mesh-delivery/modules/mesh-grants/internal/grants/part.go

package grants

import (
	"bytes"
	"context"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"git.novox.be/novox/mesh-contracts/go/seats/meshbroker"
	"slices"
	"sort"
	"strings"
	"sync"
	"time"

	"git.novox.be/novox/mesh-contracts/go/seats/nodeengine"
	oc "git.novox.be/novox/mesh-contracts/operator-and-conversation/go/seats/operatorchannel"
	"git.novox.be/novox/mesh-delivery/modules/mesh-grants/grantsstate"
	"git.novox.be/novox/mesh-sdk/go/membership"
	"mesh-grants/internal/buscompose"
)

// The part's own states, as its manifest declares them, and the keys of the two that hold one value.
const (
	StateUserList    = "user-list"
	StateMemberships = "memberships"
	StateAssertions  = "assertions"
	// StateGrants is the part's grants by module and fingerprint, the key layout of the controller's (grants.go).
	StateGrants = grantsstate.PartGrantsState
	KeyList     = "list"
	// KeyPass is the last pass's result, and KeyAsked a tool's ask to compose again now, both in StateUserList.
	KeyPass  = "pass"
	KeyAsked = "asked"
)

// States is every state the part keeps.
var States = []string{StateAccounts, StateAssertions, StateGrants, StateMemberships, StateUserList, StateWithheld}

// The events the part says.
const (
	EventUserList   = "user-list-changed"
	EventMembership = "membership-changed"
	// EventGrantWritten is said when the part wrote a grant of its state grants (ADR 0547 §5, ADR 0561 §2).
	EventGrantWritten = "grant-written"
	// EventAsserted is said when the part created a bucket, log or stream a manifest declares (issue 1790).
	EventAsserted = "asserted"
)

// Emits is every event the part says.
var Emits = []string{EventAsserted, EventGrantWritten, EventHashesAccepted, EventMembership, EventUserList}

// Store is the part's own state, by local name: the only thing the part writes.
type Store interface {
	Get(state, key string) ([]byte, uint64, bool, error)
	Put(state, key string, value []byte) (uint64, error)
	// Keys and Delete are for the wanted accounts (accounts.go) and the grants (grants.go): every other state is only put.
	Keys(state string) ([]string, error)
	Delete(state, key string) error
	// Update writes a key only while it is still at revision (0: only while it holds nothing), and refuses one changed
	// meanwhile with an error wrapping stdio.ErrChanged: the kept bindings are written so, never lost to a second writer
	// (issue 1966).
	Update(state, key string, value []byte, revision uint64) (uint64, error)
}

// Emitter says one of the part's events.
type Emitter interface {
	Emit(event string, body any) error
}

// UserListState is the value of `user-list`: the list the grants part composed (users and grants, no hashes),
// what the bus loaded, and how they differ. Inputs is the digest of all it says but At: a value is written again only
// when it changes.
type UserListState struct {
	V      int    `json:"v"`
	Mode   string `json:"mode"`
	Inputs string `json:"inputs"`
	Digest string `json:"digest"`
	Users  int    `json:"users"`
	// LeftOut are the assignments the shadow list leaves out, each with why.
	LeftOut []Gap                       `json:"left_out"`
	Compare Comparison                  `json:"compare"`
	Loaded  *nodeengine.UserListWritten `json:"loaded,omitempty"`
	// Credentials says per user whether the vault holds its hash and how its line would differ (ADR 0547 clause 9).
	Credentials Credentials `json:"credentials"`
	BusNode     string      `json:"bus_node,omitempty"`
	Gaps        []Gap       `json:"gaps"`
	At          time.Time   `json:"at"`
}

func (s UserListState) inputs() string {
	s.At, s.Inputs = time.Time{}, ""
	// When the bus node said it loaded a list is no input: the same list said again, at a later time and the key's later
	// revision, composes nothing new (issue 1126: written again every 3 s, 2026-10-28).
	if s.Loaded != nil {
		l := *s.Loaded
		l.Revision, l.At = 0, time.Time{}
		s.Loaded = &l
	}
	raw, _ := json.Marshal(s)
	sum := sha256.Sum256(raw)
	return "sha256:" + hex.EncodeToString(sum[:])
}

// Result is what one composition did.
type Result struct {
	UserList           UserListState `json:"user_list"`
	Revision           uint64        `json:"revision"`
	Written            bool          `json:"written"`
	Memberships        int           `json:"memberships"`
	MembershipsWritten int           `json:"memberships_written"`
	Published          []string      `json:"published,omitempty"`
	// KeptLoaded are the users a pass composing part of the mesh gave the union of what it composes and what the loaded list
	// and the list in force give them (keeploaded.go, issue 1966), read with grants_user_list's last pass.
	KeptLoaded   []string `json:"kept_loaded,omitempty"`
	NotPublished []string `json:"not_published,omitempty"`
	// Retired is, by `<node>.<module>`, the seat verbs its last published membership served that mesh-sdk's seat
	// catalogue no longer has (Membership.Retired): the pass keeps it, since the state memberships holds the bare
	// membership alone (issue 774).
	Retired           map[string][]string `json:"retired,omitempty"`
	Assertions        int                 `json:"assertions"`
	AssertionsWritten int                 `json:"assertions_written"`
	// Asserted is each module whose declared streams this pass created, `<module> at <digest>: created …`, and
	// NotAsserted each creation refused, `<module> <stream>: <why>` (issue 1790).
	Asserted    []string `json:"asserted,omitempty"`
	NotAsserted []string `json:"not_asserted,omitempty"`
	// Refused is every manifest the indexer holds that this pass could not read, `manifest of <module>: <why>`: its
	// module counts as placed nowhere (issue 1945).
	Refused []string `json:"refused,omitempty"`
	// RootNotFree is each machine the composition needed a root-free word for and did not get one, `<machine>: not
	// free, <why>`: a verified sender is composed as from no root-free machine there (issue 1945).
	RootNotFree []string `json:"root_not_free,omitempty"`
	// Sealed is the list sealed to the bus node as the key `user-list.sealed` names it (ADR 0547 clause 7).
	Sealed *SealedState `json:"sealed,omitempty"`
	// Consumers is what the pass found and created of the durable consumers an account needs (ADR 0561 §6).
	Consumers *ConsumersState `json:"consumers,omitempty"`
	// Accounts is what the pass wrote of the wanted accounts the vault mints from (ADR 0561 §2).
	Accounts *AccountsWritten `json:"accounts,omitempty"`
	// Grants is what the pass wrote of its grants (grants.go).
	Grants *GrantsWritten `json:"grants,omitempty"`
	// Withheld is what the pass wrote of the hearings it withholds (withheld.go).
	Withheld *WithheldWritten `json:"withheld,omitempty"`
	// PrincipalsRefused is each principal whose composed grants the composer refused, `<user>: <why>`: it keeps its grants
	// of the list in force, or stays out where it holds none (refused.go); the condition mesh-grants.principal-refused says it.
	PrincipalsRefused   []string `json:"principals_refused,omitempty"`
	Errors              []string `json:"errors,omitempty"`
	PublishedOrAsserted string   `json:"published_or_asserted"`
}

// NothingTakesEffect is what every composition says it did on the bus beyond its own state and the memberships.
const NothingTakesEffect = "memberships (in its own state memberships, unless one would leave a seat a placed module claims with no holder, or drop a capability its holder still asks for, which stays a shadow difference), each bucket, log or stream a manifest declares that the bus lacks, created once with the manifest's limits (the operator, 2026-10-26 08:54), the user list the bus node follows, the grants and the account consumers (a module's on another module's stream too); a stream or consumer the bus holds only ever gains the subjects and filters a manifest adds"

// Part is the grants part: it reads, calls and writes through these, and nothing else.
type Part struct {
	Read  Reader
	Call  Caller
	Store Store
	Emit  Emitter
	// Create creates a declared stream the bus lacks: the process's connection; nil in the tools, which ask the process.
	Create Creator
	// Logf logs what no state or condition says (an error of acting on an answer): the process's; nil in the tools.
	Logf func(string, ...any)
	// Objects is the part's object bucket of sealed lists: the process's connection; nil in the tools.
	Objects Objects
	// Purging takes keys of its own state away whole, values and all (purge.go): the process's connection; nil in the
	// tools, where a key taken away is deleted.
	Purging Purger
	// Consumers creates an account's durable consumer (consumers.go): the process's connection; nil in the tools, which
	// compare only.
	Consumers ConsumerCreator
	// Bus is where the nodes reach the bus, which each wanted account carries (accounts.go): the process's own credential's;
	// empty in the tools, which write no account.
	Bus BusAddress
	// Say says a condition to the operator: the process's; nil in the tools.
	Say func(oc.Condition) error
	Now func() time.Time
	// SeatList is the seat list reader, which gates a module whose seats the list has not named (inputs.go, SeatGate); nil
	// gates none.
	SeatList SeatGate

	cache Manifests
	mu    sync.Mutex
	last  Result
	made  creations
	// askSeen is the revision of the last ask to compose (KeyAsked) this part acted on.
	askSeen uint64
	// heldSaid is the reason of the held-list condition last said (empty: none raised); heldKnown is whether one was said
	// since the start, so the first pass after a start always says its state (a condition raised before a restart is cleared
	// by it). holdMu guards both.
	holdMu    sync.Mutex
	heldSaid  string
	heldKnown bool
	// differSaid is the account consumers' differences last said as a condition, once differKnown (consumers.go).
	differSaid  string
	differKnown bool
	// behindSaid is the account consumers far behind last said as a condition, once behindKnown; behindSeen is each one
	// found behind at the pass before, so one look at a burst raises nothing (consumers.go).
	behindSaid  string
	behindKnown bool
	behindSeen  map[string]bool
	// fullCleared is whether the old history-full key was cleared since the start (full.go, ADR 0595 §6).
	fullCleared bool
	// filterRefused is, by consumer, the refusal of the last addition of filters, so a standing refusal is said once.
	filterRefused map[string]string
	// refusedSaid is the refused principals last said as a condition, once refusedKnown (refused.go).
	refusedSaid  string
	refusedKnown bool
	// heldModulesSaid is the held modules' kept users last said as a condition, once heldModulesKnown (held.go).
	heldModulesSaid  string
	heldModulesKnown bool
	// missingSaid is the modules without a grant last said as a condition, once missingKnown (grants.go).
	missingSaid  string
	missingKnown bool
	// hashHold are the users whose hash changes held the pass's list, or nil (sealed.go).
	hashHold []string
	// brokers are the modules whose manifest claims mesh-broker at the last pass (bindings.go), guarded by mu.
	brokers map[string]bool
	// Views are the part's first views of the owners' states (firstviews.go), and FirstViewContext the process's life
	// they ask in: the process's; nil in the tools.
	Views            *Views
	FirstViewContext context.Context
	fvSaid           firstViewsSaid
}

// keepBrokers keeps the modules whose manifest claims the seat mesh-broker: the only emitters whose created, recreated
// and dropped the part applies (bindings.go).
func (p *Part) keepBrokers(in Inputs) {
	brokers := map[string]bool{}
	for module, m := range in.Manifests {
		if m.ClaimsSeat(meshbroker.Seat) {
			brokers[module] = true
		}
	}
	p.mu.Lock()
	p.brokers = brokers
	p.mu.Unlock()
}

// Brokers are the modules holding mesh-broker as the last pass read them.
func (p *Part) Brokers() map[string]bool {
	p.mu.Lock()
	defer p.mu.Unlock()
	out := map[string]bool{}
	for m := range p.brokers {
		out[m] = true
	}
	return out
}

// Ask is the value of KeyAsked: a tool's ask to compose now, and, from grants_assert, to create one module's missing
// streams again even where they were created at this manifest before.
type Ask struct {
	By     string    `json:"by"`
	At     time.Time `json:"at"`
	Assert string    `json:"assert,omitempty"`
	// Consumer is, from grants_consumer_match, one account consumer `<stream>.<name>` to bring to the composed
	// configuration by hand (ADR 0561 §6).
	Consumer string `json:"consumer,omitempty"`
	// FirstView is, from grants_first_view_again, one first view to ask once more by hand (its state), or AllFirstViews.
	FirstView string `json:"first-view,omitempty"`
}

// AllFirstViews asks every first view once more.
const AllFirstViews = "all"

// asked is a new ask, read once per ask; the zero Ask when none.
func (p *Part) asked() Ask {
	raw, rev, found, err := p.Store.Get(StateUserList, KeyAsked)
	if err != nil || !found || rev == p.askSeen {
		return Ask{}
	}
	p.askSeen = rev
	var a Ask
	_ = json.Unmarshal(raw, &a)
	return a
}

func (p *Part) now() time.Time {
	if p.Now == nil {
		return time.Now().UTC()
	}
	return p.Now().UTC()
}

// Compose composes the user list, the memberships and the assertions from what the bus holds now, compares them, and
// writes each of its own keys whose value changed, saying each change. It never stops at a refusal: a read refused is
// a gap, a write refused is said in Errors, and the next event composes again.
func (p *Part) Compose() Result {
	// No lock is held across a round trip on the bus: the reads, calls, publishes and writes below take none, and the
	// lock is taken only to keep the result, so a tool answers while a pass waits on the bus.
	res := Result{PublishedOrAsserted: NothingTakesEffect}
	defer func() {
		p.mu.Lock()
		p.last = res
		p.mu.Unlock()
	}()
	// A first view's condition the condition keeper did not take is said again at every pass until it lands.
	p.Views.SayUnsaid()
	in := p.inputs()
	p.keepBrokers(in)
	if err := p.withBound(&in); err != nil {
		res.Errors = append(res.Errors, err.Error())
	}
	for _, g := range in.Refused {
		res.Refused = append(res.Refused, g.Input+": "+g.Why)
	}
	// recs say what runs (memberships, consumers); listRecs what the user list and the grants carry: what runs and what
	// the trunk names, so the list grants a build's subjects before the want moves to it (issue 774).
	recs, _ := Records(in)
	listRecs, left := ListRecords(in)
	holdings := HoldingsOf(in)
	res.RootNotFree = WithRootFree(&listRecs, p.Call)
	recs.RootFree = listRecs.RootFree
	st := UserListState{V: 1, Mode: InForce, LeftOut: left, Loaded: in.Loaded, BusNode: in.BusNode,
		Gaps: append(append([]Gap(nil), in.Gaps...), MembershipGaps...), At: p.now()}
	if st.LeftOut == nil {
		st.LeftOut = []Gap{}
	}
	users, refused, err := ComposeEach(listRecs)
	if err != nil {
		res.Errors = append(res.Errors, "the shadow list could not be composed: "+err.Error())
	}
	// A refused principal keeps its grants of the list in force, said once per change (refused.go).
	// And a module whose manifest is held (refused, or waiting for a judge that knows a field of it) and whose want names no
	// build keeps its users of the list in force (held.go).
	loaded, loadErr := LoadedUsers(p.Call)
	inForce := p.inForceGrants()
	candidates := names(loaded)
	for user := range inForce {
		candidates = append(candidates, user)
	}
	heldUsers := heldModuleUsers(in, candidates)
	for user, why := range runnerUsersKept(listRecs, inForce) {
		heldUsers[user] = why
	}
	allRefused := mergedRefusals(refused, heldUsers)
	kept, _ := p.keptOf(allRefused)
	for _, user := range sortedKeys(refused) {
		res.PrincipalsRefused = append(res.PrincipalsRefused, user+": "+refused[user])
	}
	p.sayPrincipalsRefused(refused, kept)
	p.sayHeldModules(heldUsers, kept)
	users = withKept(users, kept)
	// A pass that composes part of the mesh keeps what the bus loaded and the list in force give each user (keeploaded.go).
	if why := in.keepsLoaded(); why != "" {
		if union := KeepLoaded(users, loaded, inForce); len(union) > 0 {
			merged := make(map[string]string, len(allRefused)+len(union))
			keptAll := make(map[string]User, len(kept)+len(union))
			for u, w := range allRefused {
				merged[u] = w
			}
			for u, k := range kept {
				keptAll[u] = k
			}
			for u, k := range union {
				merged[u], keptAll[u] = "keeps what the loaded list gives it: "+why, k
			}
			users = withKept(slices.DeleteFunc(users, func(u User) bool { _, ok := union[u.User]; return ok }), union)
			allRefused, kept = merged, keptAll
			res.KeptLoaded = sortedKeys(union)
		}
	}
	users = WithHashes(users, in.Hashes)
	st.Digest, st.Users = Digest(users), len(users)
	st.Compare = Compare(users, loaded, Placed(in)).Excusing(ExcusedBy(MultiHeld(listRecs)))
	if loadErr != nil {
		st.Compare.Unread = loadErr.Error()
	}
	st.Credentials = CredentialsOf(users, loaded, in.Hashes)
	st.Inputs = st.inputs()
	res.UserList = st
	if raw, rev, found, err := p.Store.Get(StateUserList, KeyList); err != nil {
		res.Errors = append(res.Errors, fmt.Sprintf("its own %s cannot be read: %v", StateUserList, err))
	} else {
		var was UserListState
		if found && json.Unmarshal(raw, &was) == nil && was.Inputs == st.Inputs {
			res.Revision = rev
		} else if data, err := json.Marshal(st); err == nil {
			if rev, err := p.Store.Put(StateUserList, KeyList, data); err != nil {
				res.Errors = append(res.Errors, fmt.Sprintf("its own %s cannot be written: %v", StateUserList, err))
			} else {
				res.Revision, res.Written = rev, true
				p.say(&res, EventUserList, map[string]any{"digest": st.Digest, "revision": rev, "users": st.Users,
					"differing": len(st.Compare.Differing()), "mode": InForce})
			}
		}
	}
	// The accounts the vault mints from, said in the state only it reads (ADR 0561 §2).
	if p.Store != nil {
		p.writeAccounts(&res, in, listRecs)
	}
	// The hearings withheld of what runs, for the self-check's probe D-heard (withheld.go).
	if p.Store != nil {
		p.writeWithheld(&res, in, recs)
	}
	list, n, leftOut, dropped, err := ComposeList(listRecs, in.Hashes, allRefused, kept)
	// The grants the assigner reads (ADR 0547 §4, §5): one that adds a permission only once the bus loaded a list that
	// carries it (issue 774).
	if p.Store != nil {
		inForce, unknown := p.loadedContents(in, loaded, loadErr)
		p.writeGrants(&res, in, listRecs, users, inForce, unknown)
	}
	// The list itself, sealed to the bus node and named in `user-list.sealed` (ADR 0547 clause 7).
	if err != nil {
		res.Errors = append(res.Errors, "the user list could not be composed whole: "+err.Error())
	} else {
		// The users the bus loaded: what a first list must keep where the part has not yet written one.
		var bus, lost []string
		if st.Compare.Unread == "" {
			bus = names(loaded)
			lost = holdings.LastHolders(holdings.ByUsers(loaded), holdings.ByUsers(users))
		}
		p.writeSealed(&res, list, n, leftOut, dropped, in, bus, lost, users, loaded)
	}
	ms := Memberships(listRecs)
	res.Memberships = len(ms)
	members := make([]membership.Membership, 0, len(ms))
	for _, m := range ms {
		members = append(members, m.Membership)
	}
	seated := holdings.ByMemberships(members...)
	// The memberships are written to the state alone and never said on their subject, which nothing reads any more
	// (novox/hq ADR 0571 clause 10, issue 774; unconditional since the controller's end, issue 1126).
	for _, key := range sortedKeys(ms) {
		// The last membership is the state's own (ADR 0571 clause 7), never the assignments stream's, which goes with the
		// controller's leftovers (novox/hq ADR 0590 §6).
		// The bus module's bindings of the module, as it said them created (bindings.go, ADR 0595 §1, §7).
		if binds, err := p.bindingsOf(ms[key].Module); err != nil {
			res.Errors = append(res.Errors, err.Error())
		} else if binds != nil {
			withBinds := ms[key]
			withBinds.Membership.Bindings = binds
			withBinds.Digest = digestOf(withBinds.Membership)
			ms[key] = withBinds
		}
		was, _, found, err := p.Store.Get(StateMemberships, key)
		if err != nil {
			res.NotPublished = append(res.NotPublished, key+": not published: the membership last published cannot be read from its own "+StateMemberships+": "+err.Error())
			continue
		}
		// A pass that composes part of the mesh only grows a published membership: it publishes the union of the one last
		// published and the one composed (keeploaded.go, issue 1966), so a tool runner or a seat holder goes on serving what it
		// serves and still receives what the pass that ends the hold needs.
		why := in.keepsLoaded()
		if why == "" && in.HadFailed[key] != "" {
			why = "the build it runs is unread (" + in.HadFailed[key] + ")"
		}
		if found && why != "" {
			if bare, err := membership.BareGrantsMembership(was); err == nil {
				if body, err := json.Marshal(ms[key].Membership); err == nil {
					if grown, kept, err := UnionMembership(bare, body); err == nil && len(kept) > 0 {
						var u membership.Membership
						if json.Unmarshal(grown, &u) == nil {
							withUnion := ms[key]
							withUnion.Membership = u
							withUnion.Digest = digestOf(u)
							ms[key] = withUnion
							res.NotPublished = append(res.NotPublished, key+": kept beyond this pass: "+why+", this pass composes it without "+strings.Join(kept, ", "))
						}
					}
				}
			}
		}
		m, now := publish(was, found, ms[key], holdings, seated)
		// Published is written to the state, where every runtime reads it (process.FollowMembership); only one
		// published, so a membership held back never reaches a reader.
		if now {
			body := membership.EncodeGrantsMembership(m.Membership)
			if _, err := p.Store.Put(StateMemberships, key, body); err != nil {
				res.Errors = append(res.Errors, fmt.Sprintf("its own %s %s cannot be written: %v", StateMemberships, key, err))
				m.Published, now = "not published (tried again at the next event): its own "+StateMemberships+" cannot be written", false
			} else {
				res.MembershipsWritten++
			}
		}
		// One unchanged is written bare once where the state still holds the wrapper written before 2026-10-30.
		if m.Published == MembershipUnchanged && p.changed(&res, StateMemberships, key, membership.EncodeGrantsMembership(m.Membership)) {
			res.MembershipsWritten++
		}
		switch {
		case now:
			res.Published = append(res.Published, key)
			p.say(&res, EventMembership, map[string]any{"node": m.Node, "module": m.Module, "digest": m.Digest, "published": true})
		case m.Published != MembershipUnchanged:
			res.NotPublished = append(res.NotPublished, key+": "+m.Published)
		}
		if len(m.Retired) > 0 {
			if res.Retired == nil {
				res.Retired = map[string][]string{}
			}
			res.Retired[key] = m.Retired
		}
	}
	p.clearFull()
	held, heldErr := HeldStreams(p.Call)
	var ask Ask
	if p.Create != nil || p.Consumers != nil {
		ask = p.asked()
	}
	forced := ask.Assert
	for _, module := range sortedKeys(in.Manifests) {
		if !assigned(in, module) {
			continue
		}
		a := Assert(module, in.Digests[module], in.Manifests[module], in.Nodes, held, heldErr)
		// Created only where the bus was read and lacks it, never changed where it holds it (issue 1790).
		if p.Create != nil && heldErr == nil {
			if made, raises := createMissing(p.Create, &p.made, &a, in.Manifests[module], in.Nodes, forced == module); len(made) > 0 {
				res.Asserted = append(res.Asserted, fmt.Sprintf("%s at %s: created %s", module, a.Manifest, strings.Join(made, ", ")))
				if p.Logf != nil {
					p.Logf("%s at %s: asserted on the bus: %s", module, a.Manifest, strings.Join(made, ", "))
				}
				p.say(&res, EventAsserted, map[string]any{"module": module, "manifest": a.Manifest, "created": made, "raised": raises})
			}
			for _, why := range notCreated(a) {
				res.NotAsserted = append(res.NotAsserted, module+" "+why)
			}
		}
		res.Assertions++
		data, _ := json.Marshal(a)
		if p.changed(&res, StateAssertions, module, data) {
			res.AssertionsWritten++
		}
	}
	p.consumers(&res, recs, ask.Consumer)
	p.sayFirstViews(&res)
	if ask.FirstView != "" && p.Views != nil && p.FirstViewContext != nil {
		state := ask.FirstView
		if state == AllFirstViews {
			state = ""
		}
		if asked, err := p.Views.Again(p.FirstViewContext, state, "by hand: "+ask.By); err != nil {
			res.Errors = append(res.Errors, err.Error())
		} else if p.Logf != nil {
			p.Logf("asked the first views %v once more by hand, for %s", asked, ask.By)
		}
	}
	return res
}

// Last is the result of the last pass this part made, kept under the lock alone.
func (p *Part) Last() Result {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.last
}

func assigned(in Inputs, module string) bool {
	// The node-engine runs on every node though no node is assigned it (novox/hq ADR 0563 §2, §3): its manifest is
	// placed everywhere, so its declared state is asserted and created as a placed module's.
	if module == buscompose.EngineModule {
		return true
	}
	for _, ms := range in.Assigned {
		for _, m := range ms {
			if m == module {
				return true
			}
		}
	}
	return false
}

// changed writes key when its value differs from the one held, and says whether it wrote.
func (p *Part) changed(res *Result, state, key string, data []byte) bool {
	raw, _, found, err := p.Store.Get(state, key)
	if err != nil {
		res.Errors = append(res.Errors, fmt.Sprintf("its own %s %s cannot be read: %v", state, key, err))
		return false
	}
	if found && bytes.Equal(raw, data) {
		return false
	}
	if _, err := p.Store.Put(state, key, data); err != nil {
		res.Errors = append(res.Errors, fmt.Sprintf("its own %s %s cannot be written: %v", state, key, err))
		return false
	}
	return true
}

func (p *Part) say(res *Result, event string, body any) {
	if p.Emit == nil {
		return
	}
	if err := p.Emit.Emit(event, body); err != nil {
		res.Errors = append(res.Errors, fmt.Sprintf("the event %s could not be said: %v", event, err))
	}
}

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

func names(us []User) []string {
	out := make([]string, 0, len(us))
	for _, u := range us {
		out = append(out, u.User)
	}
	return out
}

// inputs reads what the part composes from, a module whose seats the seat list has not named held back (SeatGate).
func (p *Part) inputs() Inputs {
	p.cache.mu.Lock()
	p.cache.gate = p.SeatList
	p.cache.mu.Unlock()
	return ReadInputs(p.Read, &p.cache)
}