Write each module once per merge and watch a build module's toolchains bucket on its toolchain.reported event, with no timer (issue 1444, review round 1)

modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer/integration_test.go

@@ -137,6 +137,15 @@ func startIndexer(t *testing.T, put func(context.Context, jetstream.KeyValue), b
 	if _, err := js.CreateOrUpdateConsumer(ctx, "FORGE", jetstream.ConsumerConfig{Durable: "n1_mesh-delivery-indexer", AckPolicy: jetstream.AckExplicitPolicy}); err != nil {
 		t.Fatal(err)
 	}
+	// The build modules' streams for toolchain.reported (declared by them, heard by the indexer) and its durable consumers.
+	for module, stream := range map[string]string{"mesh-build-go": "TOOLCHAINS_GO", "mesh-build-node": "TOOLCHAINS_NODE"} {
+		if _, err := js.CreateStream(ctx, jetstream.StreamConfig{Name: stream, Subjects: []string{"mesh.stream." + module + ".event.toolchain.reported"}}); err != nil {
+			t.Fatal(err)
+		}
+		if _, err := js.CreateOrUpdateConsumer(ctx, stream, jetstream.ConsumerConfig{Durable: "n1_mesh-delivery-indexer", AckPolicy: jetstream.AckExplicitPolicy}); err != nil {
+			t.Fatal(err)
+		}
+	}
 	candidates, err := js.CreateKeyValue(ctx, jetstream.KeyValueConfig{Bucket: delivery.CandidatesBucket, History: delivery.CandidatesHistory})
 	if err != nil {
 		t.Fatal(err)
@@ -271,7 +280,7 @@ func TestIndexerKeepsManifestsAtAMerge(t *testing.T) { // 1444/AC4, 1449/AC4
 
 // TestIndexerWritesAGoModuleAgainWhenTheBuildModuleReportsItsToolchain: the bucket mesh-build-go_toolchains exists (the state
 // toolchains of the build module, delivery.ToolchainsBucket) and holds no key; a merged message of mesh-catalog carrying
-// avahi's real manifest makes modules.avahi with none2 'toolchain not yet reported'; a put of the key n1.go (the build
+// avahi's real manifest makes modules.avahi with none2 'toolchain not yet reported'; the bucket made after the indexer started, a put of the key n1.go and the build module's event toolchain.reported (the build
 // module's own layout, delivery.ToolchainKey and EncodeToolchain) makes, within 5 s and with no merge, none2 "builder's
 // fingerprint not yet known".
 func TestIndexerWritesAGoModuleAgainWhenTheBuildModuleReportsItsToolchain(t *testing.T) { // 1444/AC10
@@ -289,10 +298,6 @@ func TestIndexerWritesAGoModuleAgainWhenTheBuildModuleReportsItsToolchain(t *tes
 		if _, err := builds.Put(ctx, delivery.NewestKey("avahi"), []byte("build-1")); err != nil {
 			t.Fatal(err)
 		}
-	}, func(ctx context.Context, js jetstream.JetStream) {
-		if _, err := js.CreateKeyValue(ctx, jetstream.KeyValueConfig{Bucket: delivery.ToolchainsBucket("mesh-build-go"), History: 1}); err != nil {
-			t.Fatal(err)
-		}
 	})
 	data, err := delivery.EncodeForge(delivery.Forge{Kind: delivery.ForgeMerged, Repository: "novox/mesh-catalog", Branch: "feat/x", Number: 301,
 		Head: "0123456789abcdef0123456789abcdef01234567", Merge: "89abcdef0123456789abcdef0123456789abcdef", Root: "r0",
@@ -321,6 +326,12 @@ func TestIndexerWritesAGoModuleAgainWhenTheBuildModuleReportsItsToolchain(t *tes
 	if got := none2(delivery.ToolchainNoReport); got != delivery.ToolchainNoReport {
 		t.Fatalf("none2 = %q within 5 s, want %q", got, delivery.ToolchainNoReport)
 	}
+	// The bucket is made only now, as when the build module is assigned after the indexer started (review finding 9): the
+	// indexer finds it on its own.
+	kv, err := js.CreateKeyValue(ctx, jetstream.KeyValueConfig{Bucket: delivery.ToolchainsBucket("mesh-build-go"), History: 1})
+	if err != nil {
+		t.Fatal(err)
+	}
 	key, err := delivery.ToolchainKey("n1", "go")
 	if err != nil {
 		t.Fatal(err)
@@ -329,11 +340,11 @@ func TestIndexerWritesAGoModuleAgainWhenTheBuildModuleReportsItsToolchain(t *tes
 	if err != nil {
 		t.Fatal(err)
 	}
-	kv, err := js.KeyValue(ctx, delivery.ToolchainsBucket("mesh-build-go"))
+	if _, err := kv.Put(ctx, key, value); err != nil {
-	if err != nil {
 		t.Fatal(err)
 	}
-	if _, err := kv.Put(ctx, key, value); err != nil {
+	// The build module says it wrote the key (its event on its stream): the indexer then watches the bucket it did not know.
+	if _, err := js.Publish(ctx, "mesh.stream.mesh-build-go.event.toolchain.reported", []byte(`{"node":"n1","language":"go"}`)); err != nil {
 		t.Fatal(err)
 	}
 	if got := none2("builder's fingerprint not yet known"); got != "builder's fingerprint not yet known" {

modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer/main.go

@@ -15,10 +15,8 @@ import (
 	"fmt"
 	"io"
 	"log"
-	"maps"
 	"os"
 	"os/signal"
-	"slices"
 	"strings"
 	"syscall"
 
@@ -298,10 +296,27 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 		}
 		go func() { sourceErrs <- follow(ctx, sw, each) }()
 	}
-	toolErrs, err := watchToolchains(ctx, js, ix, logger)
+	tools := newToolchainWatch(ctx, js, toolchainSink{put: ix.PutToolchain, apply: ix.ApplyToolchain, recompute: ix.Recompute}, logger)
-	if err != nil {
+	if err := tools.start(); err != nil {
 		return err
 	}
+	// A build module says each time it wrote its toolchains key: its stream, and the indexer's durable consumer on it, are
+	// made by the controller as the forge's are. Where the stream is not there yet the indexer says so and is started again
+	// with the build module, as with the gate's source.
+	for _, module := range sortedModules() {
+		stream := index.ToolchainStreams[module]
+		tcons, err := js.Consumer(ctx, stream, cred.Node+"_"+cred.Module)
+		if err != nil {
+			logger.Printf("the consumer %s_%s on %s does not exist yet, so %s's reports are not heard until the indexer starts again: %v", cred.Node, cred.Module, stream, module, err)
+			continue
+		}
+		tc, err := tcons.Consume(func(m jetstream.Msg) { tools.Reported(m) },
+			jetstream.ConsumeErrHandler(func(_ jetstream.ConsumeContext, err error) { logger.Printf("hearing the bus: %v", err) }))
+		if err != nil {
+			return err
+		}
+		defer tc.Stop()
+	}
 	cons, err := js.Consumer(ctx, forgeStream, cred.Node+"_"+cred.Module)
 	if err != nil {
 		return fmt.Errorf("the consumer %s_%s on %s: %w", cred.Node, cred.Module, forgeStream, err)
@@ -319,48 +334,7 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 		return err
 	case err := <-sourceErrs:
 		return err
-	case err := <-toolErrs:
+	case err := <-tools.errs:
 		return err
 	}
 }
-
-// watchToolchains reads the toolchains each build module reports (mesh-build-go_toolchains, mesh-build-node_toolchains: the
-// module's state toolchains, delivery.ToolchainsBucket; novox/hq ADR 0465 clause 2, issue 1444) and follows them: every
-// change of a language's agreed toolchain writes the modules of that language again (index.Indexer.Recompute). A bucket
-// that does not exist yet (its build module is not delivered) is said, and not waited for (that needs a timer, ADR 0437
-// §8): the indexer is started again with the module, as with the gate's source. The returned channel gets the error of a
-// watch that ended.
-func watchToolchains(ctx context.Context, js jetstream.JetStream, ix *index.Indexer, logger *log.Logger) (<-chan error, error) {
-	errs := make(chan error, len(index.ToolchainModules))
-	watched := 0
-	for _, module := range slices.Sorted(maps.Values(index.ToolchainModules)) {
-		name := delivery.ToolchainsBucket(module)
-		kv, err := js.KeyValue(ctx, name)
-		if errors.Is(err, jetstream.ErrBucketNotFound) {
-			logger.Printf("the bucket %s does not exist yet: no toolchain of %s is read until the indexer starts again", name, module)
-			continue
-		}
-		if err != nil {
-			return nil, fmt.Errorf("the bucket %s: %w", name, err)
-		}
-		w, err := settle(ctx, kv, func(e jetstream.KeyValueEntry) { ix.PutToolchain(module, e.Key(), e.Value(), !opOf(e)) })
-		if err != nil {
-			return nil, err
-		}
-		watched++
-		go func() {
-			errs <- follow(ctx, w, func(e jetstream.KeyValueEntry) {
-				if err := ix.ApplyToolchain(ctx, module, e.Key(), e.Value(), !opOf(e)); err != nil {
-					logger.Printf("%s.%s could not be taken into the modules: %v", name, e.Key(), err)
-				}
-			})
-		}()
-	}
-	// What the buckets held when the indexer started moves the modules once, not once per key.
-	if watched > 0 {
-		if err := ix.Recompute(ctx); err != nil {
-			logger.Printf("the modules could not be written again from the toolchains: %v", err)
-		}
-	}
-	return errs, nil
-}

modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer/toolchains.go

@@ -0,0 +1,115 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log"
+	"maps"
+	"slices"
+	"strings"
+	"sync"
+
+	"github.com/nats-io/nats.go/jetstream"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+
+	"mesh-delivery-indexer/internal/index"
+)
+
+// toolchainSink is where the toolchains the build modules report go: the indexer's own methods (index.Indexer.PutToolchain,
+// ApplyToolchain, Recompute).
+type toolchainSink struct {
+	put       func(module, key string, value []byte, deleted bool) bool
+	apply     func(ctx context.Context, module, key string, value []byte, deleted bool) error
+	recompute func(ctx context.Context) error
+}
+
+// bucketOpener is the part of the bus the toolchain watch uses.
+type bucketOpener interface {
+	KeyValue(ctx context.Context, bucket string) (jetstream.KeyValue, error)
+}
+
+// toolchainWatch reads the toolchains each build module reports (mesh-build-go_toolchains, mesh-build-node_toolchains: the
+// module's state toolchains, delivery.ToolchainsBucket; novox/hq ADR 0465 clause 2, issue 1444) and follows them: every
+// change of a language's agreed toolchain writes the modules of that language again (index.Indexer.Recompute). A bucket
+// is made when its build module is assigned, after the indexer started, and the indexer holds no timer (ADR 0437 §8):
+// it watches the buckets that exist at start, and a build module says each time it wrote its toolchains key
+// (toolchain.reported, handled by Reported); a bucket not watched yet is then watched, settled as the others.
+type toolchainWatch struct {
+	ctx     context.Context
+	js      bucketOpener
+	sink    toolchainSink
+	logger  *log.Logger
+	errs    chan error // the error of a watch that ended
+	mu      sync.Mutex
+	watched map[string]bool
+}
+
+func newToolchainWatch(ctx context.Context, js bucketOpener, sink toolchainSink, logger *log.Logger) *toolchainWatch {
+	return &toolchainWatch{ctx: ctx, js: js, sink: sink, logger: logger, errs: make(chan error, len(index.ToolchainModules)), watched: map[string]bool{}}
+}
+
+// start watches the buckets that exist now; a missing one is said and waits for its build module's event.
+func (w *toolchainWatch) start() error {
+	for _, module := range sortedModules() {
+		if err := w.ensure(module); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// ensure watches the module's toolchains bucket if it is not watched and exists (settling what it holds), then writes the
+// modules again from what the watches hold.
+func (w *toolchainWatch) ensure(module string) error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	name := delivery.ToolchainsBucket(module)
+	if !w.watched[module] {
+		kv, err := w.js.KeyValue(w.ctx, name)
+		switch {
+		case errors.Is(err, jetstream.ErrBucketNotFound):
+			w.logger.Printf("the bucket %s does not exist yet: it is watched when %s reports a toolchain", name, module)
+			return nil
+		case err != nil:
+			return fmt.Errorf("the bucket %s: %w", name, err)
+		}
+		watcher, err := settle(w.ctx, kv, func(e jetstream.KeyValueEntry) { w.sink.put(module, e.Key(), e.Value(), !opOf(e)) })
+		if err != nil {
+			return err
+		}
+		w.watched[module] = true
+		go func() {
+			w.errs <- follow(w.ctx, watcher, func(e jetstream.KeyValueEntry) {
+				if err := w.sink.apply(w.ctx, module, e.Key(), e.Value(), !opOf(e)); err != nil {
+					w.logger.Printf("%s.%s could not be taken into the modules: %v", name, e.Key(), err)
+				}
+			})
+		}()
+	}
+	return w.sink.recompute(w.ctx)
+}
+
+// Reported takes one toolchain.reported event of a build module (the message on its stream) in: the module is read off the
+// subject, mesh.stream.<module>.event.toolchain.reported, its bucket is watched if it is not, and the modules are written
+// again. A subject of no build module is terminated; a failure stays unacknowledged, and the server delivers it again.
+func (w *toolchainWatch) Reported(m delivered) {
+	module, _, _ := strings.Cut(strings.TrimPrefix(m.Subject(), "mesh.stream."), ".event.")
+	known := false
+	for _, k := range index.ToolchainModules {
+		known = known || k == module
+	}
+	if !known || !strings.HasPrefix(m.Subject(), "mesh.stream.") {
+		w.logger.Printf("%s: not a build module's event, dropped", m.Subject())
+		_ = m.Term()
+		return
+	}
+	if err := w.ensure(module); err != nil {
+		w.logger.Printf("%s: left for redelivery: %v", m.Subject(), err)
+		return
+	}
+	_ = m.Ack()
+}
+
+func sortedModules() []string { return slices.Sorted(maps.Values(index.ToolchainModules)) }

modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer/toolchains_test.go

@@ -0,0 +1,141 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"log"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/nats-io/nats.go/jetstream"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+)
+
+// A build module's bucket is made when the module is assigned, after the indexer started (issue 1444, review finding 9).
+// The fakes implement only what the watch calls: the bus's KeyValue by bucket name, and a bucket's WatchAll.
+
+type laterBuckets struct {
+	mu      sync.Mutex
+	appears map[string]jetstream.KeyValue // buckets that exist now
+	asked   map[string]int
+}
+
+func (l *laterBuckets) KeyValue(_ context.Context, name string) (jetstream.KeyValue, error) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	l.asked[name]++
+	if kv, ok := l.appears[name]; ok {
+		return kv, nil
+	}
+	return nil, jetstream.ErrBucketNotFound
+}
+
+func (l *laterBuckets) make(name string, kv jetstream.KeyValue) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	l.appears[name] = kv
+}
+
+type fakeBucket struct {
+	jetstream.KeyValue
+	held []jetstream.KeyValueEntry
+	w    *fakeWatcher
+}
+
+func (b *fakeBucket) WatchAll(context.Context, ...jetstream.WatchOpt) (jetstream.KeyWatcher, error) {
+	ch := make(chan jetstream.KeyValueEntry, 8)
+	for _, e := range b.held {
+		ch <- e
+	}
+	ch <- nil // the mark of the last value held
+	b.w = &fakeWatcher{ch: ch}
+	return b.w, nil
+}
+
+type fakeWatcher struct {
+	jetstream.KeyWatcher
+	ch chan jetstream.KeyValueEntry
+}
+
+func (w *fakeWatcher) Updates() <-chan jetstream.KeyValueEntry { return w.ch }
+func (w *fakeWatcher) Stop() error                             { return nil }
+
+type fakeEntry struct {
+	jetstream.KeyValueEntry
+	key   string
+	value []byte
+}
+
+func (e fakeEntry) Key() string                     { return e.key }
+func (e fakeEntry) Value() []byte                   { return e.value }
+func (e fakeEntry) Operation() jetstream.KeyValueOp { return jetstream.KeyValuePut }
+
+func TestABuildModulesBucketThatAppearsLaterIsWatchedWhenItsEventArrives(t *testing.T) { // 1444 (review finding 9)
+	ctx, stop := context.WithCancel(context.Background())
+	defer stop()
+	value, err := delivery.EncodeToolchain(delivery.Toolchain{Language: "go", Version: "go1.27.1", Recipe: "1", At: time.Now()})
+	if err != nil {
+		t.Fatal(err)
+	}
+	buckets := &laterBuckets{appears: map[string]jetstream.KeyValue{}, asked: map[string]int{}}
+	var mu sync.Mutex
+	var puts, applies []string
+	recomputes := 0
+	sink := toolchainSink{
+		put: func(module, key string, _ []byte, _ bool) bool {
+			mu.Lock()
+			puts = append(puts, module+" "+key)
+			mu.Unlock()
+			return true
+		},
+		apply: func(_ context.Context, module, key string, _ []byte, _ bool) error {
+			mu.Lock()
+			applies = append(applies, module+" "+key)
+			mu.Unlock()
+			return nil
+		},
+		recompute: func(context.Context) error { mu.Lock(); recomputes++; mu.Unlock(); return nil },
+	}
+	var logs bytes.Buffer
+	w := newToolchainWatch(ctx, buckets, sink, log.New(&logs, "", 0))
+	if err := w.start(); err != nil { // neither bucket exists: nothing is watched, nothing is written, nobody waits
+		t.Fatal(err)
+	}
+	if len(puts) != 0 || recomputes != 0 || !strings.Contains(logs.String(), "does not exist yet") {
+		t.Fatalf("puts %v recomputes %d log %q after a start with no bucket", puts, recomputes, logs.String())
+	}
+	// The bucket of mesh-build-go is made and already holds a key, and the module says it reported.
+	bucket := &fakeBucket{held: []jetstream.KeyValueEntry{fakeEntry{key: "novox.go", value: value}}}
+	buckets.make(delivery.ToolchainsBucket("mesh-build-go"), bucket)
+	msg := &fakeMsg{subject: "mesh.stream.mesh-build-go.event.toolchain.reported"}
+	w.Reported(msg)
+	if msg.acked != 1 || len(puts) != 1 || puts[0] != "mesh-build-go novox.go" || recomputes != 1 {
+		t.Fatalf("acked %d puts %v recomputes %d, want the settled key, one recompute and an ack", msg.acked, puts, recomputes)
+	}
+	// It is followed from now on: a key put later is applied, and a second report only recomputes.
+	bucket.w.ch <- fakeEntry{key: "ace.go", value: value}
+	for deadline := time.Now().Add(3 * time.Second); time.Now().Before(deadline); time.Sleep(5 * time.Millisecond) {
+		mu.Lock()
+		n := len(applies)
+		mu.Unlock()
+		if n == 1 {
+			break
+		}
+	}
+	if len(applies) != 1 || applies[0] != "mesh-build-go ace.go" {
+		t.Errorf("applies = %v, want the key put later", applies)
+	}
+	w.Reported(&fakeMsg{subject: "mesh.stream.mesh-build-go.event.toolchain.reported"})
+	if len(puts) != 1 || recomputes != 2 || buckets.asked[delivery.ToolchainsBucket("mesh-build-go")] != 2 {
+		t.Errorf("puts %v recomputes %d asked %v, want the bucket opened again only at the first report", puts, recomputes, buckets.asked)
+	}
+	// An event of no build module is terminated.
+	other := &fakeMsg{subject: "mesh.stream.avahi.event.toolchain.reported"}
+	w.Reported(other)
+	if other.termd != 1 || other.acked != 0 {
+		t.Errorf("terminated %d acked %d for a module that is no build module", other.termd, other.acked)
+	}
+}

modules/mesh-delivery-indexer/internal/index/modules.go

@@ -2,8 +2,6 @@ package index
 
 import (
 	"context"
-	"fmt"
-	"reflect"
 	"sort"
 	"strings"
 
@@ -57,6 +55,7 @@ func (ix *Indexer) applyMerged(ctx context.Context, f delivery.Forge) error {
 	if err != nil {
 		return err
 	}
+	overlay := map[string]delivery.Module{}
 	for _, k := range ix.Builds.Modules(f.Repository) {
 		key, err := delivery.ModulesKey(k.Module)
 		if err != nil {
@@ -77,21 +76,10 @@ func (ix *Indexer) applyMerged(ctx context.Context, f delivery.Forge) error {
 			in := inputsOf(k.Module, raw, f.Trees[k.Path], f.Locks[k.Path], ix.baseFP(ctx))
 			next.Inputs2, next.None2 = in.Kept, in.None2
 		}
-		err = write(ctx, ix.Modules, key, func(value []byte, found bool) ([]byte, bool, error) {
+		overlay[key] = next
-			if found {
-				if cur, err := delivery.DecodeModule(value); err == nil && reflect.DeepEqual(cur, withV(next)) {
-					return nil, false, nil
-				}
-			}
-			data, err := delivery.EncodeModule(next)
-			return data, false, err
-		})
-		if err != nil {
-			return fmt.Errorf("modules.%s: %w", key, err)
-		}
 	}
 	// What this merge moved reaches the modules standing on it (a base's tree, build-agent's fingerprint), in the graph's order.
-	return ix.Recompute(ctx)
+	return ix.recomputeWith(ctx, overlay)
 }
 
 // baseFP answers the new-form fingerprint of a base module as modules.<base> holds it, or why it has none.

modules/mesh-delivery-indexer/internal/index/toolchains.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"maps"
+	"reflect"
 	"slices"
 	"strings"
 
@@ -30,6 +31,11 @@ const (
 // indexer reads: the language's agreed toolchain is the one every node's entry in it agrees on.
 var ToolchainModules = map[string]string{"go": "mesh-build-go", "typescript": "mesh-build-node"}
 
+// ToolchainStreams are the stream each build module carries its toolchain.reported event on, by module, as the bus names it
+// (a stream's local name in capitals, catalogue.StreamBusName): `toolchains-go` and `toolchains-node`, declared by the
+// build modules (streams, with the event toolchain.reported) and heard by this module (consumes).
+var ToolchainStreams = map[string]string{"mesh-build-go": "TOOLCHAINS_GO", "mesh-build-node": "TOOLCHAINS_NODE"}
+
 // PutToolchain takes one change of a build module's toolchains bucket in, without writing any module: a put of a node's
 // toolchain of a language (key <node>.<language>, delivery.ToolchainKey), or its delete. It answers whether what the
 // language's toolchain is of (version and recipe) changed. A key, or a value, that is not a toolchain of the module's
@@ -120,31 +126,46 @@ type known struct {
 // the order the bucket lists them in; modules that stand on each other have a cycle as their reason. A module is written
 // only when its fingerprint or its reason changes, and only if its manifest is still the one computed from. A module with
 // no kept inputs (a message that carried no manifest) is left as it is.
-func (ix *Indexer) Recompute(ctx context.Context) error {
+func (ix *Indexer) Recompute(ctx context.Context) error { return ix.recomputeWith(ctx, nil) }
-	if ix.Modules == nil || ix.Manifests == nil {
+
+// recomputeWith is Recompute for a merge as well: the records the merge gives (overlay, by module key) stand in place
+// of what the bucket holds, are computed with the others, and are written once, with their final fingerprint or reason
+// (issue 1444: a merge writes each module once). A record of the overlay that has no kept inputs is written as it is.
+func (ix *Indexer) recomputeWith(ctx context.Context, overlay map[string]delivery.Module) error {
+	if ix.Modules == nil {
 		return nil
 	}
+	if ix.Manifests == nil {
+		return ix.writeOverlay(ctx, overlay, nil)
+	}
 	ix.recompute.Lock()
 	defer ix.recompute.Unlock()
 	keys, err := ix.Modules.Keys(ctx)
 	if err != nil {
 		return fmt.Errorf("listing the modules: %w", err)
 	}
+	for key := range overlay {
+		if !slices.Contains(keys, key) {
+			keys = append(keys, key)
+		}
+	}
 	slices.Sort(keys)
 	held := map[string]delivery.Module{}
 	all := map[string]*known{}
 	for _, key := range keys {
-		value, _, found, err := ix.Modules.Get(ctx, key)
+		cur, ok := overlay[key]
-		if err != nil {
+		if !ok {
-			return fmt.Errorf("modules.%s: %w", key, err)
+			value, _, found, err := ix.Modules.Get(ctx, key)
-		}
+			if err != nil {
-		if !found {
+				return fmt.Errorf("modules.%s: %w", key, err)
-			continue
+			}
-		}
+			if !found {
-		cur, err := delivery.DecodeModule(value)
+				continue
-		if err != nil {
+			}
-			ix.logf("modules.%s holds what is not a module, left: %v", key, err)
+			if cur, err = delivery.DecodeModule(value); err != nil {
-			continue
+				ix.logf("modules.%s holds what is not a module, left: %v", key, err)
+				continue
+			}
 		}
 		held[key] = cur
 		if cur.Manifest == "" || cur.Inputs2 == nil {
@@ -242,9 +263,25 @@ func (ix *Indexer) Recompute(ctx context.Context) error {
 		k.cur.Inputs2 = &kept
 		return r
 	}
+	finals := map[string]delivery.Module{}
 	for _, key := range slices.Sorted(maps.Keys(all)) {
 		k := all[key]
 		r := of(key, nil)
+		if _, merged := overlay[key]; merged {
+			final := overlay[key]
+			final.FP2, final.None2, final.Inputs2 = r.fp, r.none, k.cur.Inputs2
+			// What the bases and the builder gave is kept as it was while the value it gave does not change, so a merge
+			// that moves nothing for the module writes nothing (as Recompute does).
+			if value, _, found, err := ix.Modules.Get(ctx, key); err == nil && found {
+				if old, err := delivery.DecodeModule(value); err == nil && old.Inputs2 != nil && old.FP2 == r.fp && old.None2 == r.none {
+					in := *final.Inputs2
+					in.Bases, in.Builder = old.Inputs2.Bases, old.Inputs2.Builder
+					final.Inputs2 = &in
+				}
+			}
+			finals[key] = final
+			continue
+		}
 		if k.cur.FP2 == r.fp && k.cur.None2 == r.none {
 			continue
 		}
@@ -269,6 +306,30 @@ func (ix *Indexer) Recompute(ctx context.Context) error {
 			return fmt.Errorf("modules.%s: %w", key, err)
 		}
 	}
+	return ix.writeOverlay(ctx, overlay, finals)
+}
+
+// writeOverlay writes the records of a merge, each once and only if it differs from what the bucket holds; finals has
+// the ones that were computed, the others are written as the merge gave them.
+func (ix *Indexer) writeOverlay(ctx context.Context, overlay, finals map[string]delivery.Module) error {
+	for _, key := range slices.Sorted(maps.Keys(overlay)) {
+		next := overlay[key]
+		if f, ok := finals[key]; ok {
+			next = f
+		}
+		err := write(ctx, ix.Modules, key, func(value []byte, found bool) ([]byte, bool, error) {
+			if found {
+				if cur, err := delivery.DecodeModule(value); err == nil && reflect.DeepEqual(cur, withV(next)) {
+					return nil, false, nil
+				}
+			}
+			data, err := delivery.EncodeModule(next)
+			return data, false, err
+		})
+		if err != nil {
+			return fmt.Errorf("modules.%s: %w", key, err)
+		}
+	}
 	return nil
 }
 

modules/mesh-delivery-indexer/internal/index/toolchains_test.go

@@ -42,6 +42,20 @@ type mod struct {
 // merged applies one merge of the catalogue carrying these modules (every one a Go bundle's go.sum, a TypeScript bundle's
 // package-lock.json) and returns the indexer.
 func newFormIndexer(t *testing.T, mods ...mod) (*Indexer, *memStore) {
+	t.Helper()
+	ix, store, f := newFormIndexerUnmerged(t, mods...)
+	if err := ix.Apply(context.Background(), f); err != nil {
+		t.Fatal(err)
+	}
+	// build-agent's value of today's form, from a merge of the controller (modules.build-agent holds its fingerprint).
+	if err := ix.Apply(context.Background(), mergedMsg(buildAgentRepo, map[string]string{"modules/build-agent": "agent-tree-1"})); err != nil {
+		t.Fatal(err)
+	}
+	return ix, store
+}
+
+// newFormIndexerUnmerged is the indexer and the catalogue merge of these modules, not yet applied.
+func newFormIndexerUnmerged(t *testing.T, mods ...mod) (*Indexer, *memStore, delivery.Forge) {
 	t.Helper()
 	var recs [][2]any
 	trees, manifests := map[string]string{}, map[string]string{}
@@ -54,15 +68,7 @@ func newFormIndexer(t *testing.T, mods ...mod) (*Indexer, *memStore) {
 	}
 	recs = append(recs, rec(t, "build-agent-1", "build-agent", buildAgentRepo, "modules/build-agent", "main", "", "2026-10-19T09:00:00Z", aInputs))
 	ix, store, _ := manifestsIndexer(t, recs...)
-	f := withManifests(mergedMsg(catalogueRepo, trees), manifests, locks)
+	return ix, store, withManifests(mergedMsg(catalogueRepo, trees), manifests, locks)
-	if err := ix.Apply(context.Background(), f); err != nil {
-		t.Fatal(err)
-	}
-	// build-agent's value of today's form, from a merge of the controller (modules.build-agent holds its fingerprint).
-	if err := ix.Apply(context.Background(), mergedMsg(buildAgentRepo, map[string]string{"modules/build-agent": "agent-tree-1"})); err != nil {
-		t.Fatal(err)
-	}
-	return ix, store
 }
 
 // toolchainValue is a toolchains value as the build module's writer encodes it (mesh-sdk go/delivery EncodeToolchain).
@@ -194,3 +200,41 @@ func TestModulesThatStandOnEachOtherAreNamedAsACycle(t *testing.T) { // 1444 (th
 		}
 	}
 }
+
+func TestAMergeWritesEachModuleOnceWithItsFinalValueAndAReplayWritesNothing(t *testing.T) { // 1444 (review finding 8)
+	ctx := context.Background()
+	ix, mods, f := newFormIndexerUnmerged(t, mod{"avahi", catalogueManifest(t, "avahi")})
+	reportToolchain(t, ix, "mesh-build-go", "novox", "go", "go1.27.1") // agreed before the merge is heard
+	if err := ix.Apply(ctx, f); err != nil {
+		t.Fatal(err)
+	}
+	m, _ := moduleOf(t, mods, "avahi")
+	if mods.writes != 1 || m.None2 != None2Builder {
+		t.Errorf("writes = %d none2 = %q after one merge, want one write with %q", mods.writes, m.None2, None2Builder)
+	}
+	if err := ix.Apply(ctx, f); err != nil {
+		t.Fatal(err)
+	}
+	if mods.writes != 1 {
+		t.Errorf("writes = %d after the same merge again, want still 1", mods.writes)
+	}
+}
+
+func TestAMergeKeepsAFingerprintTheModuleStillHas(t *testing.T) { // 1444 (review finding 8: a merge never blanks FP2)
+	ctx := context.Background()
+	ix, mods := newFormIndexer(t, builderModules(t)...)
+	reportToolchain(t, ix, "mesh-build-go", "novox", "go", "go1.27.1")
+	before, _ := moduleOf(t, mods, "mesh-tools")
+	if before.FP2 == "" {
+		t.Fatalf("mesh-tools has no fp2: %+v", before)
+	}
+	w := mods.writes
+	// the same catalogue merged again at a later commit: nothing moved
+	_, _, f := newFormIndexerUnmerged(t, builderModules(t)...)
+	if err := ix.Apply(ctx, f); err != nil {
+		t.Fatal(err)
+	}
+	if after, _ := moduleOf(t, mods, "mesh-tools"); after.FP2 != before.FP2 || mods.writes != w {
+		t.Errorf("fp2 = %q (was %q), writes %d more", after.FP2, before.FP2, mods.writes-w)
+	}
+}

modules/mesh-delivery-indexer/manifest_test.go

@@ -84,7 +84,7 @@ func TestManifestDeclarations(t *testing.T) { // 1238/AC1, 1444 (reads the build
 	}
 	// "Reads FORGE" is a consumes of the indexer's stream: the broker grants only buckets by reads (mesh-control
 	// internal/broker/state.go:62-93).
-	if want := []string{"mesh-delivery-forge-bridge.forge.**"}; !reflect.DeepEqual(sorted(m.Consumes), want) {
+	if want := []string{"mesh-build-go.toolchain.reported", "mesh-build-node.toolchain.reported", "mesh-delivery-forge-bridge.forge.**"}; !reflect.DeepEqual(sorted(m.Consumes), want) {
 		t.Errorf("consumes = %v, want %v", m.Consumes, want)
 	}
 	if b, ok := m.OwnSecrets["broker"]; !ok || len(m.OwnSecrets) != 1 || b != "${dir:state}/broker" {

modules/mesh-delivery-indexer/module.json

@@ -3,6 +3,8 @@
   "version": "1",
   "slug": "indexer",
   "consumes": [
+    "mesh-build-go.toolchain.reported",
+    "mesh-build-node.toolchain.reported",
     "mesh-delivery-forge-bridge.forge.**"
   ],
   "state": [