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/process_unix.go

@@ -0,0 +1,56 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build !windows && !plan9
+
+package container
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+// processKiller terminates a step process together with its whole process
+// group, which is the Unix counterpart of the Windows Job Object tree-kill.
+//
+// Background: a step often launches a process tree (a shell that starts a child
+// which in turn spawns further background processes). The default
+// exec.CommandContext cancellation only kills the direct child, so cancelling a
+// job left the rest of the tree running. Because those orphans inherited the
+// step's stdout/stderr pipe, cmd.Wait() also blocked forever and the runner
+// hung.
+//
+// Steps are started with Setpgid (or Setsid for the PTY path, see
+// getSysProcAttr), which makes the step process the leader of a new process
+// group whose ID equals its PID. Signalling the negative PID delivers to every
+// process still in that group, so we can tear down the whole tree atomically on
+// cancellation, which also closes the inherited pipe handles so cmd.Wait() can
+// return.
+type processKiller struct {
+	pgid int
+}
+
+// newProcessKiller captures the process group of p (an already-started
+// process). Because the step is launched with Setpgid/Setsid, p is a group
+// leader and its PGID equals its PID; children spawned afterwards stay in the
+// same group unless they explicitly create their own.
+func newProcessKiller(p *os.Process) (*processKiller, error) {
+	return &processKiller{pgid: p.Pid}, nil
+}
+
+// Kill sends SIGKILL to the entire process group (the step process and every
+// descendant that stayed in the group). A missing group (ESRCH) means the
+// processes already exited and is not treated as an error.
+func (k *processKiller) Kill() error {
+	if k == nil || k.pgid <= 0 {
+		return nil
+	}
+	if err := syscall.Kill(-k.pgid, syscall.SIGKILL); err != nil && !errors.Is(err, syscall.ESRCH) {
+		return err
+	}
+	return nil
+}
+
+// Close is a no-op on Unix; there is no job handle to release.
+func (k *processKiller) Close() error { return nil }