tstest/natlab/vmtest: kill QEMU when test process dies (#19676)

Re-exec the test binary as a thin wrapper that holds a pipe inherited
from the test. When the test goes away (any reason, including SIGKILL,
panic, or OOM), the kernel closes the pipe write end; the wrapper sees
EOF and SIGKILLs itself, taking QEMU and its children with it.

Updates #13038

Change-Id: Ib2151098193551396c1d7bb51b07da3bd6b2cfb4

Signed-off-by: Fernando Serboncini <fserb@tailscale.com>

tstest/natlab/vmtest/qemu.go

@@ -211,7 +211,14 @@ func (e *Env) launchQEMU(name, logPath string, args []string) error {
 	}
 	cmd.Stdout = qemuLog
 	cmd.Stderr = qemuLog
+	parentPipe, err := killWithParent(cmd)
+	if err != nil {
+		devNull.Close()
+		qemuLog.Close()
+		return fmt.Errorf("killWithParent: %w", err)
+	}
 	if err := cmd.Start(); err != nil {
+		parentPipe.Close()
 		devNull.Close()
 		qemuLog.Close()
 		return fmt.Errorf("qemu for %s: %w", name, err)
@@ -224,8 +231,9 @@ func (e *Env) launchQEMU(name, logPath string, args []string) error {
 		go e.tailLogFile(e.ctx, name, logPath)
 	}
 	e.t.Cleanup(func() {
-		cmd.Process.Kill()
+		killProcessTree(cmd)
 		cmd.Wait()
+		parentPipe.Close()
 		devNull.Close()
 		qemuLog.Close()
 		// Dump tail of VM log on failure for debugging.

tstest/natlab/vmtest/qemu_wrapper.go

@@ -0,0 +1,90 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build unix
+
+package vmtest
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"os/exec"
+	"strconv"
+	"syscall"
+)
+
+// Re-exec'd as a wrapper around QEMU: when the test process dies (any
+// reason, including SIGKILL), the kernel closes the pipe write end, the
+// wrapper sees EOF, and kills QEMU's process group.
+
+const wrapperEnv = "TS_VMTEST_QEMU_WRAPPER"
+
+func init() {
+	if os.Getenv(wrapperEnv) == "" {
+		return
+	}
+	runQEMUWrapper()
+}
+
+func runQEMUWrapper() {
+	fd, err := strconv.Atoi(os.Getenv(wrapperEnv))
+	if err != nil {
+		log.Fatalf("vmtest qemu wrapper: bad %s: %v", wrapperEnv, err)
+	}
+	os.Unsetenv(wrapperEnv)
+	if len(os.Args) < 2 {
+		log.Fatalf("vmtest qemu wrapper: missing command")
+	}
+	pipeFd := os.NewFile(uintptr(fd), "parent-pipe")
+
+	// QEMU inherits our pgid (the test set Setpgid on us), so a group kill
+	// from the test reaches QEMU too. Don't set Setpgid here.
+	cmd := exec.Command(os.Args[1], os.Args[2:]...)
+	cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
+	if err := cmd.Start(); err != nil {
+		log.Fatalf("vmtest qemu wrapper: %v", err)
+	}
+
+	go func() {
+		// Block until the parent's pipe write end closes (EOF), then kill
+		// our process group (which includes QEMU and any of its children).
+		io.Copy(io.Discard, pipeFd)
+		syscall.Kill(0, syscall.SIGKILL)
+	}()
+
+	cmd.Wait()
+}
+
+// killWithParent rewrites cmd to run via a wrapper that kills it if the
+// test process dies. The returned *os.File must be kept alive until the
+// command is no longer needed; closing it makes the wrapper exit.
+func killWithParent(cmd *exec.Cmd) (*os.File, error) {
+	self, err := os.Executable()
+	if err != nil {
+		return nil, fmt.Errorf("os.Executable: %w", err)
+	}
+	r, w, err := os.Pipe()
+	if err != nil {
+		return nil, fmt.Errorf("pipe: %w", err)
+	}
+
+	cmd.ExtraFiles = append(cmd.ExtraFiles, r)
+	pipeFd := 3 + len(cmd.ExtraFiles) - 1 // stdin/stdout/stderr + ExtraFiles index
+	cmd.Args = append([]string{self, cmd.Path}, cmd.Args[1:]...)
+	cmd.Path = self
+	if cmd.Env == nil {
+		cmd.Env = os.Environ()
+	}
+	cmd.Env = append(cmd.Env, fmt.Sprintf("%s=%d", wrapperEnv, pipeFd))
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+
+	return w, nil
+}
+
+// killProcessTree SIGKILLs cmd's process group (cmd plus any descendants
+// that didn't escape it).
+func killProcessTree(cmd *exec.Cmd) error {
+	return syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
+}

tstest/natlab/vmtest/qemu_wrapper_windows.go

@@ -0,0 +1,20 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package vmtest
+
+import (
+	"os"
+	"os/exec"
+)
+
+// Stubs for Windows: no parent-death watcher, no process-group kill.
+// The test still launches QEMU; cleanup just kills the single process.
+
+func killWithParent(cmd *exec.Cmd) (*os.File, error) {
+	return os.Open(os.DevNull)
+}
+
+func killProcessTree(cmd *exec.Cmd) error {
+	return cmd.Process.Kill()
+}