package main

import (
	"context"
	"errors"
	"fmt"
	"sort"
	"strings"
	"time"

	"git.novox.be/novox/mesh-sdk/go/catalogue"
	sdkdelivery "git.novox.be/novox/mesh-sdk/go/delivery"
	"github.com/novox/mesh-controller/internal/builderkeys"
	"github.com/novox/mesh-controller/internal/conditions"
	"github.com/novox/mesh-controller/internal/inventory"
)

// A module not registered yet, and the assignment that waits for it (novox/hq issue 325, ADR 0261).
//
// On 2026-10-08 the catalogue's pull request adding `sensors` merged; a minute later `assign g14 sensors`
// answered "no module of that name: sensors", and a few minutes later the same call worked. The merge had
// asked for the build (issue 300) and the build had not finished. The answer read as "you forgot to register
// it", and nothing in it said a build was on its way.
//
// So an assignment of a module the catalogue does not hold looks further before it refuses, and says which
// of three cases it is in:
//
//   - **a build is in flight**: a build request for the module's directory has no outcome yet, or its build
//     succeeded a moment ago and is being registered. The assignment is kept as a **pending assignment**,
//     which the controller makes when the build registers the module (ADR 0261).
//   - **known, not built**: the controller asked for the directory before, and the build failed, was not
//     registered, said nothing within its bound, or could not be asked. Said, with `build` and assign's
//     own build argument, which asks for the build and keeps the assignment pending on it.
//   - **unknown**: no module registered and no build request kept by that name. Refused as before, with the
//     closest names.
//
// **Pending by default while a build is in flight** (ADR 0261), without an argument to ask for it: an
// assignment is what a person meant and is kept even when its machine does not resolve (acts.go), and
// `unassign` withdraws it. Asking for a second act once the build lands is the two acts by hand issue 300
// removed. A build is never asked for by default: it runs on another machine, and a directory whose last
// build failed is a fault to look at before it is a thing to retry.
//
// **Settling is the controller's tick, never a read** (settlingPending): `status`, the board and the summary
// only read the rows.

// buildRequestBound is how long a build request may go without an outcome before it is no longer read as in
// flight, and a pending assignment waiting for it expires. Generous: a build waits behind others on the
// build seat, and a pending assignment that waits a little longer costs nothing.
const buildRequestBound = 2 * time.Hour

// registerGrace is how long a build heard as built is read as still in flight while it is not registered:
// the outcome is recorded first and registered after it, in the same take-in. Past it, the build was
// recorded and not registered, and says so.
const registerGrace = 5 * time.Minute

// pendingShownFor is how long an ended pending assignment is listed in `status`.
const pendingShownFor = 24 * time.Hour

// settleEvery is how often the controller settles the pending assignments.
const settleEvery = time.Minute

// claimStaleAfter is how long a pending assignment may be held as applying before the tick reads the claim
// as left by a controller that stopped, and settles it from what the mesh holds.
const claimStaleAfter = 5 * time.Minute

// kindPendingEnded is a pending assignment that ended without being made: expired or refused.
const kindPendingEnded = "assignment-not-made"

// assignOptions are what an assignment was asked with beside its machine and modules.
type assignOptions struct {
	// Build asks for the build of a module known and not built, and keeps the assignment pending on it.
	Build bool
}

// The cases of a module the catalogue does not hold (novox/hq issue 325).
const (
	unknownModule = iota
	buildInFlight
	knownNotBuilt
	// keyComing is a module the indexer holds and the catalogue does not yet: registered from the builder's done key
	// of it (ADR 0501 §6), which nobody asks.
	keyComing
)

// keyPendingPrefix opens the build a pending assignment waits for when it waits for a module's key: no build is asked
// (ADR 0501 §5, §6; issue 1851, A10), so it names the module's key, not a build id.
const keyPendingPrefix = "key:"

// indexedModule is the indexer's trunk value of a module the catalogue does not hold, found when it holds one. A
// variable so a test needs no bus.
var indexedModule = func(ctx context.Context, module string) (sdkdelivery.Module, bool, error) {
	keys, closeKeys, err := openBuilderKeys()
	if err != nil {
		return sdkdelivery.Module{}, false, err
	}
	defer closeKeys()
	v, err := keys.ModuleAt(ctx, module, "")
	if errors.Is(err, builderkeys.ErrNoValue) {
		return sdkdelivery.Module{}, false, nil
	}
	return v, err == nil, err
}

// notHeld is where a module the catalogue does not hold stands: its case, the build request it was read
// from, and the sentence that says it.
type notHeld struct {
	kind    int
	request inventory.RequestOutcome
	said    string
}

// whereIs looks for a module the catalogue does not hold among the build requests the controller keeps.
// notHeldReads is what whereIs reads of the records (*inventory.Inventory, or a test's).
type notHeldReads interface {
	RequestsNamed(ctx context.Context, name string) ([]inventory.RequestOutcome, error)
	Catalogue(ctx context.Context) (map[string]catalogue.Manifest, error)
	RequestedNames(ctx context.Context) ([]string, error)
}

func whereIs(ctx context.Context, inv notHeldReads, module string, now time.Time) (notHeld, error) {
	// **Its key is coming** (ADR 0501 §6): the indexer holds it, and the controller registers it from the builder's done
	// key of it. Read before the build requests of before R4e, which nothing writes any more.
	if v, held, err := indexedModule(ctx, module); err != nil {
		return notHeld{}, fmt.Errorf("whether the indexer holds %s cannot be read: %w", module, err)
	} else if held {
		return notHeld{kind: keyComing, request: inventory.RequestOutcome{BuildRequest: inventory.BuildRequest{
			ID: keyPendingPrefix + module, Repository: v.Repository, Seat: gitSeat, Path: keyPath(v)}},
			said: fmt.Sprintf("%s is not registered yet: the indexer holds it in %s at %.8s, and it is registered from "+
				"the builder's done key of it, which no one asks", module, v.Repository, v.Commit)}, nil
	}
	requests, err := inv.RequestsNamed(ctx, module)
	if err != nil {
		return notHeld{}, err
	}
	if r, ok := inFlight(requests, now); ok {
		being := "being built"
		if r.Heard {
			being = "built and being registered"
		}
		return notHeld{kind: buildInFlight, request: r, said: fmt.Sprintf("%s is not registered yet: it is %s "+
			"from %s since %s, as build %s; it can be assigned once that build registers it", module, being,
			requestSource(r), clock(r.At), r.ID)}, nil
	}
	if len(requests) > 0 {
		return notHeld{kind: knownNotBuilt, request: requests[0], said: fmt.Sprintf("%s is known and not "+
			"registered: %s", module, whyNotBuilt(requests[0], now))}, nil
	}
	// Only what was looked at is claimed: the catalogue, and the build requests the controller keeps.
	said := fmt.Sprintf("%v: %s — the catalogue holds no module of that name, and no build request the "+
		"controller kept (the last 30 days) is for a directory of that name", inventory.ErrNoSuchModule, module)
	if near := closestNames(ctx, inv, module); len(near) > 0 {
		said += "; the closest names it holds: " + strings.Join(near, ", ")
	}
	return notHeld{kind: unknownModule, said: said}, nil
}

// notInCatalogue is the modules named that the catalogue does not hold.
func notInCatalogue(ctx context.Context, open *stores, modules []string) ([]string, error) {
	shelf, err := open.inventory.Catalogue(ctx)
	if err != nil {
		return nil, err
	}
	var missing []string
	for _, m := range modules {
		if _, ok := shelf[m]; !ok {
			missing = append(missing, m)
		}
	}
	return missing, nil
}

// openPending is the open pending assignment of a module to a machine, if there is one.
func openPending(ctx context.Context, inv *inventory.Inventory, node, module string) (*inventory.PendingAssignment, error) {
	rows, err := inv.PendingFor(ctx, node, module)
	if err != nil {
		return nil, err
	}
	for i := range rows {
		if rows[i].Open() {
			return &rows[i], nil
		}
	}
	return nil, nil
}

// notRegistered answers an assignment of a module the catalogue does not hold: pending on a build in flight,
// known and not built, or unknown (novox/hq issue 325). An answer with no error is a pending assignment kept.
func notRegistered(ctx context.Context, open *stores, node, module string, opts assignOptions) (string, error) {
	inv := open.inventory
	if _, err := inv.NodeByName(ctx, node); err != nil {
		return "", err
	}
	now := time.Now()
	where, err := whereIs(ctx, inv, module, now)
	if err != nil {
		return "", err
	}
	waiting, err := openPending(ctx, inv, node, module)
	if err != nil {
		return "", err
	}
	switch where.kind {
	case keyComing:
		lead := where.said
		if opts.Build {
			lead += "\n  build \"true\" asks nothing: the builder builds every fingerprint the indexer writes (ADR 0501 §6)"
		}
		return keepPending(ctx, open, node, module, where.request, waiting, lead)
	case buildInFlight:
		lead := where.said
		if opts.Build {
			lead += "\n  no second build was asked: this one is running"
		}
		return keepPending(ctx, open, node, module, where.request, waiting, lead)
	case knownNotBuilt:
		last := where.request
		if !opts.Build {
			also := ""
			if waiting != nil {
				also = fmt.Sprintf("\n  %s already has a pending assignment of %s, waiting for build %s; build "+
					"\"true\" asks for a new build and makes it wait for that one", node, module, waiting.Build)
			}
			return "", fmt.Errorf("%w: %s%s\n  it is registered from the builder's done key once a merge adds it to %s "+
				"(path %s) and the indexer holds it, or at the controller's terminal; assign it then",
				inventory.ErrNoSuchModule, where.said, also, last.Repository, orRoot(last.Path))
		}
		// **No build is asked** (ADR 0501 §5, §6): a module the indexer does not hold has no key coming.
		return "", fmt.Errorf("%w: %s\n  build \"true\" asks no build: the controller asks none (ADR 0501 §5); a module "+
			"is registered from the builder's done key once a merge adds it to a repository the catalogue builds from, or "+
			"at the controller's terminal, and the indexer holds no %s", inventory.ErrNoSuchModule, where.said, module)
	}
	answer := strings.TrimPrefix(where.said, inventory.ErrNoSuchModule.Error()) +
		"\n  a module in a repository is registered from the builder's done key of it, then assigned"
	if opts.Build {
		answer += "\n  build \"true\" asks for nothing: the controller asks no build (ADR 0501 §5), and the indexer " +
			"holds no module of that name"
	}
	return "", fmt.Errorf("%w%s", inventory.ErrNoSuchModule, answer)
}

// keepPending records the assignment as pending on a build request — or, where one is already open, makes it
// wait for that build — and says so.
func keepPending(ctx context.Context, open *stores, node, module string, r inventory.RequestOutcome,
	waiting *inventory.PendingAssignment, lead string) (string, error) {
	inv := open.inventory
	if waiting != nil {
		if waiting.Build == r.ID {
			return lead + fmt.Sprintf("\n  %s already waits for %s on this build: nothing changed", node, module), nil
		}
		moved, err := inv.RepointPending(ctx, waiting.ID, r.ID, r.Repository, r.Path)
		if err != nil {
			return "", err
		}
		if !moved {
			return lead + fmt.Sprintf("\n  the pending assignment of %s to %s ended while this was asked: `status` "+
				"says how", module, node), nil
		}
		return lead + fmt.Sprintf("\n  the pending assignment of %s to %s, which waited for build %s, now waits "+
			"for build %s", module, node, waiting.Build, r.ID), nil
	}
	_, err := inv.RecordPending(ctx, inventory.PendingAssignment{Node: node, Module: module, Build: r.ID,
		Repository: r.Repository, Path: r.Path})
	if errors.Is(err, inventory.ErrAlreadyPending) {
		return lead + fmt.Sprintf("\n  %s already waits for %s: nothing changed", node, module), nil
	}
	if err != nil {
		return "", err
	}
	// The controller makes it, not this act (ADR 0261): when the build registers the module, or at its next
	// tick if the module was registered between the look and the record.
	return lead + fmt.Sprintf("\n  the assignment is kept as pending: the controller assigns %s to %s when the "+
		"build registers it, and `status` lists it until then. If the build fails it expires, says why and "+
		"raises a condition; `unassign %s %s` withdraws it", module, node, node, module), nil
}

// inFlight is the newest build request for the module still to register it: no outcome yet within the
// bound, or a successful outcome heard a moment ago and being registered.
func inFlight(requests []inventory.RequestOutcome, now time.Time) (inventory.RequestOutcome, bool) {
	for _, r := range requests {
		if stillComing(r, now) {
			return r, true
		}
	}
	return inventory.RequestOutcome{}, false
}

func stillComing(r inventory.RequestOutcome, now time.Time) bool {
	if r.Heard {
		return r.Failed == "" && now.Sub(r.HeardAt) < registerGrace
	}
	return r.NotAsked == "" && now.Sub(r.At) < buildRequestBound
}

// whyNotBuilt says what came of a module's last build request. What was heard comes first: an outcome is the
// last word whatever its asker said.
func whyNotBuilt(r inventory.RequestOutcome, now time.Time) string {
	where := fmt.Sprintf("%s in %s", orRoot(r.Path), r.Repository)
	switch {
	case r.Heard && r.Failed != "":
		return fmt.Sprintf("%s; its last build, %s at %s, failed: %s", where, r.ID, clock(r.HeardAt),
			firstLine(r.Failed))
	case r.Heard && r.Module != "" && r.Module != r.Name():
		return fmt.Sprintf("%s; its last build, %s, built it as %s", where, r.ID, r.Module)
	case r.Heard:
		return fmt.Sprintf("%s; its last build, %s at %s, was recorded and not registered — `builds` says why",
			where, r.ID, clock(r.HeardAt))
	case r.NotAsked != "" && r.For == "merge":
		return fmt.Sprintf("%s; the merge that added it (%s) could not ask for its build: %s", where,
			short(r.Commit), firstLine(r.NotAsked))
	case r.NotAsked != "":
		return fmt.Sprintf("%s; build %s, asked by %s, was not handed over: %s", where, r.ID,
			askerOr(r.For), firstLine(r.NotAsked))
	case r.OutcomeUnknown != "":
		return fmt.Sprintf("%s; build %s was asked at %s, its asker stopped waiting (%s), and no outcome has "+
			"been heard in %s", where, r.ID, clock(r.At), firstLine(r.OutcomeUnknown), now.Sub(r.At).Round(time.Minute))
	default:
		return fmt.Sprintf("%s; build %s was asked at %s, and no outcome has been heard in %s", where, r.ID,
			clock(r.At), now.Sub(r.At).Round(time.Minute))
	}
}

func askerOr(who string) string {
	if who == "" {
		return "the controller"
	}
	return who
}

// requestSource is a build request's source as a person reads it: repository@commit (directory).
func requestSource(r inventory.RequestOutcome) string {
	at := short(r.Commit)
	if at == "" {
		at = r.Ref
	}
	if at == "" {
		at = "its default branch"
	}
	return fmt.Sprintf("%s@%s (%s)", r.Repository, at, orRoot(r.Path))
}

// clock is a moment as a person reads it, in the controller's local time.
func clock(t time.Time) string { return t.Local().Format("2006-01-02 15:04:05 MST") }

// closestNames is up to three names near the one asked: registered modules and directories the controller
// asked to build.
func closestNames(ctx context.Context, inv notHeldReads, name string) []string {
	var candidates []string
	if shelf, err := inv.Catalogue(ctx); err == nil {
		for m := range shelf {
			candidates = append(candidates, m)
		}
	}
	if asked, err := inv.RequestedNames(ctx); err == nil {
		candidates = append(candidates, asked...)
	}
	type scored struct {
		name     string
		distance int
	}
	seen := map[string]bool{}
	var near []scored
	limit := max(2, len(name)/3)
	for _, c := range candidates {
		if seen[c] || c == name {
			continue
		}
		seen[c] = true
		d := editDistance(name, c)
		if d <= limit || strings.Contains(c, name) || strings.Contains(name, c) {
			near = append(near, scored{c, d})
		}
	}
	sort.Slice(near, func(i, j int) bool {
		if near[i].distance != near[j].distance {
			return near[i].distance < near[j].distance
		}
		return near[i].name < near[j].name
	})
	var out []string
	for i := 0; i < len(near) && i < 3; i++ {
		out = append(out, near[i].name)
	}
	return out
}

// editDistance is the Levenshtein distance between two names.
func editDistance(a, b string) int {
	ra, rb := []rune(a), []rune(b)
	prev := make([]int, len(rb)+1)
	for j := range prev {
		prev[j] = j
	}
	for i := 1; i <= len(ra); i++ {
		cur := make([]int, len(rb)+1)
		cur[0] = i
		for j := 1; j <= len(rb); j++ {
			cost := 1
			if ra[i-1] == rb[j-1] {
				cost = 0
			}
			cur[j] = min(prev[j]+1, cur[j-1]+1, prev[j-1]+cost)
		}
		prev = cur
	}
	return prev[len(rb)]
}

// applyPending makes a pending assignment now that its module is registered (ADR 0261). Under the machine's
// hold it claims the row (waiting → applying), so a withdrawal cannot land between the look and the act,
// then assigns by the same act a person's assign is, then says what came of it: applied, or refused with the
// refusal. Returns what it said, or nothing when the row no longer waited.
func applyPending(ctx context.Context, open *stores, p inventory.PendingAssignment, by string) string {
	inv := open.inventory
	claimed, err := inv.ClaimPending(ctx, p.ID)
	if err != nil {
		return fmt.Sprintf("the pending assignment of %s to %s could not be taken for making: %v", p.Module, p.Node, err)
	}
	if !claimed {
		return ""
	}
	answer, err := assign(ctx, open, p.Node, p.Module)
	state, note := inventory.PendingApplied, ""
	switch {
	case err != nil:
		state = inventory.PendingRefused
		note = fmt.Sprintf("the pending assignment of %s to %s was refused when build %s registered it: %s",
			p.Module, p.Node, by, firstLine(err.Error()))
	case strings.Contains(answer, "already runs"):
		note = fmt.Sprintf("%s already runs %s: the pending assignment is met", p.Node, p.Module)
	default:
		// The same as a person's assign: composed, and its want follows by the assigner's rules.
		note = fmt.Sprintf("%s is assigned %s: build %s registered it (pending since %s); composed, its want follows",
			p.Node, p.Module, by, clock(p.Since))
	}
	settled, serr := inv.SettlePending(ctx, p.ID, inventory.PendingApplying, state, note)
	if serr != nil {
		return fmt.Sprintf("%s — and it could not be recorded as settled: %v", note, serr)
	}
	if !settled {
		return ""
	}
	return note
}

// expirePending ends a waiting pending assignment whose build will not register its module, with why.
func expirePending(ctx context.Context, inv *inventory.Inventory, p inventory.PendingAssignment, why string) string {
	note := fmt.Sprintf("the pending assignment of %s to %s expired: %s; nothing was assigned. Assign it again with "+
		"build \"true\" to ask for the build again", p.Module, p.Node, why)
	settled, err := inv.SettlePending(ctx, p.ID, inventory.PendingWaiting, inventory.PendingExpired, note)
	if err != nil {
		return fmt.Sprintf("%s — and it could not be recorded: %v", note, err)
	}
	if !settled {
		return ""
	}
	return note
}

// settlingPending is the controller's tick over the pending assignments (ADR 0261), every settleEvery until
// the context ends. What it does is printed to the controller's log, kept in each row's note (which `status`
// lists for a day), and raised as a condition for an assignment that was not made.
func settlingPending(ctx context.Context, open *stores) {
	for {
		for _, line := range settlePending(ctx, open, time.Now()) {
			fmt.Println(line)
		}
		for _, line := range raiseUnknownFields(ctx, open.inventory) {
			fmt.Println(line)
		}
		select {
		case <-ctx.Done():
			return
		case <-time.After(settleEvery):
		}
	}
}

// settlePending is one tick: claims left by a controller that stopped are settled from what the mesh holds;
// a module registered meanwhile is assigned; a build heard and not registered, or silent past its bound,
// ends its assignment; ended ones are raised as conditions and cleared once answered. Returns what it did.
func settlePending(ctx context.Context, open *stores, now time.Time) []string {
	inv := open.inventory
	var said []string
	say := func(format string, args ...any) { said = append(said, fmt.Sprintf(format, args...)) }

	shelf, err := inv.Catalogue(ctx)
	if err != nil {
		say("the pending assignments are not settled this time: the catalogue could not be read: %v", err)
		return said
	}
	stuck, err := inv.Stuck(ctx, now.Add(-claimStaleAfter))
	if err != nil {
		say("claims left by a stopped controller could not be read: %v", err)
	}
	for _, p := range stuck {
		assigned, err := inv.Assigned(ctx, p.Node)
		if err != nil {
			say("the claim on %s for %s could not be settled: %v", p.Node, p.Module, err)
			continue
		}
		if containsString(assigned, p.Module) {
			if _, err := inv.SettlePending(ctx, p.ID, inventory.PendingApplying, inventory.PendingApplied,
				fmt.Sprintf("%s is assigned %s (settled after the controller that made it stopped)", p.Node, p.Module)); err != nil {
				say("the claim on %s for %s could not be settled: %v", p.Node, p.Module, err)
			}
			continue
		}
		if err := inv.ReleaseClaim(ctx, p.ID); err != nil {
			say("the claim on %s for %s could not be released: %v", p.Node, p.Module, err)
		}
	}

	waiting, err := inv.Pending(ctx, time.Time{})
	if err != nil {
		say("the pending assignments could not be read to settle them: %v", err)
	}
	for _, p := range waiting {
		if _, ok := shelf[p.Module]; ok {
			if line := applyPending(ctx, open, p, p.Build); line != "" {
				said = append(said, line)
			}
			continue
		}
		if strings.HasPrefix(p.Build, keyPendingPrefix) {
			why, err := keyPendingEnded(ctx, p.Module)
			if err != nil {
				say("the key %s waits for cannot be read: %v", p.Module, err)
				continue
			}
			if why != "" {
				if line := expirePending(ctx, inv, p, why); line != "" {
					said = append(said, line)
				}
			}
			continue
		}
		requests, err := inv.RequestsNamed(ctx, p.Module)
		if err != nil {
			say("the build requests for %s could not be read: %v", p.Module, err)
			continue
		}
		var r *inventory.RequestOutcome
		for i := range requests {
			if requests[i].ID == p.Build {
				r = &requests[i]
			}
		}
		why := ""
		switch {
		case r == nil && now.Sub(p.Since) > buildRequestBound:
			why = fmt.Sprintf("build %s is no longer on record", p.Build)
		case r != nil && !stillComing(*r, now) && (r.Heard || r.NotAsked != "" || r.OutcomeUnknown != ""):
			why = whyNotBuilt(*r, now)
		case r != nil && !stillComing(*r, now):
			why = fmt.Sprintf("no outcome of build %s was heard within %s of asking", p.Build, buildRequestBound)
		}
		if why == "" {
			continue
		}
		if line := expirePending(ctx, inv, p, why); line != "" {
			said = append(said, line)
		}
	}

	said = append(said, raisePendingEnded(ctx, inv)...)
	return said
}

func containsString(xs []string, x string) bool {
	for _, y := range xs {
		if y == x {
			return true
		}
	}
	return false
}

// pendingObservation is the condition for a pending assignment that was not made: one per pending
// assignment, its row's id the last part of its id, so one row's condition is never another's.
func pendingObservation(p inventory.PendingAssignment) conditions.Observation {
	return conditions.Observation{Scope: conditions.ScopeMachine, ID: fmt.Sprintf("%s.%s.%s", p.Node, p.Module, p.ID),
		Token: kindPendingEnded, Kind: kindPendingEnded, Machine: p.Node, Severity: conditions.Warning,
		Resolver: conditions.ResolverOperator, Source: "pending assignments",
		Summary: p.Note}
}

// raisePendingEnded raises a condition for each pending assignment that expired or was refused, once, and
// clears it when it is answered: the module was assigned to that machine since, a newer pending assignment of
// it is open, or a person took it back with `unassign`. Where this process keeps no conditions, nothing is
// stamped, and the serving controller's tick raises it.
func raisePendingEnded(ctx context.Context, inv *inventory.Inventory) []string {
	keeper := conditionsFrom
	if keeper == nil {
		return nil
	}
	var said []string
	toRaise, err := inv.ToRaise(ctx)
	if err != nil {
		return []string{fmt.Sprintf("pending assignments not made could not be read to raise them: %v", err)}
	}
	for _, p := range toRaise {
		if _, err := keeper.Observe(ctx, pendingObservation(p)); err != nil {
			said = append(said, fmt.Sprintf("the condition for %s on %s could not be raised: %v", p.Module, p.Node, err))
			continue
		}
		if err := inv.MarkPending(ctx, p.ID, "raised"); err != nil {
			said = append(said, fmt.Sprintf("the condition for %s on %s was raised and not recorded: %v", p.Module, p.Node, err))
		}
	}
	toClear, err := inv.ToClear(ctx)
	if err != nil {
		return append(said, fmt.Sprintf("pending assignments raised could not be read to clear them: %v", err))
	}
	for _, p := range toClear {
		why, answered, err := pendingAnswered(ctx, inv, p)
		if err != nil {
			said = append(said, fmt.Sprintf("whether %s on %s is answered could not be read: %v", p.Module, p.Node, err))
			continue
		}
		if !answered {
			continue
		}
		if _, err := keeper.Clear(ctx, pendingObservation(p).Key(), why); err != nil {
			said = append(said, fmt.Sprintf("the condition for %s on %s could not be cleared: %v", p.Module, p.Node, err))
			continue
		}
		if err := inv.MarkPending(ctx, p.ID, "cleared"); err != nil {
			said = append(said, fmt.Sprintf("the condition for %s on %s was cleared and not recorded: %v", p.Module, p.Node, err))
		}
	}
	return append(said, clearOrphaned(ctx, keeper, inv)...)
}

// pendingAnswered says whether a pending assignment that was not made has been answered since, and how.
func pendingAnswered(ctx context.Context, inv *inventory.Inventory, p inventory.PendingAssignment) (string, bool, error) {
	if p.Acknowledged != nil {
		return "taken back with unassign", true, nil
	}
	assigned, err := inv.Assigned(ctx, p.Node)
	if err != nil {
		return "", false, err
	}
	if containsString(assigned, p.Module) {
		return p.Module + " is assigned to " + p.Node + " since", true, nil
	}
	rows, err := inv.PendingFor(ctx, p.Node, p.Module)
	if err != nil {
		return "", false, err
	}
	// A newer pending assignment answers it only while it is open or once it was made: one that itself
	// ended unmade is its own condition, and answers nothing.
	for _, r := range rows {
		if r.ID != p.ID && r.Since.After(p.Since) && (r.Open() || r.State == inventory.PendingApplied) {
			return "assigned again, pending on another build", true, nil
		}
	}
	return "", false, nil
}

// clearOrphaned clears every open condition of an assignment not made whose pending assignment is no longer
// on record: its machine was removed, which takes its pending assignments with it.
func clearOrphaned(ctx context.Context, keeper *conditions.Keeper, inv *inventory.Inventory) []string {
	open, err := keeper.Open(ctx)
	if err != nil {
		return []string{fmt.Sprintf("the open conditions could not be read to clear orphaned ones: %v", err)}
	}
	byID := map[string]string{}
	var ids []string
	for _, c := range open {
		if c.Kind != kindPendingEnded {
			continue
		}
		// `<scope>.<node>.<module>.<row>.<kind>`: the row is the part before the kind.
		parts := strings.Split(c.Key, ".")
		if len(parts) < 2 {
			continue
		}
		id := parts[len(parts)-2]
		if !strings.HasPrefix(id, "p-") {
			continue
		}
		byID[id] = c.Key
		ids = append(ids, id)
	}
	if len(ids) == 0 {
		return nil
	}
	known, err := inv.PendingKnown(ctx, ids)
	if err != nil {
		return []string{fmt.Sprintf("the pending assignments could not be read to clear orphaned conditions: %v", err)}
	}
	var said []string
	for id, key := range byID {
		if known[id] {
			continue
		}
		if _, err := keeper.Clear(ctx, key, "its pending assignment is no longer on record: its machine was removed, or the module was assigned and the wait taken back"); err != nil {
			said = append(said, fmt.Sprintf("the condition %s could not be cleared: %v", key, err))
		}
	}
	return said
}

// withdrawPending is `unassign` of a module only pending on a machine, under the machine's hold: a waiting
// pending assignment is withdrawn; one being made now is refused, never overridden; one that expired or was
// refused is taken back, which answers its condition. False when there is none.
func withdrawPending(ctx context.Context, inv *inventory.Inventory, node, module string) (string, bool, error) {
	rows, err := inv.PendingFor(ctx, node, module)
	if err != nil {
		return "", false, err
	}
	var takenBack []string
	for _, p := range rows {
		switch p.State {
		case inventory.PendingApplying:
			return "", true, fmt.Errorf("%s is being assigned %s now, as build %s registered it: nothing was "+
				"withdrawn; `unassign %s %s` again once it is assigned takes it off", node, module, p.Build, node, module)
		case inventory.PendingWaiting:
			note := fmt.Sprintf("the pending assignment of %s to %s is withdrawn; build %s goes on, and registers "+
				"%s assigned nowhere", module, node, p.Build, module)
			settled, err := inv.SettlePending(ctx, p.ID, inventory.PendingWaiting, inventory.PendingWithdrawn, note)
			if err != nil {
				return "", false, err
			}
			if !settled {
				return "", true, fmt.Errorf("the pending assignment of %s to %s changed while it was withdrawn: "+
					"`status` says how; nothing was withdrawn", module, node)
			}
			return note, true, nil
		case inventory.PendingExpired, inventory.PendingRefused:
			if p.Cleared != nil || p.Acknowledged != nil {
				continue
			}
			if err := inv.MarkPending(ctx, p.ID, "acknowledged"); err != nil {
				return "", false, err
			}
			takenBack = append(takenBack, fmt.Sprintf("build %s (%s)", p.Build, p.State))
		}
	}
	if len(takenBack) > 0 {
		return fmt.Sprintf("the pending assignment(s) of %s to %s that were not made are taken back: %s; their "+
			"conditions clear", module, node, strings.Join(takenBack, ", ")), true, nil
	}
	return "", false, nil
}

// keyPendingEnded is why a pending assignment waiting for a module's key ends: the indexer no longer holds the module,
// or the builder's key of its fingerprint failed; empty while it is coming. A variable so a test needs no bus.
var keyPendingEnded = func(ctx context.Context, module string) (string, error) {
	keys, closeKeys, err := openBuilderKeys()
	if err != nil {
		return "", err
	}
	defer closeKeys()
	v, err := keys.ModuleAt(ctx, module, "")
	if errors.Is(err, builderkeys.ErrNoValue) {
		return fmt.Sprintf("the indexer no longer holds %s", module), nil
	}
	if err != nil || v.FP2 == "" {
		return "", err
	}
	k, err := keys.Build(ctx, module, v.FP2)
	if err != nil {
		return "", err
	}
	if k.Failed() {
		return k.Says(), nil
	}
	return "", nil
}

// settlePendingOnKey assigns, now, every pending assignment of a module its done key just registered (ADR 0501 §6):
// what an outcome of a build did for one waiting on that build.
func settlePendingOnKey(ctx context.Context, open *stores, module string) []string {
	waiting, err := open.inventory.Pending(ctx, time.Time{})
	if err != nil {
		return []string{fmt.Sprintf("the pending assignments could not be read after %s was registered: %v", module, err)}
	}
	var said []string
	for _, p := range waiting {
		if p.Module == module {
			if line := applyPending(ctx, open, p, keyPendingPrefix+module); line != "" {
				said = append(said, line)
			}
		}
	}
	return said
}