Make IndexGitRepo signal whether there were changes

As described, we'd like this information to be externally available to
drive metrics for incremental indexing.

Closes #780.

cmd/zoekt-git-index/main.go

@@ -122,7 +122,7 @@ func run() int {
 			DeltaShardNumberFallbackThreshold: *deltaShardNumberFallbackThreshold,
 		}
 
-		if err := gitindex.IndexGitRepo(gitOpts); err != nil {
+		if _, err := gitindex.IndexGitRepo(gitOpts); err != nil {
 			log.Printf("indexGitRepo(%s, delta=%t): %v", dir, gitOpts.BuildOptions.IsDelta, err)
 			exitStatus = 1
 		}

gitindex/index.go

@@ -388,12 +388,16 @@ func expandBranches(repo *git.Repository, bs []string, prefix string) ([]string,
 }
 
 // IndexGitRepo indexes the git repository as specified by the options.
-func IndexGitRepo(opts Options) error {
+// The returned bool indicates whether the index was updated as a result. This
+// can be informative if doing incremental indexing.
+func IndexGitRepo(opts Options) (bool, error) {
 	return indexGitRepo(opts, gitIndexConfig{})
 }
 
 // indexGitRepo indexes the git repository as specified by the options and the provided gitIndexConfig.
-func indexGitRepo(opts Options, config gitIndexConfig) error {
+// The returned bool indicates whether the index was updated as a result. This
+// can be informative if doing incremental indexing.
+func indexGitRepo(opts Options, config gitIndexConfig) (bool, error) {
 	prepareDeltaBuild := prepareDeltaBuild
 	if config.prepareDeltaBuild != nil {
 		prepareDeltaBuild = config.prepareDeltaBuild
@@ -407,13 +411,13 @@ func indexGitRepo(opts Options, config gitIndexConfig) error {
 	// Set max thresholds, since we use them in this function.
 	opts.BuildOptions.SetDefaults()
 	if opts.RepoDir == "" {
-		return fmt.Errorf("gitindex: must set RepoDir")
+		return false, fmt.Errorf("gitindex: must set RepoDir")
 	}
 
 	opts.BuildOptions.RepositoryDescription.Source = opts.RepoDir
 	repo, err := git.PlainOpen(opts.RepoDir)
 	if err != nil {
-		return fmt.Errorf("git.PlainOpen: %w", err)
+		return false, fmt.Errorf("git.PlainOpen: %w", err)
 	}
 
 	if err := setTemplatesFromConfig(&opts.BuildOptions.RepositoryDescription, opts.RepoDir); err != nil {
@@ -422,7 +426,7 @@ func indexGitRepo(opts Options, config gitIndexConfig) error {
 
 	branches, err := expandBranches(repo, opts.Branches, opts.BranchPrefix)
 	if err != nil {
-		return fmt.Errorf("expandBranches: %w", err)
+		return false, fmt.Errorf("expandBranches: %w", err)
 	}
 	for _, b := range branches {
 		commit, err := getCommit(repo, opts.BranchPrefix, b)
@@ -431,7 +435,7 @@ func indexGitRepo(opts Options, config gitIndexConfig) error {
 				continue
 			}
 
-			return fmt.Errorf("getCommit(%q, %q): %w", opts.BranchPrefix, b, err)
+			return false, fmt.Errorf("getCommit(%q, %q): %w", opts.BranchPrefix, b, err)
 		}
 
 		opts.BuildOptions.RepositoryDescription.Branches = append(opts.BuildOptions.RepositoryDescription.Branches, zoekt.RepositoryBranch{
@@ -445,7 +449,7 @@ func indexGitRepo(opts Options, config gitIndexConfig) error {
 	}
 
 	if opts.Incremental && opts.BuildOptions.IncrementalSkipIndexing() {
-		return nil
+		return false, nil
 	}
 
 	// branch => (path, sha1) => repo.
@@ -474,7 +478,7 @@ func indexGitRepo(opts Options, config gitIndexConfig) error {
 	if !opts.BuildOptions.IsDelta {
 		repos, branchMap, branchVersions, err = prepareNormalBuild(opts, repo)
 		if err != nil {
-			return fmt.Errorf("preparing normal build: %w", err)
+			return false, fmt.Errorf("preparing normal build: %w", err)
 		}
 	}
 
@@ -507,20 +511,20 @@ func indexGitRepo(opts Options, config gitIndexConfig) error {
 
 	builder, err := build.NewBuilder(opts.BuildOptions)
 	if err != nil {
-		return fmt.Errorf("build.NewBuilder: %w", err)
+		return false, fmt.Errorf("build.NewBuilder: %w", err)
 	}
 
 	var ranks repoPathRanks
 	var meanRank float64
 	if opts.BuildOptions.DocumentRanksPath != "" {
 		data, err := os.ReadFile(opts.BuildOptions.DocumentRanksPath)
 		if err != nil {
-			return err
+			return false, err
 		}
 
 		err = json.Unmarshal(data, &ranks)
 		if err != nil {
-			return err
+			return false, err
 		}
 
 		// Compute the mean rank for this repository. Note: we overwrite the rank
@@ -564,16 +568,16 @@ func indexGitRepo(opts Options, config gitIndexConfig) error {
 		for _, key := range keys {
 			doc, err := createDocument(key, repos, branchMap, ranks, opts.BuildOptions)
 			if err != nil {
-				return err
+				return false, err
 			}
 
 			if err := builder.Add(doc); err != nil {
-				return fmt.Errorf("error adding document with name %s: %w", key.FullPath(), err)
+				return false, fmt.Errorf("error adding document with name %s: %w", key.FullPath(), err)
 			}
 		}
 	}
 
-	return builder.Finish()
+	return true, builder.Finish()
 }
 
 type repoPathRanks struct {