Ask build-agent for itself and its toolchain images, never a builder key (ADR 0520, issue 1851)

The builder writes no key of build-agent or of the toolchain bases its
builds stand on, so a walk moving one would hold its tier for ever.
Until R4g they are build-agent's own, as the builder's module is; the
bases are read from the toolchains, not copied.

cmd/mesh-controller/plan_keys.go

@@ -8,6 +8,8 @@ import (
 	"time"
 
 	sdkdelivery "git.novox.be/novox/mesh-sdk/go/delivery"
+
+	"github.com/novox/mesh-controller/internal/builder"
 	"github.com/novox/mesh-controller/internal/builderkeys"
 	"github.com/novox/mesh-controller/internal/inventory"
 )
@@ -22,13 +24,24 @@ import (
 // yet, or waiting for a reference, holds the tier and says so; a failed key fails the walk in the builder's own words.
 // **No timer**: the builder's and the indexer's writes wake the look (builderkeys.Watch, watchNewModules).
 
-// isBuildersOwn says a module is the builder's own: the one module build-agent still builds, asked on a merge, until R4g
-// (ADR 0453 §5, ADR 0498 §7).
+// isBuildersOwn says a module is build-agent's own: asked of build-agent on a merge and taken from its build record, never
+// from the builder's key, until R4g (ADR 0453 §5, ADR 0498 §7, ADR 0520). They are the builder's module, build-agent itself
+// (its image's context is another repository's) and the toolchain images build-agent's builds stand on
+// (builder.Bases): the builder writes no key of any of them, and a walk waiting for one would hold its tier for ever.
 func isBuildersOwn(module string) bool { return buildersOwn(module) }
 
-// buildersOwn is isBuildersOwn's rule: a variable only so a test of the one ask left (its bus waits, its plan key) may
-// name modules of its own as the builder's.
-var buildersOwn = func(module string) bool { return module == sdkdelivery.BuilderModule }
+// buildAgentsOwn are those modules: the builder's, build-agent's and every toolchain's base, read from the toolchains.
+var buildAgentsOwn = func() map[string]bool {
+	own := map[string]bool{sdkdelivery.BuilderModule: true, builder.AgentModule: true}
+	for _, b := range builder.Bases() {
+		own[b] = true
+	}
+	return own
+}()
+
+// buildersOwn is isBuildersOwn's rule: a variable only so a test of the asks left (their bus waits, their plan key) may
+// name modules of its own as build-agent's.
+var buildersOwn = func(module string) bool { return buildAgentsOwn[module] }
 
 // planKeyReads is what a walk reads of the indexer and the builder (builderkeys.Reader, or a test's).
 type planKeyReads interface {