fix(agent): ensure SSH server shutdown with process groups (#17227)

Fix hanging workspace shutdowns caused by orphaned SSH child processes.
Key changes:

- Create process groups for non-PTY SSH sessions
- Send SIGHUP to entire process group for proper termination
- Add 5-second timeout to prevent indefinite blocking

Fixes #17108

agent/agent.go

@@ -1773,15 +1773,22 @@ func (a *agent) Close() error {
 	a.setLifecycle(codersdk.WorkspaceAgentLifecycleShuttingDown)
 
 	// Attempt to gracefully shut down all active SSH connections and
-	// stop accepting new ones.
-	err := a.sshServer.Shutdown(a.hardCtx)
+	// stop accepting new ones. If all processes have not exited after 5
+	// seconds, we just log it and move on as it's more important to run
+	// the shutdown scripts. A typical shutdown time for containers is
+	// 10 seconds, so this still leaves a bit of time to run the
+	// shutdown scripts in the worst-case.
+	sshShutdownCtx, sshShutdownCancel := context.WithTimeout(a.hardCtx, 5*time.Second)
+	defer sshShutdownCancel()
+	err := a.sshServer.Shutdown(sshShutdownCtx)
 	if err != nil {
-		a.logger.Error(a.hardCtx, "ssh server shutdown", slog.Error(err))
-	}
-	err = a.sshServer.Close()
-	if err != nil {
-		a.logger.Error(a.hardCtx, "ssh server close", slog.Error(err))
+		if errors.Is(err, context.DeadlineExceeded) {
+			a.logger.Warn(sshShutdownCtx, "ssh server shutdown timeout", slog.Error(err))
+		} else {
+			a.logger.Error(sshShutdownCtx, "ssh server shutdown", slog.Error(err))
+		}
 	}
+
 	// wait for SSH to shut down before the general graceful cancel, because
 	// this triggers a disconnect in the tailnet layer, telling all clients to
 	// shut down their wireguard tunnels to us. If SSH sessions are still up,

agent/agentssh/agentssh.go

@@ -582,6 +582,12 @@ func (s *Server) sessionStart(logger slog.Logger, session ssh.Session, env []str
 func (s *Server) startNonPTYSession(logger slog.Logger, session ssh.Session, magicTypeLabel string, cmd *exec.Cmd) error {
 	s.metrics.sessionsTotal.WithLabelValues(magicTypeLabel, "no").Add(1)
 
+	// Create a process group and send SIGHUP to child processes,
+	// otherwise context cancellation will not propagate properly
+	// and SSH server close may be delayed.
+	cmd.SysProcAttr = cmdSysProcAttr()
+	cmd.Cancel = cmdCancel(session.Context(), logger, cmd)
+
 	cmd.Stdout = session
 	cmd.Stderr = session.Stderr()
 	// This blocks forever until stdin is received if we don't
@@ -926,7 +932,12 @@ func (s *Server) CreateCommand(ctx context.Context, script string, env []string,
 // Serve starts the server to handle incoming connections on the provided listener.
 // It returns an error if no host keys are set or if there is an issue accepting connections.
 func (s *Server) Serve(l net.Listener) (retErr error) {
-	if len(s.srv.HostSigners) == 0 {
+	// Ensure we're not mutating HostSigners as we're reading it.
+	s.mu.RLock()
+	noHostKeys := len(s.srv.HostSigners) == 0
+	s.mu.RUnlock()
+
+	if noHostKeys {
 		return xerrors.New("no host keys set")
 	}
 
@@ -1054,27 +1065,36 @@ func (s *Server) Close() error {
 	}
 	s.closing = make(chan struct{})
 
+	ctx := context.Background()
+
+	s.logger.Debug(ctx, "closing server")
+
+	// Stop accepting new connections.
+	s.logger.Debug(ctx, "closing all active listeners", slog.F("count", len(s.listeners)))
+	for l := range s.listeners {
+		_ = l.Close()
+	}
+
 	// Close all active sessions to gracefully
 	// terminate client connections.
+	s.logger.Debug(ctx, "closing all active sessions", slog.F("count", len(s.sessions)))
 	for ss := range s.sessions {
 		// We call Close on the underlying channel here because we don't
 		// want to send an exit status to the client (via Exit()).
 		// Typically OpenSSH clients will return 255 as the exit status.
 		_ = ss.Close()
 	}
-
-	// Close all active listeners and connections.
-	for l := range s.listeners {
-		_ = l.Close()
-	}
+	s.logger.Debug(ctx, "closing all active connections", slog.F("count", len(s.conns)))
 	for c := range s.conns {
 		_ = c.Close()
 	}
 
-	// Close the underlying SSH server.
+	s.logger.Debug(ctx, "closing SSH server")
 	err := s.srv.Close()
 
 	s.mu.Unlock()
+
+	s.logger.Debug(ctx, "waiting for all goroutines to exit")
 	s.wg.Wait() // Wait for all goroutines to exit.
 
 	s.mu.Lock()
@@ -1082,15 +1102,35 @@ func (s *Server) Close() error {
 	s.closing = nil
 	s.mu.Unlock()
 
+	s.logger.Debug(ctx, "closing server done")
+
 	return err
 }
 
-// Shutdown gracefully closes all active SSH connections and stops
-// accepting new connections.
-//
-// Shutdown is not implemented.
-func (*Server) Shutdown(_ context.Context) error {
-	// TODO(mafredri): Implement shutdown, SIGHUP running commands, etc.
+// Shutdown stops accepting new connections. The current implementation
+// calls Close() for simplicity instead of waiting for existing
+// connections to close. If the context times out, Shutdown will return
+// but Close() may not have completed.
+func (s *Server) Shutdown(ctx context.Context) error {
+	ch := make(chan error, 1)
+	go func() {
+		// TODO(mafredri): Implement shutdown, SIGHUP running commands, etc.
+		// For now we just close the server.
+		ch <- s.Close()
+	}()
+	var err error
+	select {
+	case <-ctx.Done():
+		err = ctx.Err()
+	case err = <-ch:
+	}
+	// Re-check for context cancellation precedence.
+	if ctx.Err() != nil {
+		err = ctx.Err()
+	}
+	if err != nil {
+		return xerrors.Errorf("close server: %w", err)
+	}
 	return nil
 }
 

agent/agentssh/agentssh_test.go

@@ -21,6 +21,7 @@ import (
 	"go.uber.org/goleak"
 	"golang.org/x/crypto/ssh"
 
+	"cdr.dev/slog"
 	"cdr.dev/slog/sloggers/slogtest"
 
 	"github.com/coder/coder/v2/agent/agentexec"
@@ -147,51 +148,92 @@ func (*fakeEnvInfoer) ModifyCommand(cmd string, args ...string) (string, []strin
 func TestNewServer_CloseActiveConnections(t *testing.T) {
 	t.Parallel()
 
-	ctx := context.Background()
-	logger := slogtest.Make(t, &slogtest.Options{IgnoreErrors: true})
-	s, err := agentssh.NewServer(ctx, logger, prometheus.NewRegistry(), afero.NewMemMapFs(), agentexec.DefaultExecer, nil)
-	require.NoError(t, err)
-	defer s.Close()
-	err = s.UpdateHostSigner(42)
-	assert.NoError(t, err)
-
-	ln, err := net.Listen("tcp", "127.0.0.1:0")
-	require.NoError(t, err)
-
-	var wg sync.WaitGroup
-	wg.Add(2)
-	go func() {
-		defer wg.Done()
-		err := s.Serve(ln)
-		assert.Error(t, err) // Server is closed.
-	}()
-
-	pty := ptytest.New(t)
-
-	doClose := make(chan struct{})
-	go func() {
-		defer wg.Done()
-		c := sshClient(t, ln.Addr().String())
-		sess, err := c.NewSession()
-		assert.NoError(t, err)
-		sess.Stdin = pty.Input()
-		sess.Stdout = pty.Output()
-		sess.Stderr = pty.Output()
-
-		assert.NoError(t, err)
-		err = sess.Start("")
+	prepare := func(ctx context.Context, t *testing.T) (*agentssh.Server, func()) {
+		t.Helper()
+		logger := slogtest.Make(t, &slogtest.Options{IgnoreErrors: true}).Leveled(slog.LevelDebug)
+		s, err := agentssh.NewServer(ctx, logger, prometheus.NewRegistry(), afero.NewMemMapFs(), agentexec.DefaultExecer, nil)
+		require.NoError(t, err)
+		defer s.Close()
+		err = s.UpdateHostSigner(42)
 		assert.NoError(t, err)
 
-		close(doClose)
-		err = sess.Wait()
-		assert.Error(t, err)
-	}()
+		ln, err := net.Listen("tcp", "127.0.0.1:0")
+		require.NoError(t, err)
 
-	<-doClose
-	err = s.Close()
-	require.NoError(t, err)
+		waitConns := make([]chan struct{}, 4)
 
-	wg.Wait()
+		var wg sync.WaitGroup
+		wg.Add(1 + len(waitConns))
+
+		go func() {
+			defer wg.Done()
+			err := s.Serve(ln)
+			assert.Error(t, err) // Server is closed.
+		}()
+
+		for i := 0; i < len(waitConns); i++ {
+			waitConns[i] = make(chan struct{})
+			go func(ch chan struct{}) {
+				defer wg.Done()
+				c := sshClient(t, ln.Addr().String())
+				sess, err := c.NewSession()
+				assert.NoError(t, err)
+				pty := ptytest.New(t)
+				sess.Stdin = pty.Input()
+				sess.Stdout = pty.Output()
+				sess.Stderr = pty.Output()
+
+				// Every other session will request a PTY.
+				if i%2 == 0 {
+					err = sess.RequestPty("xterm", 80, 80, nil)
+					assert.NoError(t, err)
+				}
+				// The 60 seconds here is intended to be longer than the
+				// test. The shutdown should propagate.
+				err = sess.Start("/bin/bash -c 'trap \"sleep 60\" SIGTERM; sleep 60'")
+				assert.NoError(t, err)
+
+				close(ch)
+				err = sess.Wait()
+				assert.Error(t, err)
+			}(waitConns[i])
+		}
+
+		for _, ch := range waitConns {
+			<-ch
+		}
+
+		return s, wg.Wait
+	}
+
+	t.Run("Close", func(t *testing.T) {
+		t.Parallel()
+		ctx := testutil.Context(t, testutil.WaitMedium)
+		s, wait := prepare(ctx, t)
+		err := s.Close()
+		require.NoError(t, err)
+		wait()
+	})
+
+	t.Run("Shutdown", func(t *testing.T) {
+		t.Parallel()
+		ctx := testutil.Context(t, testutil.WaitMedium)
+		s, wait := prepare(ctx, t)
+		err := s.Shutdown(ctx)
+		require.NoError(t, err)
+		wait()
+	})
+
+	t.Run("Shutdown Early", func(t *testing.T) {
+		t.Parallel()
+		ctx := testutil.Context(t, testutil.WaitMedium)
+		s, wait := prepare(ctx, t)
+		ctx, cancel := context.WithCancel(ctx)
+		cancel()
+		err := s.Shutdown(ctx)
+		require.ErrorIs(t, err, context.Canceled)
+		wait()
+	})
 }
 
 func TestNewServer_Signal(t *testing.T) {

agent/agentssh/exec_other.go

@@ -0,0 +1,24 @@
+//go:build !windows
+
+package agentssh
+
+import (
+	"context"
+	"os/exec"
+	"syscall"
+
+	"cdr.dev/slog"
+)
+
+func cmdSysProcAttr() *syscall.SysProcAttr {
+	return &syscall.SysProcAttr{
+		Setsid: true,
+	}
+}
+
+func cmdCancel(ctx context.Context, logger slog.Logger, cmd *exec.Cmd) func() error {
+	return func() error {
+		logger.Debug(ctx, "cmdCancel: sending SIGHUP to process and children", slog.F("pid", cmd.Process.Pid))
+		return syscall.Kill(-cmd.Process.Pid, syscall.SIGHUP)
+	}
+}

agent/agentssh/exec_windows.go

@@ -0,0 +1,21 @@
+package agentssh
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"syscall"
+
+	"cdr.dev/slog"
+)
+
+func cmdSysProcAttr() *syscall.SysProcAttr {
+	return &syscall.SysProcAttr{}
+}
+
+func cmdCancel(ctx context.Context, logger slog.Logger, cmd *exec.Cmd) func() error {
+	return func() error {
+		logger.Debug(ctx, "cmdCancel: sending interrupt to process", slog.F("pid", cmd.Process.Pid))
+		return cmd.Process.Signal(os.Interrupt)
+	}
+}