Build a change once: the check builds what its merge moves, the walk takes it by key

A merge built every moved module a second time although its check had judged
the same tree (issue 649). The check now builds into the scratch namespace
under a reuse key (source fingerprint plus the builder's own digest), and the
walk of the trunk commit copies a build of the same key by digest instead of
building, saying per module why when it cannot (novox/hq ADR 0350).

cmd/mesh-controller/checks.go

@@ -282,10 +282,59 @@ func checkRequestFor(ctx context.Context, open *stores, p link.PullUpdated, scop
 		Source:     sourceOnSeat(source),
 		Check: &link.CheckRequest{Owner: p.Owner, Repo: p.Repo, Number: p.Number, Base: p.Base,
 			Paths: p.Paths, Beside: beside, Modules: scope.Modules, Dependents: scope.Dependents, New: scope.New,
-			Manifests: scope.Manifests, Judge: scope.Judge},
+			Manifests: scope.Manifests, Judge: scope.Judge, Builds: checkBuildsOf(p, scope, entries, clone)},
 	}, nil
 }
 
+// checkBuildsOf are the modules a merge of the change would build, in the walk's order, as the check builds them
+// for the walk to take (novox/hq ADR 0350 §1): a module built from the repository checked, at the branch it merges
+// into, from the merge the check makes; any other — a module standing on one — from its own repository at the
+// branch it follows, as the walk builds it. A module whose build reads the repository checked through a context,
+// or that the merge deletes, cannot be built as the walk would: said, and nothing after it is built. A change that
+// moves no module builds nothing.
+func checkBuildsOf(p link.PullUpdated, scope checkScope, entries []inventory.Entry,
+	clone func(inventory.Source) (string, error)) []link.CheckBuild {
+	if len(scope.Reach.Plan.Tiers) == 0 {
+		return nil
+	}
+	m := link.SourceMoved{Owner: p.Owner, Repo: p.Repo, Base: p.Base, CloneURL: p.CloneURL}
+	byName := map[string]inventory.Entry{}
+	for _, e := range entries {
+		byName[e.Manifest.Module] = e
+	}
+	skip := map[string]string{}
+	for _, e := range scope.Reach.Packaging {
+		skip[e.Manifest.Module] = "its build reads " + p.Owner + "/" + p.Repo + " through a context, which a check " +
+			"cannot hand the merge"
+	}
+	for _, e := range scope.Reach.Deleted {
+		skip[e.Manifest.Module] = "the merge deletes it"
+	}
+	var out []link.CheckBuild
+	for tier, names := range scope.Reach.Plan.Tiers {
+		for _, name := range names {
+			e, known := byName[name]
+			b := link.CheckBuild{Module: name, Tier: tier, Skip: skip[name]}
+			switch {
+			case !known:
+				b.Skip = "the catalogue does not hold it"
+			case b.Skip != "":
+			case sourceIs(e.Source, m):
+				b.Merged, b.Path = true, e.Source.Path
+			default:
+				url, err := clone(e.Source)
+				if err != nil {
+					b.Skip = "its repository cannot be cloned: " + err.Error()
+					break
+				}
+				b.Repository, b.Path, b.Ref = url, e.Source.Path, followedBranch(e.Source.Ref)
+			}
+			out = append(out, b)
+		}
+	}
+	return out
+}
+
 // besideRefs is the ref each repository is cloned at beside a check, by the directory it is found under:
 // a core repository at the commit the mesh runs of the module built from it (`running`) — but the
 // catalogue, whose checkout beside is what tests read its files from, at its main, what the next merge
@@ -382,6 +431,7 @@ func checkedOf(result link.BuildResult) link.Checked {
 		// What was judged (novox/hq ADR 0323 §3): the head merged into the trunk commit, and the commits of the
 		// repositories read beside it.
 		c.Trunk, c.Beside, c.Took = result.Check.Trunk, result.Check.Beside, result.Check.Took
+		c.Built = result.Check.Built
 		if c.Gate == nil {
 			// A build seat from before the layers: its verdict is the gate's.
 			c.Gate = &link.CheckLayer{Verdict: c.Verdict, Summary: c.Summary}