Judge an ask by the builder's say the walk read, never a second read (issue 1886, review round 1)

A second read that failed, or that turned true in between, refused the ask and
failed the whole plan.

cmd/mesh-controller/builder_says_test.go

@@ -181,3 +181,43 @@ func TestTheReferenceWritersAreAskedOfBuildAgentWhateverTheBuilderSays(t *testin
 		t.Fatalf("build-agent was asked %v", *asked)
 	}
 }
+
+// 1886 review round 1 (ADR 0522 §2): the guard of the one ask function judges by the say askModule read, never a second
+// read: a second read that fails, or that says otherwise, fails no plan.
+func TestTheAskGuardUsesTheSayTheWalkRead(t *testing.T) {
+	was := askedModuleOf
+	t.Cleanup(func() { askedModuleOf = was })
+	askedModuleOf = func(_ context.Context, _ buildSource, path string) (string, error) {
+		return strings.TrimPrefix(path, "modules/"), nil
+	}
+	reads := 0
+	wasSays := builderSaysReferences
+	t.Cleanup(func() { builderSaysReferences = wasSays })
+	for _, second := range []struct {
+		name string
+		says bool
+		err  error
+	}{{"a second read that fails", false, errors.New("the bus did not answer")}, {"a say that turned true", true, nil}} {
+		reads = 0
+		builderSaysReferences = func(context.Context) (bool, error) {
+			reads++
+			if reads == 1 {
+				return false, nil
+			}
+			return second.says, second.err
+		}
+		wasAsk := askABuild
+		askABuild = func(ctx context.Context, source buildSource, path, _ string) (string, error) {
+			if err := onlyTheBuildersOwn(ctx, source, path); err != nil {
+				return "", err
+			}
+			return "build-9", nil
+		}
+		p, byName := aWalkOfOne("sudo")
+		askModule(t.Context(), p, "sudo", byName)
+		askABuild = wasAsk
+		if s := p.Modules["sudo"]; p.State == inventory.PlanFailed || s.Build != "build-9" || s.Key {
+			t.Errorf("%s: the walk %s, the module %+v", second.name, p.State, s)
+		}
+	}
+}

cmd/mesh-controller/plan_keys.go

@@ -51,6 +51,20 @@ type planKeyReads interface {
 	SaysReferences(ctx context.Context) (bool, error)
 }
 
+// builderSayKey carries, on an ask's context, the builder's say the walk read before asking (withBuilderSay).
+type builderSayKey struct{}
+
+// withBuilderSay hands the say a walk read to the ask's guard, so the guard never reads it a second time.
+func withBuilderSay(ctx context.Context, says bool) context.Context {
+	return context.WithValue(ctx, builderSayKey{}, says)
+}
+
+// builderSayOf is the say a walk handed the ask, and whether it handed one.
+func builderSayOf(ctx context.Context) (says, handed bool) {
+	says, handed = ctx.Value(builderSayKey{}).(bool)
+	return says, handed
+}
+
 // builderSaysReferences is whether a running builder instance says, in its own bucket of instances, that the keys it
 // writes name references (issue 1886). Until one does, a walk asks build-agent for every module, as before R4e: the
 // keys the running builder writes could never be delivered from. A variable so a test needs no bus.

cmd/mesh-controller/release_plan.go

@@ -457,8 +457,16 @@ func onlyTheBuildersOwn(ctx context.Context, source buildSource, path string) er
 	if isBuildersOwn(module) {
 		return nil
 	}
-	// **While no running builder says it writes references, build-agent builds every module** (issue 1886).
-	if says, err := builderSaysReferences(ctx); err == nil && !says {
+	// **While no running builder says it writes references, build-agent builds every module** (issue 1886): judged by the
+	// say the walk read when it has one, read here only for an ask that carries none.
+	says, read := builderSayOf(ctx)
+	if !read {
+		var err error
+		if says, err = builderSaysReferences(ctx); err != nil {
+			says = true // not known: refused below, as before the say existed
+		}
+	}
+	if !says {
 		return nil
 	}
 	if module == "" {
@@ -547,6 +555,8 @@ func askModule(ctx context.Context, p *inventory.Plan, name string, byName map[s
 			takeFromKey(p, state, e, now)
 			return
 		}
+		// The ask's guard judges by this say, never by a second read that could fail or differ (review of issue 1886).
+		ctx = withBuilderSay(ctx, says)
 	}
 	source := buildSource{Repository: e.Source.Repository, Seat: e.Source.Seat}
 	fmt.Printf("  tier %d: ", p.Tier)