Refactor git command stdio pipe (#36422)

Most potential deadlock problems should have been fixed, and new code is
unlikely to cause new problems with the new design.

Also raise the minimum Git version required to 2.6.0 (released in 2015)

modules/git/catfile_batch_reader.go

@@ -18,46 +18,40 @@ import (
 )
 
 type catFileBatchCommunicator struct {
-	cancel context.CancelFunc
-	reader *bufio.Reader
-	writer io.Writer
-
+	cancel      context.CancelFunc
+	reqWriter   io.Writer
+	respReader  *bufio.Reader
 	debugGitCmd *gitcmd.Command
 }
 
 func (b *catFileBatchCommunicator) Close() {
 	if b.cancel != nil {
 		b.cancel()
-		b.reader = nil
-		b.writer = nil
 		b.cancel = nil
 	}
 }
 
 // newCatFileBatch opens git cat-file --batch in the provided repo and returns a stdin pipe, a stdout reader and cancel function
-func newCatFileBatch(ctx context.Context, repoPath string, cmdCatFile *gitcmd.Command) *catFileBatchCommunicator {
-	// We often want to feed the commits in order into cat-file --batch, followed by their trees and subtrees as necessary.
+func newCatFileBatch(ctx context.Context, repoPath string, cmdCatFile *gitcmd.Command) (ret *catFileBatchCommunicator) {
 	ctx, ctxCancel := context.WithCancelCause(ctx)
 
-	var batchStdinWriter io.WriteCloser
-	var batchStdoutReader io.ReadCloser
-	cmdCatFile = cmdCatFile.
-		WithDir(repoPath).
-		WithStdinWriter(&batchStdinWriter).
-		WithStdoutReader(&batchStdoutReader)
+	// We often want to feed the commits in order into cat-file --batch, followed by their trees and subtrees as necessary.
+	stdinWriter, stdoutReader, pipeClose := cmdCatFile.MakeStdinStdoutPipe()
+	ret = &catFileBatchCommunicator{
+		debugGitCmd: cmdCatFile,
+		cancel:      func() { ctxCancel(nil) },
+		reqWriter:   stdinWriter,
+		respReader:  bufio.NewReaderSize(stdoutReader, 32*1024), // use a buffered reader for rich operations
+	}
 
-	err := cmdCatFile.StartWithStderr(ctx)
+	err := cmdCatFile.WithDir(repoPath).StartWithStderr(ctx)
 	if err != nil {
 		log.Error("Unable to start git command %v: %v", cmdCatFile.LogString(), err)
 		// ideally here it should return the error, but it would require refactoring all callers
 		// so just return a dummy communicator that does nothing, almost the same behavior as before, not bad
-		return &catFileBatchCommunicator{
-			writer: io.Discard,
-			reader: bufio.NewReader(bytes.NewReader(nil)),
-			cancel: func() {
-				ctxCancel(err)
-			},
-		}
+		ctxCancel(err)
+		pipeClose()
+		return ret
 	}
 
 	go func() {
@@ -66,19 +60,10 @@ func newCatFileBatch(ctx context.Context, repoPath string, cmdCatFile *gitcmd.Co
 			log.Error("cat-file --batch command failed in repo %s, error: %v", repoPath, err)
 		}
 		ctxCancel(err)
+		pipeClose()
 	}()
 
-	// use a buffered reader to read from the cat-file --batch (StringReader.ReadString)
-	batchReader := bufio.NewReaderSize(batchStdoutReader, 32*1024)
-
-	return &catFileBatchCommunicator{
-		writer: batchStdinWriter,
-		reader: batchReader,
-		cancel: func() {
-			ctxCancel(nil)
-		},
-		debugGitCmd: cmdCatFile,
-	}
+	return ret
 }
 
 // catFileBatchParseInfoLine reads the header line from cat-file --batch