Ask a held first build again when a delivery of build-agent ends (issue 1173)

ADR 0482 section 5: the build waiting for a build-agent that reads its manifest is asked once the plan delivering it ends.

cmd/mesh-controller/first_builds.go

@@ -310,3 +310,58 @@ func probeFirstBuilds(ctx context.Context, d *doctor) ([]conditions.Observation,
 	}
 	return firstBuildObservations(waiting, time.Now().UTC()), nil
 }
+
+// askHeldFirstBuildsAgain asks again the first build of each first merge held for a build-agent that reads its manifest,
+// when the plan that ended delivered build-agent (novox/hq issue 1173): once for each end, since the ask answers the merge
+// and a refusal that comes again holds it anew. Answers how many merges it asked for.
+func askHeldFirstBuildsAgain(ctx context.Context, open *stores, p inventory.Plan) int {
+	if !deliversBuildAgent(p) {
+		return 0
+	}
+	actingOnMerges.Lock()
+	defer actingOnMerges.Unlock()
+	inv := open.inventory
+	waiting, err := inv.WaitingMerges(ctx)
+	if err != nil {
+		fmt.Printf("%s delivered build-agent, and the first merges held for it could not be read: %v\n", p.ID, err)
+		return 0
+	}
+	asked := 0
+	for _, w := range waiting {
+		k := announcedOf(w)
+		if k.Held == nil {
+			continue
+		}
+		entries, err := inv.Catalogued(ctx)
+		if err != nil {
+			fmt.Printf("%s delivered build-agent, and the catalogue could not be read: %v\n", p.ID, err)
+			return asked
+		}
+		read, err := readForPlanning(ctx, inv)
+		if err != nil {
+			fmt.Printf("%s delivered build-agent, and what the builds read could not be read: %v\n", p.ID, err)
+			return asked
+		}
+		added, why, err := firstBuildsOf(ctx, inv, k.SourceMoved, entries, read)
+		if err != nil || len(added) == 0 {
+			fmt.Printf("%s delivered build-agent, and the merge %.8s of %s held for it asks no build now (%v): it stays "+
+				"waiting\n", p.ID, w.Commit, w.Repository, err)
+			continue
+		}
+		fmt.Printf("%s delivered build-agent: the first build of %s, held for the word %q, is asked again\n", p.ID,
+			k.Held.Module, k.Held.Word)
+		if err := askFirstBuilds(ctx, inv, k.SourceMoved, time.Now().UTC(), added, why); err != nil {
+			fmt.Printf("  and could not be asked: %v\n", err)
+			continue
+		}
+		asked++
+	}
+	return asked
+}
+
+// deliversBuildAgent says a delivery plan ended delivered with build-agent built in it: the reader a held first build
+// waits for is the one now running.
+func deliversBuildAgent(p inventory.Plan) bool {
+	m := p.Modules["build-agent"]
+	return p.WalkSaid == inventory.WalkEnded && p.Outcome() == inventory.WalkDelivered && m != nil && m.State == "built"
+}