mesh-host/internal/declaration/declaration.go

// Package declaration is what the host is told a machine should be.
//
// Data, never instructions. The vocabulary is finite, versioned, and anything outside it
// refuses the whole declaration rather than being skipped — a host that applied most of what
// it was sent and reported success is a node that looks configured and is not
// (novox/hq ADR 0005).
package declaration

import (
	"bytes"
	"encoding/json"
	"fmt"
	"io"
	"net"
	"reflect"
	"regexp"
	"slices"
	"sort"
	"strconv"
	"strings"
)

// Version is the vocabulary this host speaks. A declaration naming any other version is
// refused: an older host handed a newer vocabulary must not quietly do half of it.
const Version = 1

// Type names a kind of resource. Every addition widens what a compromised control plane can
// express, so the list is a security artefact and grows deliberately.
type Type string

const (
	TypeDirectory Type = "directory"
	TypeFile      Type = "file"
	TypeService   Type = "service"
	TypePackage   Type = "package"
	TypeContainer Type = "container"
	TypeAction    Type = "action"

	// TypeUser is a login on the machine. Added because most of what a person actually installs
	// is not a service: a shell, a terminal, a chat client, a desktop. All of those are a package
	// plus configuration **in somebody's home**, and a mesh with no notion of a user can only
	// manage /etc.
	TypeUser Type = "user"

	// TypeArchive is a set of files fetched by digest and unpacked. A desktop theme is hundreds
	// of files; inlining them would make every declaration enormous and rewrite the lot whenever
	// one changed.
	TypeArchive Type = "archive"

	// TypeNetwork is a named network on this machine, for a module whose containers must reach
	// each other by name. Created if absent, removed when no longer declared — which is the whole
	// reason it is a shape rather than an action, because an action leaves nothing the host can
	// undo and the network would outlive the module (novox/hq ADR 0029).
	TypeNetwork Type = "network"

	// TypeAccess is a pre-existing, operator-owned path a module is granted use of but does not
	// own (novox/hq ADR 0051). The opposite of a directory on every axis the host acts on: the
	// host creates, chowns and reconciles a directory, and removes it when it is empty; it does
	// none of that to an access. It confirms the path is present — refusing clearly if the
	// operator has not provided it, rather than creating it as a bind mount source would
	// (04-ISSUES/026) — and leaves everything about it alone. Several modules declaring one
	// access is ordinary, because none of them owns it.
	TypeAccess Type = "access"

	// TypeProcess is the module's own code, run on the machine, in one of three modes.
	//
	// **The intent, not the mechanism.** Until this, an author decided the hosting before they
	// could declare anything: code of their own meant a `container` built from an image, a script
	// meant a `service` and a unit somebody else had to install. Same intent — run this — with
	// the choice baked into which kind was picked.
	//
	// **And three modes rather than three kinds**, exactly as a container has. A first draft of
	// this added a `daemon` for the long-running case alone, which would have meant a new kind for
	// each of the others — a scheduled task, a run-once migration, a health check. They are one
	// thing run at different cadences, and that is a field, not a vocabulary entry. Every addition
	// to this vocabulary widens what a compromised control plane can express.
	//
	// What a module declares is a bundle and a command. The mesh unpacks the bundle where it keeps
	// such things and runs it — as a unit that stays up, as a step that must finish, or on a
	// cadence. Tools, hooks and event consumers are not separate modes: they are loaded by a tool
	// host, which is itself a process that stays up.
	TypeProcess Type = "process"

	// TypeOpening is a port the mesh needs reachable on an adopted node, converged through the
	// firewall found there in that firewall's own terms (novox/hq ADR 0100). A state, not a
	// command: the host adds the rule it marks as the mesh's when it is missing, and removes only
	// what it marked — which is what lets it travel over the link.
	TypeOpening Type = "opening"
)

// Resource is one thing that should be true of the machine.
//
// A struct per kind rather than one struct carrying every field, because the decoder is then
// what rejects a field the kind does not have: a `file` carrying an `image` is refused because
// File has no such field, not because a list somewhere remembered to say so. The one-struct
// form needs every kind revisited whenever a field is added, and the kind nobody revisits
// silently accepts a field the host will never read.
type Resource interface {
	// Identity is the name the control plane keeps stable across declarations. Not a position
	// and not a hash of the content: it is what lets the store say *this is the same resource
	// I applied last time*, which is what makes removal possible at all.
	Identity() string
	// Kind is the resource's type, for the store and for reporting.
	Kind() Type
	// Target is what the resource acts on, for a person reading a report.
	Target() string

	validate(where string, allowActions bool) []string
}

// The `Type` field on each kind below exists only to absorb the JSON `"type"` key, which the
// strict decoder would otherwise refuse. `Kind()` returns the constant and is what anything
// else should read.

// Directory is a directory that should exist, with a mode.
type Directory struct {
	ID   string `json:"id"`
	Type Type   `json:"type"`
	Path string `json:"path"`
	Mode string `json:"mode,omitempty"`
	// Owner is the user this belongs to, by name. Absent means root, which is what everything
	// managed was until users existed.
	Owner string `json:"owner,omitempty"`
}

func (d *Directory) Identity() string { return d.ID }
func (d *Directory) Kind() Type       { return TypeDirectory }
func (d *Directory) Target() string   { return d.Path }

func (d *Directory) validate(where string, _ bool) []string {
	var problems []string
	if d.Path == "" {
		problems = append(problems, where+": a directory needs a path")
	}
	return append(problems, checkMode(where, d.Mode)...)
}

// File is a file with literal content. The host renders nothing.
type File struct {
	ID      string `json:"id"`
	Type    Type   `json:"type"`
	Path    string `json:"path"`
	Content string `json:"content"`
	Mode    string `json:"mode,omitempty"`

	// CreateOnce says the content is a seed: written when the file is absent, and left alone —
	// content, mode and owner — whenever it is present.
	//
	// **Two intentions had one vocabulary** (novox/hq issue 035, ADR 0087). "This file has this
	// content, for ever" is what an ordinary file says, and the host holds the machine to it. A
	// module that needs a file to exist before a program first starts — an access list the
	// program then persists into, a bootstrap configuration it rewrites — needs the other thing,
	// and with only the first available, every reconcile restored the seed behind the running
	// program and erased what had grown in it, reporting success. What grows in a seeded file
	// is somebody else's work the mesh asked for; the mesh removes nothing it did not create
	// (ADR 0030), and it does not overwrite that either.
	CreateOnce bool `json:"create-once,omitempty"`

	// Into says the file is shared with software the mesh did not install, and the content is
	// the mesh's part of it: written into what is there, never over it (novox/hq ADR 0102). Only
	// "json" is spoken — the content is a JSON object whose keys the host sets in the file's
	// object, keeping every other key as it found it and recording what each of its keys held
	// before, so undeclaring the file gives those back.
	//
	// "block" is the same idea for a file that is not structured (novox/hq issue 128): the
	// content is the mesh's lines, and the host owns only the region between `# BEGIN mesh <id>`
	// and `# END mesh <id>`, keeping every line outside it byte for byte. The machine's hosts file
	// is the case that needed it — on a workstation the distribution, a local development tool and
	// the operator all write into it, and the mesh writing it whole took their lines away at the
	// next change to the mesh's names, silently. Marked blocks are the shape the other tools in
	// that file already use, and `#` is the comment character of every file this serves.
	//
	// Written into, in either format, the file's mode and owner are the machine's: a declared mode
	// and owner apply only to a file the host creates, and a file that was there keeps its own.
	Into string `json:"into,omitempty"`

	// At is where a file written into a block has its region added when the file does not hold
	// one yet: "end", the default, or "start". A region already there stays where it is, whatever
	// this says — moving it would move the lines around it, and those are the machine's.
	//
	// **Some files give a line its meaning by what stands above it.** dhcpcd's configuration scopes
	// every line after `interface X` to that interface, and a real one ends with exactly that — an
	// interface and its static address. A region added at the end would make the mesh's global
	// options (`nohook resolv.conf`, `denyinterfaces mesh0`) options of one interface, and dhcpcd
	// would read them without complaint. At the start, nothing stands above them.
	At string `json:"at,omitempty"`

	// Sealed is content encrypted to this node's sealing key, for a file the mesh must deliver
	// without being able to read.
	//
	// The one thing here the host cannot simply write. Everything else in a declaration is
	// visible to whatever carried it — the broker relays the message, and the message is signed
	// so it cannot be forged, but signing does not make it unreadable. A password travelling in
	// `content` would be a password the broker sees, which is the transitive trust the design
	// refuses everywhere else (novox/hq ADR 0004).
	//
	// Exclusive with Content: a file is one or the other, so that "was this secret" is answerable
	// by looking rather than by knowing which field won.
	Sealed string `json:"sealed,omitempty"`

	// Secrets are sealed values put into Content where it says `${secret:name}`.
	//
	// **The one place a secret and a configuration meet, and it happens on the machine.** A
	// program that wants its token inside a JSON document cannot be given a file that is entirely
	// a token, and the mesh cannot compose the document itself — it discarded the value
	// (novox/hq ADR 0024). So the module supplies the document with a hole in it, the mesh
	// delivers the value sealed, and the host is the only thing that ever sees both.
	//
	// **Substitution is textual and the host learns no formats.** That is deliberate: a mechanism
	// that understood JSON would be asked to understand YAML next, and then INI, which is how the
	// arrangement this replaces became something nobody could hold in their head. The module knows
	// its own format, because it wrote the rest of the file.
	//
	// The sharp edge, stated rather than discovered: a value containing a quote or a backslash
	// will not be escaped for whatever syntax surrounds it.
	Secrets map[string]string `json:"secrets,omitempty"`

	// Bytes is content that is not text, base64-encoded — a wallpaper, a font, an icon.
	//
	// A third way of saying what is in a file, and the three are exclusive. It would have been
	// tempting to let Content carry base64 and add a flag, and then "what is in this file" would
	// depend on a field somewhere else.
	Bytes string `json:"bytes,omitempty"`

	// Owner is the user this belongs to, by name. Absent means root.
	Owner string `json:"owner,omitempty"`
}

// Secret reports whether this file arrived sealed, which is what decides both that it must be
// opened before writing and that its contents must never appear in a report.
func (f *File) Secret() bool { return f.Sealed != "" }

func (f *File) Identity() string { return f.ID }
func (f *File) Kind() Type       { return TypeFile }
func (f *File) Target() string   { return f.Path }

// placeholder is what Content says where a sealed value belongs: ${secret:name}.
var placeholder = regexp.MustCompile(`\$\{secret:([a-z0-9][a-z0-9-]*)\}`)

// SecretsUsed are the names Content asks for, in the order they first appear.
func (f *File) SecretsUsed() []string {
	var used []string
	seen := map[string]bool{}
	for _, m := range placeholder.FindAllStringSubmatch(f.Content, -1) {
		if !seen[m[1]] {
			seen[m[1]] = true
			used = append(used, m[1])
		}
	}
	return used
}

func (f *File) validate(where string, _ bool) []string {
	var problems []string
	if f.Path == "" {
		problems = append(problems, where+": a file needs a path")
	}
	switch f.Into {
	case "":
	case IntoJSON:
		var object map[string]json.RawMessage
		if err := json.Unmarshal([]byte(f.Content), &object); err != nil || object == nil {
			problems = append(problems, where+
				": a file written into JSON carries a JSON object of the keys it sets")
		}
	case IntoBlock:
		// The markers are how the host finds its region again. A marker in the content would be
		// a second region, or the end of this one, the next time the file is read — and the host
		// would then rewrite, or on undeclare take out, lines that were never the mesh's.
		for _, line := range strings.Split(f.Content, "\n") {
			if strings.HasPrefix(line, BlockBegin) || strings.HasPrefix(line, BlockEnd) {
				problems = append(problems, fmt.Sprintf(
					"%s: a file written into a block carries the mesh's lines, and %q is a marker the "+
						"host keeps for itself", where, strings.TrimRight(line, "\r")))
				break
			}
		}
		// The id is written into the markers, so it has to stay on one line — and whitespace at
		// either end of it is whitespace the host would have to match exactly in a line some
		// editor may trim.
		if strings.ContainsAny(f.ID, "\r\n") {
			problems = append(problems, where+
				": a file written into a block names its region by its id, and this id spans lines")
		} else if strings.TrimSpace(f.ID) != f.ID {
			problems = append(problems, where+
				": a file written into a block names its region by its id, and this id begins or ends in whitespace")
		}
	default:
		problems = append(problems, fmt.Sprintf(
			"%s: into %q; a file is written into \"json\" or \"block\", or omits it to be written whole",
			where, f.Into))
	}
	switch {
	case f.At == "":
	case f.Into != IntoBlock:
		problems = append(problems, fmt.Sprintf(
			"%s: at %q; only a file written into a block has a place its region is added", where, f.At))
	case f.At != AtStart && f.At != AtEnd:
		problems = append(problems, fmt.Sprintf(
			"%s: at %q; a block is added at \"start\" or \"end\", or omits it to be added at the end",
			where, f.At))
	}
	if f.Into == IntoJSON || f.Into == IntoBlock {
		if f.Sealed != "" || f.Bytes != "" || len(f.Secrets) > 0 || f.CreateOnce {
			problems = append(problems, where+
				": a file written into says only its part, in content — not sealed, bytes, "+
				"secrets or create-once")
		}
	}
	var said []string
	for name, value := range map[string]string{
		"content": f.Content, "sealed": f.Sealed, "bytes": f.Bytes,
	} {
		if value != "" {
			said = append(said, name)
		}
	}
	if len(said) > 1 {
		sort.Strings(said)
		problems = append(problems, where+
			": a file says what is in it exactly once, and this says it as "+
			strings.Join(said, " and ")+
			" — otherwise nobody can tell by looking which one landed on the machine")
	}
	// A file whose content names a secret must be given exactly the secrets it names.
	//
	// **Both directions, and both are refusals rather than warnings.** A placeholder with nothing
	// to fill it would write `${secret:x}` into a configuration file, which the program reads as
	// a value and fails on somewhere unrelated. A secret nobody uses means whoever wrote this
	// believes a credential is in a file where it is not.
	if len(f.Secrets) > 0 && f.Content == "" {
		problems = append(problems, where+
			": secrets were given and there is no content to put them in")
	}
	used := f.SecretsUsed()
	for _, name := range used {
		if f.Secrets[name] == "" {
			problems = append(problems, fmt.Sprintf(
				"%s: the content asks for the secret %q and none was given", where, name))
		}
	}
	for name := range f.Secrets {
		if !slices.Contains(used, name) {
			problems = append(problems, fmt.Sprintf(
				"%s: the secret %q was given and the content never asks for it", where, name))
		}
	}
	return append(problems, checkMode(where, f.Mode)...)
}

// User is a login on the machine.
//
// The thing that makes a shell, a chat client or a desktop expressible at all: each is a package
// plus configuration in somebody's home, and until this the mesh could only own /etc.
//
// It also makes "zsh is my login shell" **declared state** rather than an action. `chsh` is a
// command, the link may not carry one (novox/hq ADR 0005), and a shell that could only be set by
// hand would be a shell the mesh cannot manage — which is most of the reason to manage a machine
// at all.
type User struct {
	ID   string `json:"id"`
	Type Type   `json:"type"`
	Name string `json:"name"`

	// Shell this user logs in with. Absent means the host asserts nothing and leaves whatever is
	// there — the same rule Service.Boot follows, for the same reason: a field that always
	// asserts cannot express "I do not care".
	Shell string `json:"shell,omitempty"`

	// Groups this user must be in. Additive: the host puts the user in these and does not remove
	// it from others, because a machine's own groups are not the mesh's to know about.
	Groups []string `json:"groups,omitempty"`

	// Home directory. Absent means the system's default for a new user, and is not changed for
	// one that exists — moving somebody's home is not something a declaration should do quietly.
	Home string `json:"home,omitempty"`
}

// Network is a named network on this machine.
//
// **A name and nothing else.** Not a driver, a subnet or a gateway: each of those is something a
// module would have to know about the machine it lands on, and a module naming a subnet is a
// module that collides with whatever else chose the same one. The runtime picks; the mesh names
// (novox/hq ADR 0029).
type Network struct {
	ID   string `json:"id"`
	Type Type   `json:"type"`
	Name string `json:"name"`
}

func (n *Network) Identity() string { return n.ID }
func (n *Network) Kind() Type       { return TypeNetwork }
func (n *Network) Target() string   { return n.Name }

func (n *Network) validate(where string, _ bool) []string {
	var problems []string
	if n.Name == "" {
		problems = append(problems, where+": a network needs a name")
	}
	// The runtimes accept more than this, and the mesh does not: a name with a slash or a colon
	// in it reads as a reference to something else entirely wherever it is later printed.
	for _, r := range n.Name {
		if (r < 'a' || r > 'z') && (r < 'A' || r > 'Z') && (r < '0' || r > '9') &&
			r != '-' && r != '_' && r != '.' {
			problems = append(problems, where+
				": a network name is letters, digits, dashes, underscores and dots, and "+
				n.Name+" is not")
			break
		}
	}
	return problems
}

// Modes an access may be granted at. Plain words, not the octal a directory's mode is: an access
// is not a thing the host chmods, it is a statement of how this module reaches what the operator
// owns.
const (
	AccessRead      = "read"
	AccessReadWrite = "read-write"
)

// Access is a pre-existing, operator-owned path this module is granted use of but does not own.
//
// **The distinction 04-ISSUES/036 and 026 turn on.** A `directory` resource is the mesh's own —
// it creates it, sets its owner and mode, and removes it when empty ([ADR 0030](novox/hq)). A
// media library, a download spool is the operator's: it existed before the mesh, several modules
// read and write it at once, and the mesh must not create, chown, reconcile or remove it. The
// host confirms it is there and mounts it; nothing else.
type Access struct {
	ID   string `json:"id"`
	Type Type   `json:"type"`
	Path string `json:"path"`
	// Mode is how this module reaches the path: read or read-write. Absent narrows to read.
	Mode string `json:"mode,omitempty"`
}

func (a *Access) Identity() string { return a.ID }
func (a *Access) Kind() Type       { return TypeAccess }
func (a *Access) Target() string   { return a.Path }

func (a *Access) validate(where string, _ bool) []string {
	var problems []string
	if !strings.HasPrefix(a.Path, "/") {
		problems = append(problems, where+": an access needs an absolute path, and "+
			a.Path+" is not one")
	}
	switch a.Mode {
	case "", AccessRead, AccessReadWrite:
	default:
		problems = append(problems, fmt.Sprintf(
			"%s: an access is %q or %q, not %q", where, AccessRead, AccessReadWrite, a.Mode))
	}
	return problems
}

func (u *User) Identity() string { return u.ID }
func (u *User) Kind() Type       { return TypeUser }
func (u *User) Target() string   { return u.Name }

func (u *User) validate(where string, _ bool) []string {
	var problems []string
	if u.Name == "" {
		problems = append(problems, where+": a user needs a name")
	}
	if u.Shell != "" && !strings.HasPrefix(u.Shell, "/") {
		problems = append(problems, where+
			": a login shell is an absolute path, and "+u.Shell+" is not one")
	}
	if u.Home != "" && !strings.HasPrefix(u.Home, "/") {
		problems = append(problems, where+": a home directory is an absolute path")
	}
	return problems
}

// Archive is a set of files, fetched by digest and unpacked.
//
// For the case inlining cannot serve: a theme, an icon set, a tree of configuration. Hundreds of
// files inlined would make every declaration enormous and rewrite all of it when one changed.
//
// **Pinned by digest, and the digest is checked before anything is unpacked.** The same discipline
// the bootstrap uses for images, and for the same reason — this is fetched over a network the
// mesh does not control, and a reference that can be made to point elsewhere is not a reference.
type Archive struct {
	ID   string `json:"id"`
	Type Type   `json:"type"`
	// Source is where to fetch it from.
	Source string `json:"source"`
	// Digest is sha256 of the archive, as "sha256:<hex>".
	Digest string `json:"digest"`
	// Path is the directory it is unpacked into.
	Path string `json:"path"`
	// Owner is the user the unpacked files belong to. Absent means root.
	Owner string `json:"owner,omitempty"`
}

func (a *Archive) Identity() string { return a.ID }
func (a *Archive) Kind() Type       { return TypeArchive }
func (a *Archive) Target() string   { return a.Path }

func (a *Archive) validate(where string, _ bool) []string {
	var problems []string
	if a.Source == "" {
		problems = append(problems, where+": an archive needs somewhere to fetch it from")
	}
	if a.Path == "" {
		problems = append(problems, where+": an archive needs somewhere to unpack into")
	}
	if !strings.HasPrefix(a.Digest, "sha256:") || len(a.Digest) != len("sha256:")+64 {
		// Refused rather than fetched and trusted. Everything else pinned in this vocabulary is
		// pinned by digest, and an archive that was not would be the one way in.
		problems = append(problems, where+
			": an archive is pinned by digest, as sha256:<64 hex characters>")
	}
	return problems
}

// Process is the module's own code, run on the machine, in one of three modes.
//
// The difference from Service is who owns the unit: a Service puts an EXISTING unit into a state
// and deliberately does not install one, which is right for software that ships its own. This is
// the mesh's own code — a bundle it built — so there is no unit until the mesh writes it, and
// nothing else will.
//
// The difference from Container is the hosting, and a module should not have to choose: what this
// says is what to run, and the machine's own supervisor is how. Code that genuinely needs a
// container's isolation declares a container and says so.
//
// **Three modes, matching a container's**, because they are the same thing at different cadences:
// stays up, runs once, runs on a schedule. A module's scheduled task, its run-once migration, its
// health check and its tool host are all this — and each being its own resource kind would be four
// entries in a vocabulary where every entry widens what a compromised control plane can express.
type Process struct {
	ID   string `json:"id"`
	Type Type   `json:"type"`
	// Name is what the unit is called, and what an operator will see in the process table.
	Name string `json:"name"`
	// Source is where to fetch the bundle from, and Digest is what it must hash to. The same
	// discipline as an archive, for the same reason: this crosses a network the mesh does not
	// control.
	Source string `json:"source"`
	Digest string `json:"digest"`
	// Run is the command, relative to the unpacked bundle. The first element is the program.
	//
	// **Named by the module, never inferred.** Guessing an entrypoint from which files exist makes
	// a daemon change what it runs when somebody adds a file.
	Run []string `json:"run"`
	// Env and EnvFile are what it runs with. A file rather than inline values is how a credential
	// reaches a daemon without passing through the declaration.
	Env     map[string]string `json:"env,omitempty"`
	EnvFile []string          `json:"env-file,omitempty"`
	// User is who it runs as. Absent means root, which is what the mesh's own modules need for
	// the things they do to a machine.
	User string `json:"user,omitempty"`
	// RestartOn names resources whose change means this must be restarted — the same rule a
	// service follows, and for the same reason: a running process does not re-read its
	// configuration, so replacing a file and finding the process already up leaves the machine
	// behaving the way it did before while every check passes.
	RestartOn []string `json:"restart-on,omitempty"`

	// RunOnce marks code the host runs to completion rather than leaves running: a migration, a
	// seed, a first-boot step. What follows it is gated on it finishing, because a step that did
	// not make the machine ready must not be followed by the thing that needed it.
	RunOnce bool `json:"run-once,omitempty"`

	// Schedule runs it on a cadence — a five-field cron expression (novox/hq ADR 0053). The
	// recurring twin of RunOnce: the same code, run again rather than left running.
	//
	// Exclusive with RunOnce and with RestartOn, for the same reason a container's is: something
	// that runs once does not run on a schedule, and something that is not running cannot be
	// restarted when a file changes.
	Schedule string `json:"schedule,omitempty"`
}

func (d *Process) Identity() string { return d.ID }
func (d *Process) Kind() Type       { return TypeProcess }
func (d *Process) Target() string   { return d.Name }

// ProcessNameProblem says what is wrong with a process name, or nothing.
//
// The name becomes a unit name, a file under the unit directory and a directory under the mesh's
// own — the one removing the process deletes, whole (novox/hq ADR 0118). So a name that is not one
// plain path element is refused: with a separator it writes somewhere nobody meant, and "." or
// ".." IS the mesh's directory or its parent — removing a process named ".." would delete every
// bundle the mesh has, and more. One with a leading dash is read by the service manager as an
// option, not a unit. Exported because the removal checks the recorded name again: a record is
// what the host wrote, and a host of an older version wrote it under looser rules.
func ProcessNameProblem(name string) string {
	switch {
	case name == "":
		return "a process needs a name, which is what its unit is called"
	case strings.ContainsAny(name, "/ \t"):
		return "a process name becomes a unit name, so it cannot contain a path separator or a space"
	case name == "." || name == "..":
		return fmt.Sprintf("a process name becomes a directory under the mesh's own, and %q would be "+
			"that directory or its parent", name)
	case strings.HasPrefix(name, "-"):
		return "a process name cannot begin with a dash: the service manager would read it as an option"
	}
	return ""
}

func (d *Process) validate(where string, _ bool) []string {
	var problems []string
	if problem := ProcessNameProblem(d.Name); problem != "" {
		problems = append(problems, where+": "+problem)
	}
	if d.Source == "" {
		problems = append(problems, where+": a process needs somewhere to fetch its bundle from")
	}
	if !strings.HasPrefix(d.Digest, "sha256:") || len(d.Digest) != len("sha256:")+64 {
		// The same rule an archive follows, and for the same reason: this crosses a network the
		// mesh does not control, and a reference that can be made to point elsewhere is not one.
		problems = append(problems, where+
			": a process bundle is pinned by digest, as sha256:<64 hex characters>")
	}
	if len(d.Run) == 0 {
		problems = append(problems, where+": a process needs to say what to run")
	}
	for _, part := range d.Run {
		if part == "" {
			problems = append(problems, where+": a process command has an empty element")
			break
		}
	}
	// **A newline cannot be represented in a unit's environment, so it is refused rather than
	// mangled.** Everything else a unit file reinterprets — a percent specifier, whitespace
	// splitting assignments, a quote ending one early — can be escaped. A newline cannot: it ends
	// the line, and what follows is read as a unit DIRECTIVE. A value carrying one could write
	// ExecStart= and have the machine run something nobody declared.
	//
	// Refused here, near whoever wrote it, rather than at the far end of a declaration.
	for key, value := range d.Env {
		if strings.ContainsAny(value, "\n\r") {
			problems = append(problems, fmt.Sprintf(
				"%s: the value of %s contains a line break, which cannot be written into a unit's "+
					"environment — what followed it would be read as a unit directive", where, key))
		}
		if key == "" {
			problems = append(problems, where+": an environment value with no name")
		}
	}

	if d.Schedule != "" {
		if d.RunOnce {
			problems = append(problems, where+
				": a process runs once or on a schedule, not both")
		}
		if len(d.RestartOn) > 0 {
			problems = append(problems, where+
				": a scheduled process is not running between its fires, so there is nothing to "+
				"restart when something it reads changes")
		}
		if _, err := ParseCron(d.Schedule); err != nil {
			problems = append(problems, where+": "+err.Error())
		}
	}
	return problems
}

// Service is a unit the host puts into a state. It does not install the unit.
//
// Two states, and they are orthogonal rather than one scale. A unit can be enabled and stopped
// (it will come back at boot), or disabled and running (started by hand, gone after a reboot).
// Folding them into one field would make the second expressible only by accident.
type Service struct {
	ID   string `json:"id"`
	Type Type   `json:"type"`
	Unit string `json:"unit"`
	// State is "running" or "stopped" — or absent, and then **the unit's lifecycle is the
	// machine's; the mesh only reflects its triggers** (novox/hq ADR 0117). The uplink modules
	// declare the machine's own network manager this way: the mesh writes into its configuration
	// and needs it to read that again, and nothing more. Stated, the host would start the manager
	// on a machine that uses another one — two managers fighting over the same links — and, when
	// the module was unassigned, stop it: the machine's network, the channel the mesh itself
	// arrives on, gone at the moment of a routine change. So a service without a state is never
	// started, stopped, enabled or disabled, is reloaded or restarted only when a trigger changed
	// and it is already running, and undeclared is simply forgotten. It says nothing unless it
	// names a trigger, and it may not say boot or takes-over, which are both lifecycle.
	State string `json:"state,omitempty"`
	// Boot is "enabled" or "disabled" — whether the unit starts at boot. Optional: absent means
	// the host asserts nothing about it and leaves whatever is there.
	//
	// Without this the host could start a unit and not make it survive a reboot, which is a
	// declaration that reports success and stops being true at the next power cut.
	Boot string `json:"boot,omitempty"`

	// RestartOn names resources whose change means this service must be restarted.
	//
	// Because a running service does not re-read its configuration. Replace the file, find the
	// service already running, do nothing, and the machine keeps behaving the way it did before —
	// while every check passes, because the file is right and the service is up. That is not
	// hypothetical: it is how a third node joining a mesh left the first two carrying a network
	// that no longer existed, and every part of it reported success.
	//
	// This is declared state rather than a command. The declaration says the running service must
	// reflect these files; the host works out that it does not and acts. A *command* to restart
	// would be an action, and the link may not carry one (novox/hq ADR 0005) — so this is not a
	// way around that rule, it is the shape the rule leaves.
	RestartOn []string `json:"restart-on,omitempty"`

	// ReloadOn names resources whose change means this service must be reloaded — for a service
	// that re-reads its configuration when told to, where a restart would stop what it runs: the
	// container runtime, whose restart stops every container on the machine (novox/hq ADR 0102).
	// A change that is also in RestartOn restarts it, which covers a reload.
	ReloadOn []string `json:"reload-on,omitempty"`

	// TakesOver names the found tunnel this service replaces (novox/hq ADR 0105): before this unit
	// is started, the named unit is stopped and disabled — never flushed — and its configuration
	// file is kept like any held file — until the take is proven by a peer's handshake, and then
	// retired, its original staying kept (novox/hq ADR 0119). Only on an adopted node, and only
	// said by the controller, which knows the found tunnel's key is this node's own: without that,
	// starting this unit on the found one's port would drop every peer's packets.
	TakesOver *TakeOver `json:"takes-over,omitempty"`
}

// TakeOver is a found tunnel a service replaces: its interface, the unit that raised it, and its
// configuration file.
type TakeOver struct {
	Interface string `json:"interface"`
	Unit      string `json:"unit"`
	Config    string `json:"config"`
}

// Stateless reports whether the unit's lifecycle is the machine's, and the mesh only reflects the
// service's triggers (novox/hq ADR 0117).
func (s *Service) Stateless() bool { return s.State == "" }

func (s *Service) Identity() string { return s.ID }
func (s *Service) Kind() Type       { return TypeService }
func (s *Service) Target() string   { return s.Unit }

func (s *Service) validate(where string, _ bool) []string {
	var problems []string
	if s.Unit == "" {
		problems = append(problems, where+": a service needs a unit")
	}
	switch {
	case s.State == "running" || s.State == "stopped":
	case s.State != "":
		problems = append(problems, fmt.Sprintf(
			"%s: state %q; a service is \"running\" or \"stopped\", or omits state to leave the "+
				"unit's lifecycle to the machine", where, s.State))
	case s.Boot != "" || s.TakesOver != nil:
		problems = append(problems, where+": a service that omits state leaves the unit's lifecycle "+
			"to the machine, and boot and takes-over are both its lifecycle")
	case len(s.RestartOn) == 0 && len(s.ReloadOn) == 0:
		problems = append(problems, where+": a service that omits state leaves the unit's lifecycle "+
			"to the machine, and names no restart-on or reload-on — it declares nothing")
	}
	if s.Boot != "" && s.Boot != "enabled" && s.Boot != "disabled" {
		problems = append(problems, fmt.Sprintf(
			"%s: boot %q; a service is \"enabled\" or \"disabled\" at boot, or omits it to "+
				"leave the machine's own setting alone", where, s.Boot))
	}
	if t := s.TakesOver; t != nil {
		switch {
		case t.Unit == "" || t.Config == "" || t.Interface == "":
			problems = append(problems, where+": takes-over names the found tunnel's interface, unit "+
				"and config, and this leaves one out")
		case t.Unit == s.Unit:
			problems = append(problems, fmt.Sprintf("%s: takes-over names %s, which is this service's own unit",
				where, t.Unit))
		case s.State != "running" && s.State != "":
			problems = append(problems, where+": a service that takes over a tunnel is running — stopping "+
				"the found one for a service that will not run would leave the peers with nothing")
		}
	}
	return problems
}

// What a file is written into (novox/hq ADR 0102): a JSON object whose keys the mesh sets, or a
// text file in which the mesh owns one marked block of lines (novox/hq issue 128).
const (
	IntoJSON  = "json"
	IntoBlock = "block"
)

// The lines that delimit the mesh's region in a file written into a block, each followed by the
// resource's id. Exact lines, never patterns: another tool's `# BEGIN …` block in the same file is
// that tool's, and a marker that merely resembled the mesh's must not be taken for it.
const (
	BlockBegin = "# BEGIN mesh "
	BlockEnd   = "# END mesh "
)

// Where a file written into a block has its region added, when it has none yet.
const (
	AtStart = "start"
	AtEnd   = "end"
)

// BlockMarkers are the two lines, without their line ends, that delimit a resource's region.
func BlockMarkers(id string) (begin, end string) {
	return BlockBegin + id, BlockEnd + id
}

// Opening is a port reachable on an adopted node, from where, and on which path.
//
// **From** is everywhere or mesh — the private network, by its interface. **Path** is incoming,
// for something listening on the machine, or forwarded, for a published container port: the found
// firewall sees a published port after the runtime has translated it, so a forwarded opening names
// the container's own port in To as well as the machine's in Port.
type Opening struct {
	ID       string `json:"id"`
	Type     Type   `json:"type"`
	Port     int    `json:"port"`
	Protocol string `json:"protocol"`
	From     string `json:"from"`
	Path     string `json:"path"`
	To       int    `json:"to,omitempty"`
}

// Where an opening admits from, and the path it is on.
const (
	FromEverywhere = "everywhere"
	FromMesh       = "mesh"
	PathIncoming   = "incoming"
	PathForwarded  = "forwarded"
)

func (o *Opening) Identity() string { return o.ID }
func (o *Opening) Kind() Type       { return TypeOpening }

func (o *Opening) Target() string {
	if o.Path == PathForwarded {
		return fmt.Sprintf("%s/%d forwarded to %d from %s", o.Protocol, o.Port, o.To, o.From)
	}
	return fmt.Sprintf("%s/%d %s from %s", o.Protocol, o.Port, o.Path, o.From)
}

func (o *Opening) validate(where string, _ bool) []string {
	var problems []string
	if o.Port < 1 || o.Port > 65535 {
		problems = append(problems, fmt.Sprintf("%s: an opening's port is 1-65535, not %d", where, o.Port))
	}
	if o.Protocol != "tcp" && o.Protocol != "udp" {
		problems = append(problems, fmt.Sprintf("%s: an opening is tcp or udp, not %q", where, o.Protocol))
	}
	if o.From != FromEverywhere && o.From != FromMesh {
		problems = append(problems, fmt.Sprintf(
			"%s: an opening is from %q or %q, not %q", where, FromEverywhere, FromMesh, o.From))
	}
	switch o.Path {
	case PathIncoming:
		if o.To != 0 {
			problems = append(problems, where+
				": an incoming opening names no container port; only a forwarded one does")
		}
	case PathForwarded:
		if o.To < 1 || o.To > 65535 {
			problems = append(problems, where+
				": a forwarded opening names the container's port it reaches, as to, 1-65535")
		}
	default:
		problems = append(problems, fmt.Sprintf(
			"%s: an opening's path is %q or %q, not %q", where, PathIncoming, PathForwarded, o.Path))
	}
	if !strings.HasPrefix(o.ID, AdoptionPrefix) {
		problems = append(problems, fmt.Sprintf(
			"%s: an opening is the mesh's own, so its id starts %q", where, AdoptionPrefix))
	}
	return problems
}

// Package is a package that should be present.
//
// Present is the whole of what it asserts, never a version: version is the package manager's
// business and the mesh does not hold a second opinion about it.
type Package struct {
	ID      string `json:"id"`
	Type    Type   `json:"type"`
	Package string `json:"package"`
}

func (p *Package) Identity() string { return p.ID }
func (p *Package) Kind() Type       { return TypePackage }
func (p *Package) Target() string   { return p.Package }

func (p *Package) validate(where string, _ bool) []string {
	if p.Package == "" {
		return []string{where + ": a package needs a package name"}
	}
	return nil
}

// Container is a container that should be running, from an image pinned by digest.
type Container struct {
	ID   string `json:"id"`
	Type Type   `json:"type"`
	Name string `json:"name"`
	// Image is pinned by digest (novox/hq ADR 0006) — a tag moves and a digest does not.
	Image string            `json:"image"`
	Env   map[string]string `json:"env,omitempty"`
	// EnvFile names files the runtime reads environment from, in order.
	//
	// **Because a secret may not travel in Env.** A declaration reaches a node over the broker,
	// and `env` is plain text in it — so a password there is a password the broker sees, which is
	// the transitive trust refused everywhere else (novox/hq ADR 0004). A sealed file reaches the
	// machine unreadable, the host writes it, and the runtime reads it: the mesh never holds it
	// and neither does anything between them.
	//
	// It is also simply how third-party software takes credentials. Nothing that ships in a
	// container will read a path the mesh invented; every one of them reads its environment.
	EnvFile []string `json:"env-file,omitempty"`
	Ports   []string `json:"ports,omitempty"`
	Volumes []string `json:"volumes,omitempty"`
	Args    []string `json:"args,omitempty"`
	// Names this container can reach, as `name:address`.
	//
	// **Because a container does not inherit the machine's names.** It gets its own `/etc/hosts`
	// holding only its own hostname, so every internal name the mesh wrote for this machine is
	// invisible to the thing the machine is running. That was hit for real: a database client on
	// one node could not resolve another node, on a mesh where both names were correct and
	// present on both machines.
	//
	// **A file rather than a resolver, which is the decision the mesh already made about names**
	// and this extends rather than overturns: it works on every runtime, needs no package, and
	// has no failure mode of its own. A resolver becomes necessary when names are wanted that are
	// not one-per-node — service names, wildcards — and that is still not true.
	//
	// Set by the mesh, not by a module: which machines exist is a fact about the mesh, and a
	// module that listed them would be a module that goes stale when one joins.
	Hosts []string `json:"hosts,omitempty"`

	// Network is the container's network, passed to the runtime unchanged.
	//
	// Needed because the control plane must reach the store and the broker on the machine it was
	// raised on, before there is any mesh to arrange that. The alternative was publishing ports
	// and guessing an address that works from inside a container, which is the same thing with a
	// worse failure mode.
	Network string `json:"network,omitempty"`

	// Dns is the resolvers this container asks, passed to the runtime unchanged.
	//
	// **Because some software refuses to run behind the runtime's forwarding resolver.** A mail
	// server's admin demands a DNSSEC-validating resolver, and the runtime's own (127.0.0.11)
	// forwards to whatever the machine has — so a module that ships its own validating resolver
	// must be able to point its other containers at it. Addresses, not names: the runtime's flag
	// takes only addresses, which is also why IP below exists — the resolver has to be somewhere
	// its siblings can name before any of them can resolve anything.
	Dns []string `json:"dns,omitempty"`

	// IP is this container's address on its network, passed to the runtime unchanged.
	//
	// Only meaningful on a user-defined network, and refused by the runtime elsewhere. Exists for
	// exactly one shape: a container others must reach *before* name resolution works — a
	// module's own DNS resolver being the case that forced it (see Dns).
	IP string `json:"ip,omitempty"`

	// RestartOn names resources whose change means this container must be recreated — the same
	// field a service has, for the same reason (novox/hq 04-ISSUES/009). A container reads a
	// mounted file once at start; a changed file leaves the running process holding the old value,
	// while every check passes because the file on disk is right. The host recreates the container
	// when one of these resources changed this pass, even if the spec matches.
	//
	// What a running container reads at creation — its env-files, and a file mounted into it
	// directly — is part of its spec by content since novox/hq 04-ISSUES/103, and needs no naming
	// here. A directory mounted into it is NOT looked inside, not even for files the host wrote
	// there: whether a service reads such a file once or watches it live is the service's, and
	// RestartOn is how a module says "once, at start" — a config the host renders under the
	// module's state directory, a step whose result it consumes. On a run-once step it means *run
	// again*: a step that fetches a fact from a provider names the binding it reads, and is run
	// again when the provider moved (novox/hq ADR 0099).
	RestartOn []string `json:"restart-on,omitempty"`

	// RunOnce marks a container the host runs to completion rather than leaves running: a step,
	// not a service (novox/hq ADR 0052). The host runs it, requires it to exit 0, and records that
	// it did — and because the declaration is applied in order and a failed step halts the apply,
	// whatever is declared after a run-once container starts only once the step has finished. It is
	// how a module runs its own code at first boot — seed a store, migrate, health-gate — under its
	// own account (ADR 0047), before the container that depends on it. The record that it ran is
	// the digest of this declaration, so a re-apply does not re-run it unless the declaration
	// changed.
	RunOnce bool `json:"run-once,omitempty"`

	// Schedule marks a container the host runs on a recurring cadence — a five-field cron
	// expression (novox/hq ADR 0053). It is the recurring twin of RunOnce: the same container, run
	// to completion, but again and again on the clock rather than once. Installing it does not run
	// it — the schedule is state that is present, like a running service, so the apply is current as
	// soon as it is recorded and does NOT gate what follows. The host's scheduler fires the
	// container when the cron is due, re-established from this declaration each apply because the
	// declaration is the source of truth (ADR 0018). A run that exits non-zero is recorded and never
	// fails the apply or flips the node's state; a run still going when the next is due is skipped
	// rather than stacked. It is exclusive with RunOnce and with restart-on: a container runs once
	// and gates, runs on a cadence, or stays up — never two of these.
	Schedule string `json:"schedule,omitempty"`
}

func (c *Container) Identity() string { return c.ID }
func (c *Container) Kind() Type       { return TypeContainer }
func (c *Container) Target() string   { return c.Name }

func (c *Container) validate(where string, _ bool) []string {
	var problems []string
	if c.Name == "" {
		problems = append(problems, where+": a container needs a name")
	}
	// A run-once step may name what it reads under restart-on. For a step the word means *run
	// again*: what a container reads is part of its digest, so a step whose named resource changed
	// is a different step and runs again (novox/hq ADR 0099). Not refused.
	// A container runs once and gates, on a cadence, or stays up — never two of these
	// (novox/hq ADR 0053). run-once and schedule are the two "runs to completion" lifecycles and
	// contradict each other, and a scheduled step does not stay running to be brought back by
	// restart-on either. Refused here on arrival, as the control plane refuses it near its author.
	if c.RunOnce && c.Schedule != "" {
		problems = append(problems, where+": a container is run-once or scheduled, not both; "+
			"run-once runs once and gates what follows, a schedule runs it again on a cadence")
	}
	if c.Schedule != "" && len(c.RestartOn) > 0 {
		problems = append(problems, where+": a scheduled container cannot also declare restart-on; "+
			"it runs to completion on its cadence rather than staying running to be restarted")
	}
	if c.Schedule != "" {
		if _, err := ParseCron(c.Schedule); err != nil {
			problems = append(problems, where+": "+err.Error())
		}
	}
	// The runtime's flags take addresses, and a name here would be handed to it verbatim and
	// refused at create — after the old container was already removed. Refused on arrival instead.
	for _, d := range c.Dns {
		if net.ParseIP(d) == nil {
			problems = append(problems, where+": dns "+strconv.Quote(d)+" is not an address; "+
				"the runtime's resolver flag takes only addresses")
		}
	}
	if c.IP != "" {
		if net.ParseIP(c.IP) == nil {
			problems = append(problems, where+": ip "+strconv.Quote(c.IP)+" is not an address")
		}
		if c.Network == "" {
			problems = append(problems, where+": an ip needs a network; the runtime refuses a "+
				"static address anywhere but a user-defined one")
		}
	}
	return append(problems, checkImage(where, c.Image)...)
}

// Action runs something the bundle declared, and the host never learns what it means.
type Action struct {
	ID      string   `json:"id"`
	Type    Type     `json:"type"`
	Command []string `json:"command"`
	// Verify is not optional and is not a courtesy. It is the read-back AND the idempotency
	// check: the host does not know what a database is, so "is it already there" is a question
	// only the declaration can ask (novox/hq ADR 0005).
	Verify []string `json:"verify"`
	// In names a container to run inside. Empty means the machine itself.
	In string `json:"in,omitempty"`
}

func (a *Action) Identity() string { return a.ID }
func (a *Action) Kind() Type       { return TypeAction }

func (a *Action) Target() string {
	target := strings.Join(a.Command, " ")
	if a.In != "" {
		return "in " + a.In + ": " + target
	}
	return target
}

func (a *Action) validate(where string, allowActions bool) []string {
	// The bound the whole security argument rests on (novox/hq ADR 0005).
	if !allowActions {
		return []string{where +
			": an action arrived over the link, and the link may not carry one. The host " +
			"applies declarations of known shape; a command to run is not one. A bundle may " +
			"carry an action because it arrives with the binary — anyone able to put a " +
			"hostile action there could have put it in the host itself"}
	}
	var problems []string
	if len(a.Command) == 0 {
		problems = append(problems, where+": an action needs a command")
	}
	if len(a.Verify) == 0 {
		problems = append(problems, where+
			": an action needs a verify. An action that runs and reports success without "+
			"reading anything back is the fault this host exists to prevent, and verify is "+
			"also how the host knows whether the action is already done")
	}
	return problems
}

// newOf returns an empty resource of a kind, or nil if the kind is unknown.
//
// This is the whole vocabulary, in one place. A kind that is not here cannot be declared.
func newOf(t Type) Resource {
	switch t {
	case TypeDirectory:
		return &Directory{}
	case TypeFile:
		return &File{}
	case TypeService:
		return &Service{}
	case TypePackage:
		return &Package{}
	case TypeContainer:
		return &Container{}
	case TypeAction:
		return &Action{}
	case TypeNetwork:
		return &Network{}
	case TypeUser:
		return &User{}
	case TypeArchive:
		return &Archive{}
	case TypeAccess:
		return &Access{}
	case TypeProcess:
		return &Process{}
	case TypeOpening:
		return &Opening{}
	}
	return nil
}

// Vocabulary is every kind this host speaks.
func Vocabulary() []Type {
	return []Type{
		TypeAccess, TypeAction, TypeArchive, TypeContainer, TypeDirectory, TypeFile,
		TypeNetwork, TypeOpening, TypePackage, TypeProcess, TypeService, TypeUser,
	}
}

// Declaration is what a machine should be, in the order it should be made so.
type Declaration struct {
	Version int
	// For names the node this is meant for. A host with an identity refuses one addressed
	// elsewhere; a host without one — the first node, applying the bundle it carries — has
	// nothing to check against.
	For string
	// Resources, in the order they are applied. The host does not sort them: ordering is a
	// decision, and deciding is not what the host does (novox/hq ADR 0005).
	Resources []Resource

	// Adoption says this node is adopted, and which of its modules have been taken. Nil is a
	// converged node — which is every node the mesh raised before adoption existed, and so the
	// only form an older controller ever sends (novox/hq ADR 0100).
	Adoption *Adoption
}

// Adoption is a node's mode, as the controller records it: the node is adopted, and these are
// the modules taken on it so far (novox/hq ADR 0100).
//
// **Authoritative, and only ever stated by the controller.** A host does not work out whether it
// is adopted; it is told, in every declaration, so a host restarted from the declaration it kept
// is in the same mode it was in before.
//
// Untaken names, per module assigned here and not yet taken, the ids of its resources. Any kind
// may be listed: a file, a directory, a service's unit or a container can already be on the
// machine, and an action run inside a held container reaches what was found (novox/hq ADR 0103);
// the host decides per kind what can be held. The host cannot split a resource id into its
// module, because module names may contain dots, so the controller says which ids belong to which
// module rather than leaving the host to guess.
type Adoption struct {
	Taken   []string            `json:"taken"`
	Untaken map[string][]string `json:"untaken,omitempty"`
}

// AdoptionPrefix is the id prefix of what the mesh itself declares because a node is adopted —
// its openings and its guard. Nothing under it belongs to a module, so none of it is ever held.
const AdoptionPrefix = "adoption."

// UntakenModuleOf says which untaken module declares a resource, if any.
func (a *Adoption) UntakenModuleOf(id string) (string, bool) {
	if a == nil {
		return "", false
	}
	for module, ids := range a.Untaken {
		if slices.Contains(ids, id) {
			return module, true
		}
	}
	return "", false
}

// checkAdoption holds what an adoption says against the resources beside it. Every problem is a
// refusal: a host that misread which module is untaken would replace a predecessor's service the
// operator never took.
func checkAdoption(a *Adoption, resources []Resource, allowActions bool) []string {
	if a == nil {
		return nil
	}
	if allowActions {
		// The bundle is carried with the binary and raises a foundation before any mesh exists.
		// Whether a node is adopted is the controller's record, and a bundle that claimed it would
		// be the host deciding its own mode (novox/hq ADR 0100).
		return []string{"a carried bundle says the node is adopted, and only the mesh can say " +
			"that: a node's mode is the controller's record, sent in every declaration"}
	}
	kinds := map[string]Type{}
	for _, r := range resources {
		kinds[r.Identity()] = r.Kind()
	}
	var problems []string
	for _, module := range a.Taken {
		if _, both := a.Untaken[module]; both {
			problems = append(problems, fmt.Sprintf(
				"adoption: the module %q is said to be both taken and untaken", module))
		}
	}
	owner := map[string]string{}
	modules := make([]string, 0, len(a.Untaken))
	for module := range a.Untaken {
		modules = append(modules, module)
	}
	sort.Strings(modules)
	for _, module := range modules {
		for _, id := range a.Untaken[module] {
			if strings.HasPrefix(id, AdoptionPrefix) {
				problems = append(problems, fmt.Sprintf(
					"adoption: %q is the mesh's own and belongs to no module, so it cannot be untaken", id))
				continue
			}
			if first, twice := owner[id]; twice {
				problems = append(problems, fmt.Sprintf(
					"adoption: %q is said to belong to both %q and %q", id, first, module))
				continue
			}
			owner[id] = module
			if _, declared := kinds[id]; !declared {
				problems = append(problems, fmt.Sprintf(
					"adoption: %q of the untaken module %q is not in this declaration", id, module))
			}
		}
	}
	return problems
}

// RefusalError refuses a whole declaration, naming every problem at once.
//
// Every problem rather than the first: a caller fixing one at a time learns the next only by
// running again, and a declaration is generated, so a person reading this is debugging the
// generator.
type RefusalError struct {
	Problems []string
}

func (e *RefusalError) Error() string {
	return fmt.Sprintf(
		"this declaration is refused, and none of it was applied:\n  - %s\n\n"+
			"A host that applied the parts it understood would leave a machine that looks "+
			"configured and is not.",
		strings.Join(e.Problems, "\n  - "))
}

// Parse reads a declaration that arrived over the link, and refuses anything it does not fully
// understand — including any action, which the link may not carry (novox/hq ADR 0005).
func Parse(raw []byte) (*Declaration, error) { return parse(raw, false) }

// ParseTrusted reads a declaration from a source already as privileged as the host itself: the
// bundle it carries, or a file handed to it by someone who is running it as root.
//
// Actions are permitted here and nowhere else. The asymmetry is deliberate and is the entire
// content of ADR 0005: refusing actions from the bundle buys nothing, because whoever built the
// bundle built the binary; refusing them from the link buys the bound on what a compromised
// control plane can express.
func ParseTrusted(raw []byte) (*Declaration, error) { return parse(raw, true) }

// envelope is the declaration with its resources still unread.
//
// Two passes, because which fields are legal depends on the "type" inside each resource. The
// first pass takes the envelope and each resource's bytes; the second decodes each one into
// the struct for its kind, strictly.
type envelope struct {
	Version  int       `json:"declaration"`
	For      string    `json:"for,omitempty"`
	Adoption *Adoption `json:"adoption,omitempty"`
	// OwnsNothing is the control plane saying, explicitly, that this node's declaration is empty
	// on purpose — it owns nothing the mesh put there (novox/hq issue 127). Without it an empty
	// resources list is refused as a likely mistake; with it the node applies the empty
	// declaration and drops what it last held. The two are distinguished because a truncated or
	// mis-composed body arrives as empty too, and a host that could not tell them apart would let
	// a bug quietly strip a machine.
	OwnsNothing bool              `json:"owns_nothing,omitempty"`
	Resources   []json.RawMessage `json:"resources"`
}

func parse(raw []byte, allowActions bool) (*Declaration, error) {
	var env envelope
	if err := strictDecode(raw, &env); err != nil {
		return nil, &RefusalError{Problems: []string{"not a declaration: " + err.Error()}}
	}

	if env.Version != Version {
		// Everything below assumes the vocabulary, so there is nothing further to say.
		return nil, &RefusalError{Problems: []string{fmt.Sprintf(
			"declaration version %d; this host speaks version %d. Refused whole rather than "+
				"partly, so a newer vocabulary is never half-applied by an older host",
			env.Version, Version)}}
	}

	d := &Declaration{Version: env.Version, For: env.For, Adoption: env.Adoption}
	var problems []string

	if len(env.Resources) == 0 && !env.OwnsNothing {
		problems = append(problems, "no resources. An empty declaration is a mistake, not a "+
			"machine with nothing on it — say so with an explicit empty list if that is meant")
	}

	seen := map[string]int{}
	for i, rawResource := range env.Resources {
		// Peek, leniently. This pass only needs to know which struct to decode into; reading
		// strictly here would report an unknown field before knowing which fields are known.
		var head struct {
			ID   string `json:"id"`
			Type Type   `json:"type"`
		}
		_ = json.Unmarshal(rawResource, &head)

		where := fmt.Sprintf("resource %d", i)
		if head.ID != "" {
			where = fmt.Sprintf("resource %q", head.ID)
		}

		if head.ID == "" {
			problems = append(problems, where+": no id. Identity is what lets the host know "+
				"this is the same resource it applied last time")
		} else if first, ok := seen[head.ID]; ok {
			problems = append(problems, fmt.Sprintf(
				"%s: id already used by resource %d. Two resources with one identity cannot "+
					"both be tracked", where, first))
		} else {
			seen[head.ID] = i
		}

		resource := newOf(head.Type)
		if resource == nil {
			problems = append(problems, fmt.Sprintf(
				"%s: unknown type %q. This host understands %s", where, head.Type, vocabulary()))
			continue
		}

		// A field the kind does not have is refused, and the struct is what says so — there
		// is no list of exclusions for anyone to keep current.
		//
		// Asked separately rather than taken from the decoder's error, because the decoder
		// stops at the first unknown field and this record promises every problem at once. A
		// caller fixing one field at a time learns the next only by running again.
		if unknown := unknownFields(rawResource, resource); len(unknown) > 0 {
			for _, field := range unknown {
				problems = append(problems, fmt.Sprintf(
					"%s: a %s does not use %q, and it is set. Refused rather than ignored",
					where, head.Type, field))
			}
			continue
		}
		if err := json.Unmarshal(rawResource, resource); err != nil {
			problems = append(problems, fmt.Sprintf("%s: %s", where, err))
			continue
		}

		problems = append(problems, resource.validate(where, allowActions)...)
		d.Resources = append(d.Resources, resource)
	}
	problems = append(problems, checkAdoption(env.Adoption, d.Resources, allowActions)...)
	if env.Adoption == nil {
		for _, r := range d.Resources {
			if r.Kind() == TypeOpening {
				// On a converged node the mesh's own filter admits what is declared, and the
				// found firewall is retired; an opening there would be a rule in a firewall the
				// mesh has disabled (novox/hq ADR 0100).
				problems = append(problems, fmt.Sprintf(
					"resource %q: an opening is for an adopted node, and this declaration does not "+
						"say the node is adopted", r.Identity()))
			}
			if svc, ok := r.(*Service); ok && svc.TakesOver != nil {
				// A tunnel is taken over on an adopted node, where what is found is kept: on a
				// converged one there is nothing found to take over, and stopping a unit the
				// mesh did not declare would be the host deciding (novox/hq ADR 0105).
				problems = append(problems, fmt.Sprintf(
					"resource %q: taking over a tunnel is for an adopted node, and this declaration "+
						"does not say the node is adopted", r.Identity()))
			}
		}
	}

	if len(problems) > 0 {
		return nil, &RefusalError{Problems: problems}
	}
	return d, nil
}

func strictDecode(raw []byte, into any) error {
	// DisallowUnknownFields is the whole point rather than strictness for its own sake: a
	// field the host does not know is a thing the control plane believes it asked for.
	dec := json.NewDecoder(bytes.NewReader(raw))
	dec.DisallowUnknownFields()
	if err := dec.Decode(into); err != nil {
		return err
	}
	// And **nothing after it**. A decoder reads one value and stops, so a file holding a
	// declaration followed by anything at all — a truncated rewrite, two declarations
	// concatenated, a stray line from whatever wrote the file — parses as the first value and the
	// rest is never looked at.
	//
	// That is the same fault this host refuses everywhere else, in its quietest form: the machine
	// applies something, reports success, and what it applied is not what the file says. Found
	// when a test harness appended a line to a bundle by accident and every apply kept working.
	if _, err := dec.Token(); err != io.EOF {
		return fmt.Errorf(
			"there is more in this file after the declaration ends. Refused whole: a file with " +
				"something after it may be a truncated rewrite or two declarations run together, " +
				"and applying the first would be applying something nobody wrote")
	}
	return nil
}

// unknownFields names every JSON key the kind's struct has no field for.
//
// The struct's own tags are the list of what is legal, so adding a field to a kind is the
// whole of adding it — there is nowhere else that has to agree.
func unknownFields(raw []byte, into Resource) []string {
	var got map[string]json.RawMessage
	if err := json.Unmarshal(raw, &got); err != nil {
		return nil // not an object; the decode below will say so properly
	}

	known := map[string]bool{}
	t := reflect.TypeOf(into).Elem()
	for i := 0; i < t.NumField(); i++ {
		name, _, _ := strings.Cut(t.Field(i).Tag.Get("json"), ",")
		if name != "" && name != "-" {
			known[name] = true
		}
	}

	var unknown []string
	for field := range got {
		if !known[field] {
			unknown = append(unknown, field)
		}
	}
	sort.Strings(unknown)
	return unknown
}

func checkMode(where, mode string) []string {
	if mode == "" {
		return nil
	}
	if len(mode) != 4 || mode[0] != '0' {
		return []string{fmt.Sprintf(
			"%s: mode %q; write it as four octal digits such as \"0644\", so it means the "+
				"same thing here as it does in the manifest it came from", where, mode)}
	}
	for _, c := range mode[1:] {
		if c < '0' || c > '7' {
			return []string{fmt.Sprintf("%s: mode %q is not octal", where, mode)}
		}
	}
	return nil
}

// checkImage insists on content, not on a name.
//
// A tag moves and a digest does not. The bundle's whole claim is that what it names is exact
// (novox/hq ADR 0006), and a bundle pinning `postgres:17` pins nothing — it names whatever
// that tag points at on the day the host happens to run.
//
// **Two forms say something exact, and only one of them needs a registry.** `name@sha256:…` is a
// manifest digest, which a registry assigns on push. A bare `sha256:…` is an image the machine
// already holds, addressed by the digest of its own configuration — equally immutable, equally
// unforgeable, and requiring nothing to have served it.
//
// That second form is what a first machine needs. The mesh's own control plane exists in no public
// registry and never will: it is built from source, and until this mesh has a registry of its own
// there is nowhere to push it to and therefore no manifest digest to name it by. Insisting on one
// would mean a registry has to exist before the thing that lets a mesh have a registry can start —
// which is not a pin, it is a dependency the rule accidentally created. A machine that built an
// image, or was handed one, can name it by what it is.
func checkImage(where, image string) []string {
	if image == "" {
		return []string{where + ": a container needs an image"}
	}
	// An image this machine holds, named by the digest of its own configuration.
	if strings.HasPrefix(image, "sha256:") {
		if len(image) != len("sha256:")+64 {
			return []string{fmt.Sprintf(
				"%s: image id %q is not a sha256 digest", where, image)}
		}
		return nil
	}
	name, digest, found := strings.Cut(image, "@")
	if !found || name == "" {
		return []string{fmt.Sprintf(
			"%s: image %q is not pinned. Write it as name@sha256:… — or as sha256:… for an image "+
				"this machine already holds. A tag moves, and a bundle that pinned a tag would "+
				"not be pinned", where, image)}
	}
	if !strings.HasPrefix(digest, "sha256:") || len(digest) != len("sha256:")+64 {
		return []string{fmt.Sprintf(
			"%s: image digest %q is not a sha256 digest", where, digest)}
	}
	return nil
}

func vocabulary() string {
	kinds := Vocabulary()
	names := make([]string, 0, len(kinds))
	for _, t := range kinds {
		names = append(names, string(t))
	}
	sort.Strings(names)
	return strings.Join(names, ", ")
}

// ParseFileTrusted reads a declaration from a file somebody handed this host.
//
// The same as ParseTrusted, and it allows whole-line `//` comments first. A pinned, hand-authored
// artefact that nobody can annotate is one nobody can review — the foundation bundle is mostly
// explanation of why each digest is what it is.
//
// **Only for a file, never for the link.** Over the link the format stays exactly JSON, because
// a wire format with a second thing to strip is a wire format with a second thing to disagree
// about.
//
// It exists because there were two readers for one file: the bundle stripped comments and `apply`
// did not, so the example bundle in this repository could be built into a binary and not applied
// from disk. The failure was `invalid character '/'`, which names the symptom and not the cause.
func ParseFileTrusted(raw []byte) (*Declaration, error) {
	return ParseTrusted(stripComments(raw))
}

// stripComments removes whole lines beginning with `//`.
//
// Only whole lines: anything cleverer would need to know where strings begin and end, and a
// parser that half-understands its input is worse than one that does not try. A `//` inside a
// value — every image reference has one — is untouched.
func stripComments(raw []byte) []byte {
	var kept []string
	for _, line := range strings.Split(string(raw), "\n") {
		if strings.HasPrefix(strings.TrimSpace(line), "//") {
			continue
		}
		kept = append(kept, line)
	}
	return []byte(strings.Join(kept, "\n"))
}