Improve checkBranchName (#17901)

The current implementation of checkBranchName is highly inefficient
involving opening the repository, the listing all of the branch names
checking them individually before then using using opened repo to get
the tags.

This PR avoids this by simply walking the references from show-ref
instead of opening the repository (in the nogogit case).

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/git/repo_branch_nogogit.go

@@ -61,14 +61,29 @@ func (repo *Repository) IsBranchExist(name string) bool {
 	return repo.IsReferenceExist(BranchPrefix + name)
 }
 
-// GetBranches returns branches from the repository, skipping skip initial branches and
+// GetBranchNames returns branches from the repository, skipping skip initial branches and
 // returning at most limit branches, or all branches if limit is 0.
-func (repo *Repository) GetBranches(skip, limit int) ([]string, int, error) {
+func (repo *Repository) GetBranchNames(skip, limit int) ([]string, int, error) {
 	return callShowRef(repo.Ctx, repo.Path, BranchPrefix, "--heads", skip, limit)
 }
 
+// WalkReferences walks all the references from the repository
+func WalkReferences(ctx context.Context, repoPath string, walkfn func(string) error) (int, error) {
+	return walkShowRef(ctx, repoPath, "", 0, 0, walkfn)
+}
+
 // callShowRef return refs, if limit = 0 it will not limit
 func callShowRef(ctx context.Context, repoPath, prefix, arg string, skip, limit int) (branchNames []string, countAll int, err error) {
+	countAll, err = walkShowRef(ctx, repoPath, arg, skip, limit, func(branchName string) error {
+		branchName = strings.TrimPrefix(branchName, prefix)
+		branchNames = append(branchNames, branchName)
+
+		return nil
+	})
+	return
+}
+
+func walkShowRef(ctx context.Context, repoPath, arg string, skip, limit int, walkfn func(string) error) (countAll int, err error) {
 	stdoutReader, stdoutWriter := io.Pipe()
 	defer func() {
 		_ = stdoutReader.Close()
@@ -77,7 +92,11 @@ func callShowRef(ctx context.Context, repoPath, prefix, arg string, skip, limit
 
 	go func() {
 		stderrBuilder := &strings.Builder{}
-		err := NewCommandContext(ctx, "show-ref", arg).RunInDirPipeline(repoPath, stdoutWriter, stderrBuilder)
+		args := []string{"show-ref"}
+		if arg != "" {
+			args = append(args, arg)
+		}
+		err := NewCommandContext(ctx, args...).RunInDirPipeline(repoPath, stdoutWriter, stderrBuilder)
 		if err != nil {
 			if stderrBuilder.Len() == 0 {
 				_ = stdoutWriter.Close()
@@ -94,10 +113,10 @@ func callShowRef(ctx context.Context, repoPath, prefix, arg string, skip, limit
 	for i < skip {
 		_, isPrefix, err := bufReader.ReadLine()
 		if err == io.EOF {
-			return branchNames, i, nil
+			return i, nil
 		}
 		if err != nil {
-			return nil, 0, err
+			return 0, err
 		}
 		if !isPrefix {
 			i++
@@ -112,39 +131,42 @@ func callShowRef(ctx context.Context, repoPath, prefix, arg string, skip, limit
 			_, err = bufReader.ReadSlice(' ')
 		}
 		if err == io.EOF {
-			return branchNames, i, nil
+			return i, nil
 		}
 		if err != nil {
-			return nil, 0, err
+			return 0, err
 		}
 
 		branchName, err := bufReader.ReadString('\n')
 		if err == io.EOF {
 			// This shouldn't happen... but we'll tolerate it for the sake of peace
-			return branchNames, i, nil
+			return i, nil
 		}
 		if err != nil {
-			return nil, i, err
+			return i, err
 		}
-		branchName = strings.TrimPrefix(branchName, prefix)
+
 		if len(branchName) > 0 {
 			branchName = branchName[:len(branchName)-1]
 		}
-		branchNames = append(branchNames, branchName)
+		err = walkfn(branchName)
+		if err != nil {
+			return i, err
+		}
 		i++
 	}
 	// count all refs
 	for limit != 0 {
 		_, isPrefix, err := bufReader.ReadLine()
 		if err == io.EOF {
-			return branchNames, i, nil
+			return i, nil
 		}
 		if err != nil {
-			return nil, 0, err
+			return 0, err
 		}
 		if !isPrefix {
 			i++
 		}
 	}
-	return branchNames, i, nil
+	return i, nil
 }