Keep a check build whose key is unchanged instead of building it again

A fresh check asked when only the trunk moved rebuilt every image, though
the reuse key said nothing it is built from changed: a controller check
took about 12 minutes instead of 5.5. The check now keeps a build kept
under its key when a verdict of the build seat names that key and every
digest, names it as the build's own check so a walk takes it, and the
cleanup keeps it while any check names it (novox/hq issue 703).

cmd/mesh-controller/check_builds_test.go

@@ -214,3 +214,28 @@ func TestAWalksLeaveNamesTheBuildsItsPullRequestsVerdictsMade(t *testing.T) {
 		t.Fatalf("verdicts that could not be read named %+v", got.Builds)
 	}
 }
+
+// A check is handed the builds the seat's verdicts name for the modules it builds, read once, and none when they
+// cannot be read (novox/hq issue 703).
+func TestACheckIsHandedTheBuildsTheVerdictsNameForItsModules(t *testing.T) {
+	was := checkedBuildsOfModules
+	t.Cleanup(func() { checkedBuildsOfModules = was })
+	var asked [][]string
+	named := []link.CheckedBuild{{Pull: link.PullRef{Owner: "novox", Repo: "shop", Number: 7}, Check: "build-1",
+		Built: link.CheckBuilt{Module: "app", Key: "r1:1"}}}
+	checkedBuildsOfModules = func(_ context.Context, modules []string) ([]link.CheckedBuild, error) {
+		asked = append(asked, modules)
+		return named, nil
+	}
+	builds := []link.CheckBuild{{Module: "app"}, {Module: "web", Skip: "the merge deletes it"}}
+	if got := namedForTheCheck(t.Context(), builds); !reflect.DeepEqual(got, named) ||
+		!reflect.DeepEqual(asked, [][]string{{"app"}}) {
+		t.Fatalf("the check is handed %+v, read %v", got, asked)
+	}
+	checkedBuildsOfModules = func(context.Context, []string) ([]link.CheckedBuild, error) {
+		return nil, errors.New("no bus")
+	}
+	if got := namedForTheCheck(t.Context(), builds); got != nil {
+		t.Fatalf("verdicts that could not be read named %+v", got)
+	}
+}