Refactor git command context & pipeline (#36406)

Less and simpler code, fewer bugs

modules/git/gitcmd/error.go

@@ -0,0 +1,78 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitcmd
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+type RunStdError interface {
+	error
+	Unwrap() error
+	Stderr() string
+}
+
+type runStdError struct {
+	err    error  // usually the low-level error like `*exec.ExitError`
+	stderr string // git command's stderr output
+	errMsg string // the cached error message for Error() method
+}
+
+func (r *runStdError) Error() string {
+	// FIXME: GIT-CMD-STDERR: it is a bad design, the stderr should not be put in the error message
+	// But a lot of code only checks `strings.Contains(err.Error(), "git error")`
+	if r.errMsg == "" {
+		r.errMsg = fmt.Sprintf("%s - %s", r.err.Error(), strings.TrimSpace(r.stderr))
+	}
+	return r.errMsg
+}
+
+func (r *runStdError) Unwrap() error {
+	return r.err
+}
+
+func (r *runStdError) Stderr() string {
+	return r.stderr
+}
+
+func ErrorAsStderr(err error) (string, bool) {
+	var runErr RunStdError
+	if errors.As(err, &runErr) {
+		return runErr.Stderr(), true
+	}
+	return "", false
+}
+
+func StderrHasPrefix(err error, prefix string) bool {
+	stderr, ok := ErrorAsStderr(err)
+	if !ok {
+		return false
+	}
+	return strings.HasPrefix(stderr, prefix)
+}
+
+func IsErrorExitCode(err error, code int) bool {
+	var exitError *exec.ExitError
+	if errors.As(err, &exitError) {
+		return exitError.ExitCode() == code
+	}
+	return false
+}
+
+func IsErrorSignalKilled(err error) bool {
+	var exitError *exec.ExitError
+	return errors.As(err, &exitError) && exitError.String() == "signal: killed"
+}
+
+func IsErrorCanceledOrKilled(err error) bool {
+	// When "cancel()" a git command's context, the returned error of "Run()" could be one of them:
+	// - context.Canceled
+	// - *exec.ExitError: "signal: killed"
+	// TODO: in the future, we need to use unified error type from gitcmd.Run to check whether it is manually canceled
+	return errors.Is(err, context.Canceled) || IsErrorSignalKilled(err)
+}