Accept a merged message with no trees, so a merge that leaves a repository with no module.json still reaches the indexer (issue 1126)

go/delivery/source.go

@@ -688,9 +688,10 @@ func (f Forge) check() error {
 		if f.Root == "" {
 			return fmt.Errorf("delivery: a merged message needs the repository's root tree")
 		}
-		if len(f.Trees) == 0 {
-			return fmt.Errorf("delivery: a merged message needs the tree of at least one directory holding a module.json")
-		}
+		// No trees is a merge that leaves the repository with no module.json anywhere (a documents repository, or one
+		// whose last module was removed): it is written all the same, so the indexer takes the merge in and drops the
+		// repository's modules (novox/hq issue 1126). An older writer always sent at least one tree, so none is unambiguous;
+		// such a message carries no manifests, locks or paths (checkInputs below: each names a directory of its trees).
 		for dir, tree := range f.Trees {
 			if tree == "" {
 				return fmt.Errorf("delivery: a merged message's tree of the directory %q is empty", dir)

go/delivery/source_test.go

@@ -272,9 +272,15 @@ func TestSourceValuesRefused(t *testing.T) { // 1200/AC4
 	_, err = EncodeForge(f)
 	wantErr("merged without root", err, "root")
 	f = sampleForges()[1]
-	f.Trees = nil
+	f.Trees, f.Manifests, f.Locks, f.Paths = nil, nil, nil, nil
+	if data, err := EncodeForge(f); err != nil {
+		t.Errorf("a merged message of a repository with no module.json is refused: %v", err)
+	} else if got, err := DecodeForge(data); err != nil || len(got.Trees) != 0 {
+		t.Errorf("a merged message with no trees comes back as %+v, %v", got, err)
+	}
+	f.Manifests = map[string]string{"modules/a": "{}"}
 	_, err = EncodeForge(f)
-	wantErr("merged without trees", err, "tree")
+	wantErr("merged with a manifest and no trees", err, "modules/a")
 	f = sampleForges()[1]
 	f.Trees = map[string]string{"modules/a": ""}
 	_, err = EncodeForge(f)