// A container by this name from a previous, different declaration must not linger and be
	// mistaken for this run. Removing a name that is not there is the state we want, so its error
	// is ignored.
	_, _ = run(ctx, cri, "rm", "-f", r.Name)

	// Run in the foreground so the runtime waits for the container and hands back its exit code.
	// No --detach and no --restart: a step that is restarted is not a step.
	args := foregroundRunArgs(r, want)

	if _, err := run(ctx, cri, args...); err != nil {
		// A non-zero exit or a runtime that could not start it. Either way the step did not make
		// the machine ready, so the apply must not go on to the container that needs it.
		return out, fmt.Errorf("run-once step %s did not complete: %w", r.Name, err)
	}

	// It completed. Remove the exited container so a later apply is not confused by a stopped one;
	// the record that it ran is the digest below, which the caller persists after the fact.
	_, _ = run(ctx, cri, "rm", "-f", r.Name)

	out.Action = "created"
	out.Detail = "run-once step completed"
	out.wrote = want
	return out, nil
}

// foregroundRunArgs builds a `docker run` that runs a container to completion and hands back its
// exit code — no --detach, no --restart, because a step that is restarted is not a step. Shared by
// a run-once step (novox/hq ADR 0052) and by one fire of a scheduled step (ADR 0053), which are the
// same "run the container and let it exit" up to how often it happens.
func foregroundRunArgs(r *declaration.Container, want string) []string {
	args := []string{"run", "--name", r.Name}
	for _, file := range r.EnvFile {
		args = append(args, "--env-file", file)
	}
	if r.Network != "" {
		args = append(args, "--network", r.Network)
	}
	args = append(args, "--label", specLabel+"="+want, "--label", idLabel+"="+r.ID)
	for _, k := range sortedKeys(r.Env) {
		args = append(args, "--env", k+"="+r.Env[k])
	}
	for _, v := range r.Volumes {
		args = append(args, "--volume", v)
	}
	for _, h := range r.Hosts {
		args = append(args, "--add-host", h)
	}
	args = append(args, r.Image)
	args = append(args, r.Args...)
	return args
}

// ensureImage makes the pinned image present on the node without running anything.
//
// A service or a run-once container gets its image as a side effect of `docker run` — the first run
// pulls it. A scheduled step is installed but deliberately never run at apply (novox/hq ADR 0053), so
// nothing would pull its image until the first scheduled fire: the image is absent from the node
// right after a successful apply, the first run pays the whole pull latency, and tooling that expects
// the image present after apply finds it missing. This fetches the same bytes `docker run` would, and
// stops short of starting the container.
//
// Idempotent, and it reads back (novox/hq ADR 0018): an image already present is left as is, and a
// pull that reported success but left nothing there is a failure, not a convergence.
func ensureImage(ctx context.Context, cri, image string, run Runner) error {
	if _, err := run(ctx, cri, "image", "inspect", image); err == nil {
		return nil
	}
	// An image named by the digest of its own configuration is one this machine was supposed to
	// already hold — built here, or handed over. There is no registry that answers for it, so
	// pulling would fail somewhere that names a network problem instead of a missing image.
	if strings.HasPrefix(image, "sha256:") {
		return fmt.Errorf(
			"image %s is not on this machine, and an image named by its own digest cannot be "+
				"fetched: nothing serves it. Build it here, or load it, before applying this",
			image)
	}
	if _, err := run(ctx, cri, "pull", image); err != nil {
		return fmt.Errorf("pulling image %s: %w", image, err)
	}
	if _, err := run(ctx, cri, "image", "inspect", image); err != nil {
		return fmt.Errorf("image %s is not present after pulling it: %w", image, err)
	}
	return nil
}

// applySchedule installs a scheduled step: it records the schedule as present and reports the node
// current, without running anything (novox/hq ADR 0053).
//
// This is the deliberate inversion of run-once. A run-once step gates the apply — it runs to
// completion here and a non-zero exit halts everything after it — because "seed the store before the
// broker starts" is a precondition of convergence. A scheduled step is the opposite: it runs *after*
// the machine is up, on its own clock, and a single failed run is an ordinary operational event. So
// installing it is pure state: the schedule is present, like a running service, and the apply is
// current at once. The recurring run is fired by the host's Scheduler off the clock (see
// schedule.go), re-established from the applied declaration each pass because the declaration is the
// source of truth (ADR 0018) — never from here, and never persisted beyond what the mesh already
// owns.
//
// The marker is the declaration's digest, exactly as for run-once, so a re-apply of the same
// declaration reports the schedule unchanged and a changed image, environment or cadence reports it
// re-installed. A failed run touches none of this: it happens entirely in the Scheduler, outside the
// apply and the store, which is why a routine job's failure can never flip the node's state.
func applySchedule(r *declaration.Container, want string, previous store.Applied) (Outcome, error) {
	out := begin(r)
	out.wrote = want
	switch {
	case previous.Wrote == want:
		out.Action = "unchanged"
		out.Detail = "scheduled step; already installed for this declaration"
	case previous.Wrote != "":
		out.Action = "updated"
		out.Detail = "scheduled step re-installed; its image, environment or cadence changed"
	default:
		out.Action = "created"
		out.Detail = "scheduled step installed; the host runs it on its cadence"
	}
	return out, nil
}

// restartedBy is which of the named resources changed this pass — the reason a container or service
// must be brought back rather than left as it is (novox/hq 04-ISSUES/009).
func restartedBy(restartOn []string, changed map[string]bool) []string {
	var which []string
	for _, id := range restartOn {
		if changed[id] {
			which = append(which, id)
		}
	}
	return which
}

// sameReads is whether two records of what a container reads name the same files holding the
// same content — a file added, dropped or changed makes them differ.
func sameReads(a, b map[string]string) bool {
	if len(a) != len(b) {
		return false
	}
	for path, digest := range a {
		if b[path] != digest {
			return false
		}
	}
	return true
}

func sortedKeys(m map[string]string) []string {
	keys := make([]string, 0, len(m))
	for k := range m {
		keys = append(keys, k)
	}
	sort.Strings(keys)
	return keys
}

// applyAction runs something the bundle declared, and never learns what it means.
//
// Verify does double duty, and that is the design rather than a convenience: it is both the
// idempotency check and the read-back. Running it first is how the host knows whether there is
// anything to do — it does not know what a database is, so "is the database there" is a
// question only the declaration can ask. Running it again afterwards is how the host knows the
// command had the effect it claimed (novox/hq ADR 0005).
func applyAction(ctx context.Context, r *declaration.Action, run Runner) (Outcome, error) {
	out := begin(r)

	if _, err := runAction(ctx, r, r.Verify, run); err == nil {
		out.Action = "unchanged"
		out.Detail = "already true"
		return out, nil
	}

	if _, err := runAction(ctx, r, r.Command, run); err != nil {
		return out, fmt.Errorf("running the action: %w", err)
	}

	if _, err := runAction(ctx, r, r.Verify, run); err != nil {
		return out, fmt.Errorf(
			"the action ran without error and its own verify still fails: %w\n\n"+
				"The command reported success and the thing it was for did not happen, which "+
				"is exactly what verify exists to catch", err)
	}

	out.Action = "created"
	out.Detail = "verify was false and is now true"
	return out, nil
}

// runAction runs one of an action's command lines, on the machine or inside a container.
func runAction(ctx context.Context, r *declaration.Action, argv []string, run Runner) (string, error) {
	if len(argv) == 0 {
		return "", errors.New("no command")
	}
	if r.In != "" {
		return run(ctx, "docker", append([]string{"exec", r.In}, argv...)...)
	}
	return run(ctx, argv[0], argv[1:]...)
}

// Container runtimes the host knows how to ask.
//
// Two, because two exist on machines the mesh runs on. The list is short on purpose: each entry
// is a claim that its probe and its CLI have been checked, not that a binary of that name might
// work (novox/hq ADR 0005).
//
// The probe differs and the rest does not, which is what makes this a lookup rather than an
// interface. `docker info --format {{.ServerVersion}}` fails on podman — the field does not
// exist in its report — while `run`, `inspect --format` and `rm -f` are identical, including
// docker's own Go template syntax for reading state and labels.
var containerRuntimes = []struct {
	command string
	// probe asks the runtime for its version in the form THAT runtime understands. It must
	// prove the runtime is FUNCTIONING, never that a binary is on disk
	// (novox/hq 04-ISSUES/007).
	probe []string
}{
	{command: "docker", probe: []string{"info", "--format", "{{.ServerVersion}}"}},
	{command: "podman", probe: []string{"info", "--format", "{{.Version.Version}}"}},
}

// containerRuntime returns the runtime this machine actually has, or says there is none.
//
// Detected rather than declared, because a machine already carrying one keeps it: adoption
// takes over what is there rather than replacing it (novox/hq research 012). Which runtime a
// machine has is reported upward in the profile; what to install on a machine with none is the
// control plane's decision, not this one's.
func containerRuntime(ctx context.Context, run Runner) (string, error) {
	var tried []string
	for _, rt := range containerRuntimes {
		if _, err := run(ctx, rt.command, rt.probe...); err == nil {
			return rt.command, nil
		}
		tried = append(tried, rt.command)
	}
	return "", fmt.Errorf(
		"no container runtime answers on this machine (tried %s)", strings.Join(tried, ", "))
}

// digestOf is how this host recognises what it wrote.
//
// A digest rather than the content: the store is read on every reconcile and sits beside the
// state on disk, and keeping every managed file twice would make it grow with the machine rather
// than with the number of resources.
func digestOf(content string) string {
	sum := sha256.Sum256([]byte(content))
	return hex.EncodeToString(sum[:])
}

// DigestOf names a declaration by its bytes, exactly as the mesh names what it sends: sha256 of
// the raw bytes, hex. The two sides never digest different things.
func DigestOf(raw []byte) string {
	sum := sha256.Sum256(raw)
	return hex.EncodeToString(sum[:])
}

// holds is the machine's own ports a resource occupies.
//
// **What the declaration binds, not what is open.** A machine's open ports are a moving target —
// something a person started, a connection the kernel handed out — and assigning around them would
// mean a port that was free when it was asked for and taken when it was used. What a resource
// declares is stable, and it is the half the mesh can be responsible for.
func holds(resource declaration.Resource) []int {
	container, ok := resource.(*declaration.Container)
	if !ok {
		return nil
	}
	var out []int
	for _, mapping := range container.Ports {
		// "8080:80", or "127.0.0.1:8080:80" when an address was named. The machine's port is the
		// one before the last colon; the last is inside the container and is not the machine's.
		parts := strings.Split(mapping, ":")
		if len(parts) < 2 {
			continue
		}
		port, err := strconv.Atoi(strings.TrimSpace(parts[len(parts)-2]))
		if err != nil {
			continue
		}
		out = append(out, port)
	}
	return out
}

// declaredDigest is what a resource currently says it should be.
//
// **Content, not identity.** It exists so a container can be told apart by what it reads: a file
// whose text changed must produce a different digest, or the container mounting it compares equal
// to one started against the old text. Only the shapes something can read are digested; for
// everything else the identity is enough, because nothing mounts a package.
func declaredDigest(r declaration.Resource) string {
	var material string
	switch res := r.(type) {
	case *declaration.File:
		// The content as declared, before any sealing is opened — two machines are given different
		// ciphertext for the same secret, and digesting that would make an unchanged file look
		// changed on every apply and restart the container reading it for ever.
		material = res.Content
	case *declaration.Directory:
		material = res.Path + "\n" + res.Mode
	default:
		return ""
	}
	return fmt.Sprintf("%x", sha256.Sum256([]byte(material)))
}

// removeService gives a unit back the state the host first found it in, and nothing more.
//
// **Removes what it made, gives back what it changed, leaves what was the machine's**
// (novox/hq ADR 0118). A service resource never installs a unit; it puts one that already existed
// into a state. So undeclaring it cannot mean stopping it — that is how unassigning the private
// network stopped the container runtime and every container with it, how unassigning sshd would
// have stopped ssh, and how an uplink module would have taken a machine off its only link
// (novox/hq issue 130). It means undoing what the mesh did to it: a unit found running is left
// running; a unit the mesh started — the packet filter a converge loaded, which returning to
// adopted must unload — is stopped again, and disabled again if the mesh enabled it.
//
// **Never started on the way out.** A unit the mesh stopped is not started again when its
// declaration goes: starting something is a decision, and the operator makes it.
func removeService(ctx context.Context, sys system.System, a store.Applied, run Runner) (string, string, error) {
	if a.Stateless {
		return "forgotten", "its state was never the mesh's", nil
	}
	if a.Found == nil {
		// Recorded before the host kept what it found. Not knowing, it leaves the unit as it is:
		// a unit left running can be stopped by the operator, and one stopped by mistake may be
		// the link the operator would use to do it.
		return "forgotten", "the unit is the machine's; left as it is", nil
	}
	if a.Found.State != "stopped" && (a.Found.Boot == "" || a.Found.Boot == "enabled") {
		return "forgotten", "it was running before the mesh; left as it is", nil
	}
	// Something is to be given back, so the unit must still be there to give it to. One since
	// uninstalled is already as far from the mesh as it can be, and saying so keeps a record of it
	// from failing every apply.
	if _, err := sys.ServiceState(ctx, run, a.Target); err != nil {
		if strings.Contains(err.Error(), "does not exist on this machine") {
			return "forgotten", "the unit no longer exists", nil
		}
		return "", "", err
	}
	var gave []string
	if a.Found.State == "stopped" {
		if err := sys.SetServiceState(ctx, run, a.Target, "stopped"); err != nil {
			return "", "", fmt.Errorf("stopping %s, which the mesh started: %w", a.Target, err)
		}
		gave = append(gave, "stopped again")
	}
	if a.Found.Boot == "disabled" {
		if err := sys.SetServiceBoot(ctx, run, a.Target, "disabled"); err != nil {
			return "", "", fmt.Errorf("disabling %s, which the mesh enabled: %w", a.Target, err)
		}
		gave = append(gave, "disabled at boot again")
	}
	return "restored", strings.Join(gave, ", ") + ", as the host found it", nil
}