fix: propagate correct agent exit code (#21718)

The reaper (PID 1) now returns the child's exit code instead of always
exiting 0. Signal termination uses the standard Unix convention of 128 +
signal number.

fixes #21661

agent/reaper/reaper_stub.go

@@ -7,6 +7,6 @@ func IsInitProcess() bool {
 	return false
 }
 
-func ForkReap(_ ...Option) error {
-	return nil
+func ForkReap(_ ...Option) (int, error) {
+	return 0, nil
 }

agent/reaper/reaper_test.go

@@ -32,12 +32,13 @@ func TestReap(t *testing.T) {
 	}
 
 	pids := make(reap.PidCh, 1)
-	err := reaper.ForkReap(
+	exitCode, err := reaper.ForkReap(
 		reaper.WithPIDCallback(pids),
 		// Provide some argument that immediately exits.
 		reaper.WithExecArgs("/bin/sh", "-c", "exit 0"),
 	)
 	require.NoError(t, err)
+	require.Equal(t, 0, exitCode)
 
 	cmd := exec.Command("tail", "-f", "/dev/null")
 	err = cmd.Start()
@@ -65,6 +66,36 @@ func TestReap(t *testing.T) {
 	}
 }
 
+//nolint:paralleltest
+func TestForkReapExitCodes(t *testing.T) {
+	if testutil.InCI() {
+		t.Skip("Detected CI, skipping reaper tests")
+	}
+
+	tests := []struct {
+		name         string
+		command      string
+		expectedCode int
+	}{
+		{"exit 0", "exit 0", 0},
+		{"exit 1", "exit 1", 1},
+		{"exit 42", "exit 42", 42},
+		{"exit 255", "exit 255", 255},
+		{"SIGKILL", "kill -9 $$", 128 + 9},
+		{"SIGTERM", "kill -15 $$", 128 + 15},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			exitCode, err := reaper.ForkReap(
+				reaper.WithExecArgs("/bin/sh", "-c", tt.command),
+			)
+			require.NoError(t, err)
+			require.Equal(t, tt.expectedCode, exitCode, "exit code mismatch for %q", tt.command)
+		})
+	}
+}
+
 //nolint:paralleltest // Signal handling.
 func TestReapInterrupt(t *testing.T) {
 	// Don't run the reaper test in CI. It does weird
@@ -84,13 +115,17 @@ func TestReapInterrupt(t *testing.T) {
 	defer signal.Stop(usrSig)
 
 	go func() {
-		errC <- reaper.ForkReap(
+		exitCode, err := reaper.ForkReap(
 			reaper.WithPIDCallback(pids),
 			reaper.WithCatchSignals(os.Interrupt),
 			// Signal propagation does not extend to children of children, so
 			// we create a little bash script to ensure sleep is interrupted.
 			reaper.WithExecArgs("/bin/sh", "-c", fmt.Sprintf("pid=0; trap 'kill -USR2 %d; kill -TERM $pid' INT; sleep 10 &\npid=$!; kill -USR1 %d; wait", os.Getpid(), os.Getpid())),
 		)
+		// The child exits with 128 + SIGTERM (15) = 143, but the trap catches
+		// SIGINT and sends SIGTERM to the sleep process, so exit code varies.
+		_ = exitCode
+		errC <- err
 	}()
 
 	require.Equal(t, <-usrSig, syscall.SIGUSR1)

agent/reaper/reaper_unix.go

@@ -40,7 +40,10 @@ func catchSignals(pid int, sigs []os.Signal) {
 // the reaper and an exec.Command waiting for its process to complete.
 // The provided 'pids' channel may be nil if the caller does not care about the
 // reaped children PIDs.
-func ForkReap(opt ...Option) error {
+//
+// Returns the child's exit code (using 128+signal for signal termination)
+// and any error from Wait4.
+func ForkReap(opt ...Option) (int, error) {
 	opts := &options{
 		ExecArgs: os.Args,
 	}
@@ -53,7 +56,7 @@ func ForkReap(opt ...Option) error {
 
 	pwd, err := os.Getwd()
 	if err != nil {
-		return xerrors.Errorf("get wd: %w", err)
+		return 1, xerrors.Errorf("get wd: %w", err)
 	}
 
 	pattrs := &syscall.ProcAttr{
@@ -72,7 +75,7 @@ func ForkReap(opt ...Option) error {
 	//#nosec G204
 	pid, err := syscall.ForkExec(opts.ExecArgs[0], opts.ExecArgs, pattrs)
 	if err != nil {
-		return xerrors.Errorf("fork exec: %w", err)
+		return 1, xerrors.Errorf("fork exec: %w", err)
 	}
 
 	go catchSignals(pid, opts.CatchSignals)
@@ -82,5 +85,18 @@ func ForkReap(opt ...Option) error {
 	for xerrors.Is(err, syscall.EINTR) {
 		_, err = syscall.Wait4(pid, &wstatus, 0, nil)
 	}
-	return err
+
+	// Convert wait status to exit code using standard Unix conventions:
+	// - Normal exit: use the exit code
+	// - Signal termination: use 128 + signal number
+	var exitCode int
+	switch {
+	case wstatus.Exited():
+		exitCode = wstatus.ExitStatus()
+	case wstatus.Signaled():
+		exitCode = 128 + int(wstatus.Signal())
+	default:
+		exitCode = 1
+	}
+	return exitCode, err
 }

cli/agent.go

@@ -136,7 +136,7 @@ func workspaceAgent() *serpent.Command {
 				// to do this else we fork bomb ourselves.
 				//nolint:gocritic
 				args := append(os.Args, "--no-reap")
-				err := reaper.ForkReap(
+				exitCode, err := reaper.ForkReap(
 					reaper.WithExecArgs(args...),
 					reaper.WithCatchSignals(StopSignals...),
 				)
@@ -145,8 +145,8 @@ func workspaceAgent() *serpent.Command {
 					return xerrors.Errorf("fork reap: %w", err)
 				}
 
-				logger.Info(ctx, "reaper process exiting")
-				return nil
+				logger.Info(ctx, "reaper child process exited", slog.F("exit_code", exitCode))
+				return ExitError(exitCode, nil)
 			}
 
 			// Handle interrupt signals to allow for graceful shutdown,