fix(cleanup): kill Unix step process group on cancel to avoid hang (#1025)

Cancelling a job on a Linux/macOS host runner can leave the spawned process
tree running and hang the runner — the same failure mode fixed for Windows in
#1011, just on the other platforms.

Steps are launched as process-group leaders (`Setpgid`, or `Setsid` for the PTY
path), but the default `exec.CommandContext` cancellation only kills the
**direct child**. When a step launches a shell that starts a child which in turn
spawns further background processes, cancelling the job leaves the descendants
running. Because those orphans inherited the step's stdout/stderr pipe, the read
end never hits EOF and `cmd.Wait()` blocks forever.

Because the step executor never returns:
- the orphaned processes keep running (the cancelled work is not actually
  stopped), and
- end-of-job cleanup is never reached, so the runner appears to go offline / stop
  picking up jobs.

## Fix

Apply the same tree-kill approach as Windows, using the Unix counterpart of a Job
Object: the **process group**.

- Add a Unix `processKiller` (`process_unix.go`) that captures the step's PGID
  (== PID, since the step is launched as a group leader) and sends `SIGKILL` to
  the whole group on cancellation. This also closes the inherited pipe handles so
  `cmd.Wait()` can return. `ESRCH` (group already gone) is not treated as an error.
- Restrict the previous no-op stub (`process_other.go`) to `plan9` and have it
  fall back to a single-process kill, preserving plan9's prior behaviour.
- Wire `cmd.Cancel` (tree kill) and `cmd.WaitDelay` (10s) **unconditionally** in
  `exec()` instead of Windows-only. `WaitDelay` also covers a step that
  backgrounds a process holding the pipe open after the main process exits.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1025
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

act/container/host_environment.go

@@ -323,28 +323,28 @@ func (e *HostEnvironment) exec(ctx context.Context, command []string, cmdline st
 	cmd.Dir = wd
 	cmd.SysProcAttr = getSysProcAttr(cmdline, false)
 
-	// On Windows a step often launches a process tree (a shell that starts a
-	// child which spawns further GUI or background processes). The default
-	// context cancellation only kills the direct child, leaving the rest of the
-	// tree running; and because the orphans inherit cmd's stdout/stderr pipe,
-	// cmd.Wait() would block forever, hanging the runner. Kill the whole tree
-	// via a Job Object on cancellation, and bound the wait so a leftover pipe
-	// writer can never hang Wait indefinitely.
+	// A step often launches a process tree (a shell that starts a child which
+	// spawns further background or GUI processes). The default context
+	// cancellation only kills the direct child, leaving the rest of the tree
+	// running; and because the orphans inherit cmd's stdout/stderr pipe,
+	// cmd.Wait() would block forever, hanging the runner. Kill the whole tree on
+	// cancellation — via a Job Object on Windows and the process group on Unix
+	// (see processKiller) — and bound the wait so a leftover pipe writer can
+	// never hang Wait indefinitely.
 	var killer atomic.Pointer[processKiller]
-	if runtime.GOOS == "windows" {
-		cmd.Cancel = func() error {
-			if k := killer.Load(); k != nil {
-				return k.Kill()
-			}
-			if cmd.Process != nil {
-				return cmd.Process.Kill()
-			}
-			return nil
+	cmd.Cancel = func() error {
+		if k := killer.Load(); k != nil {
+			return k.Kill()
 		}
-		// Once the step process has exited, give its I/O pipes at most this long
-		// to drain before Wait force-closes them and returns (Go's WaitDelay).
-		cmd.WaitDelay = 10 * time.Second
+		if cmd.Process != nil {
+			return cmd.Process.Kill()
+		}
+		return nil
 	}
+	// Once the step process has exited, give its I/O pipes at most this long to
+	// drain before Wait force-closes them and returns (Go's WaitDelay). This
+	// also covers a step that backgrounds a process holding the pipe open.
+	cmd.WaitDelay = 10 * time.Second
 
 	var ppty *os.File
 	var tty *os.File
@@ -375,17 +375,16 @@ func (e *HostEnvironment) exec(ctx context.Context, command []string, cmdline st
 	if err := cmd.Start(); err != nil {
 		return err
 	}
-	if runtime.GOOS == "windows" {
-		// Assign the started process to a Job Object so cmd.Cancel can kill the
-		// whole descendant tree. Children spawned afterwards are auto-included.
-		// On failure (e.g. nested-job restrictions) we fall back to the default
-		// single-process kill; WaitDelay + end-of-job cleanup still apply.
-		if k, kerr := newProcessKiller(cmd.Process); kerr != nil {
-			common.Logger(ctx).Warnf("process tree kill setup failed, falling back to single-process kill: %v", kerr)
-		} else {
-			killer.Store(k)
-			defer k.Close()
-		}
+	// Capture the started process for tree-kill on cancellation: a Job Object on
+	// Windows (children spawned afterwards are auto-included) and the process
+	// group on Unix. On failure (e.g. Windows nested-job restrictions) we fall
+	// back to the default single-process kill; WaitDelay + end-of-job cleanup
+	// still apply.
+	if k, kerr := newProcessKiller(cmd.Process); kerr != nil {
+		common.Logger(ctx).Warnf("process tree kill setup failed, falling back to single-process kill: %v", kerr)
+	} else {
+		killer.Store(k)
+		defer k.Close()
 	}
 	err = cmd.Wait()
 	if err != nil {