fix(process): reap entire process group on cmd.Cancel (#39143)

Signed-off-by: Royce Remer <royceremer@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/process/command_unix_test.go

@@ -0,0 +1,67 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build unix
+
+package process
+
+import (
+	"bufio"
+	"context"
+	"os"
+	"strconv"
+	"strings"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCommandContextCancelKillProcessGroup(t *testing.T) {
+	ctx, cancel := context.WithCancel(t.Context())
+	defer cancel()
+
+	// When executing external commands, there can be multiple subprocesses involved.
+	// e.g.: Gitea ->  "git upload-pack" -> "git pack-objects".
+	// If the context is canceled (HTTP client disconnects), all subprocesses must terminate.
+
+	// Spawn a shell that itself spawns a long-lived background process and
+	// prints its PID — mimicking git upload-pack spawning git pack-objects.
+
+	r, w, err := os.Pipe()
+	require.NoError(t, err)
+
+	cmd := CommandContext(ctx, "sh", "-c", "sleep 600 & echo $!; wait")
+	cmd.Stdout = w
+	require.NoError(t, cmd.Start())
+	_ = w.Close() // parent keeps only the read end
+
+	t.Cleanup(func() {
+		// make sure our test doesn't leave a zombie process even if test fails
+		_ = syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
+		_ = cmd.Wait()
+	})
+
+	// Block until the shell prints the grandchild PID.
+	scanner := bufio.NewScanner(r)
+	require.True(t, scanner.Scan(), "expected grandchild PID on stdout")
+	grandchildPID, err := strconv.Atoi(strings.TrimSpace(scanner.Text()))
+	require.NoError(t, err)
+	_ = r.Close()
+
+	// Sanity: grandchild must be alive before we cancel.
+	grandchild, err := os.FindProcess(grandchildPID)
+	require.NoError(t, err)
+	require.NoError(t, grandchild.Signal(syscall.Signal(0)), "grandchild should be alive before cancel")
+
+	// Cancel the context
+	cancel()
+	_ = cmd.Wait()
+
+	// Subprocess should not exist after context cancel (killed by process group)
+	assert.Eventually(t, func() bool {
+		return grandchild.Signal(syscall.Signal(0)) != nil
+	}, 5*time.Second, 10*time.Millisecond)
+}