fix(cli): allow coder ssh --stdio to exit when parent process dies (#21583)

Relates to https://github.com/coder/internal/issues/1217

Adds a background goroutine in `--stdio` mode to check if the parent PID
is still alive and exit if it is no longer present.

🤖 Implemented using Mux + Claude Opus 4.5, reviewed and refactored by
me.

cli/ssh.go

@@ -24,6 +24,7 @@ import (
 	"github.com/gofrs/flock"
 	"github.com/google/uuid"
 	"github.com/mattn/go-isatty"
+	"github.com/shirou/gopsutil/v4/process"
 	"github.com/spf13/afero"
 	gossh "golang.org/x/crypto/ssh"
 	gosshagent "golang.org/x/crypto/ssh/agent"
@@ -84,6 +85,9 @@ func (r *RootCmd) ssh() *serpent.Command {
 
 		containerName string
 		containerUser string
+
+		// Used in tests to simulate the parent exiting.
+		testForcePPID int64
 	)
 	cmd := &serpent.Command{
 		Annotations: workspaceCommand,
@@ -175,6 +179,24 @@ func (r *RootCmd) ssh() *serpent.Command {
 			ctx, cancel := context.WithCancel(ctx)
 			defer cancel()
 
+			// When running as a ProxyCommand (stdio mode), monitor the parent process
+			// and exit if it dies to avoid leaving orphaned processes. This is
+			// particularly important when editors like VSCode/Cursor spawn SSH
+			// connections and then crash or are killed - we don't want zombie
+			// `coder ssh` processes accumulating.
+			// Note: using gopsutil to check the parent process as this handles
+			// windows processes as well in a standard way.
+			if stdio {
+				ppid := int32(os.Getppid())             // nolint:gosec
+				checkParentInterval := 10 * time.Second // Arbitrary interval to not be too frequent
+				if testForcePPID > 0 {
+					ppid = int32(testForcePPID)                  // nolint:gosec
+					checkParentInterval = 100 * time.Millisecond // Shorter interval for testing
+				}
+				ctx, cancel = watchParentContext(ctx, quartz.NewReal(), ppid, process.PidExistsWithContext, checkParentInterval)
+				defer cancel()
+			}
+
 			// Prevent unnecessary logs from the stdlib from messing up the TTY.
 			// See: https://github.com/coder/coder/issues/13144
 			log.SetOutput(io.Discard)
@@ -775,6 +797,12 @@ func (r *RootCmd) ssh() *serpent.Command {
 			Value:       serpent.BoolOf(&forceNewTunnel),
 			Hidden:      true,
 		},
+		{
+			Flag:        "test.force-ppid",
+			Description: "Override the parent process ID to simulate a different parent process. ONLY USE THIS IN TESTS.",
+			Value:       serpent.Int64Of(&testForcePPID),
+			Hidden:      true,
+		},
 		sshDisableAutostartOption(serpent.BoolOf(&disableAutostart)),
 	}
 	return cmd
@@ -1662,3 +1690,33 @@ func normalizeWorkspaceInput(input string) string {
 		return input // Fallback
 	}
 }
+
+// watchParentContext returns a context that is canceled when the parent process
+// dies. It polls using the provided clock and checks if the parent is alive
+// using the provided pidExists function.
+func watchParentContext(ctx context.Context, clock quartz.Clock, originalPPID int32, pidExists func(context.Context, int32) (bool, error), interval time.Duration) (context.Context, context.CancelFunc) {
+	ctx, cancel := context.WithCancel(ctx) // intentionally shadowed
+
+	go func() {
+		ticker := clock.NewTicker(interval)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				alive, err := pidExists(ctx, originalPPID)
+				// If we get an error checking the parent process (e.g., permission
+				// denied, the process is in an unknown state), we assume the parent
+				// is still alive to avoid disrupting the SSH connection. We only
+				// cancel when we definitively know the parent is gone (alive=false, err=nil).
+				if !alive && err == nil {
+					cancel()
+					return
+				}
+			}
+		}
+	}()
+
+	return ctx, cancel
+}

cli/ssh_internal_test.go

@@ -312,6 +312,102 @@ type fakeCloser struct {
 	err    error
 }
 
+func TestWatchParentContext(t *testing.T) {
+	t.Parallel()
+
+	t.Run("CancelsWhenParentDies", func(t *testing.T) {
+		t.Parallel()
+		ctx := testutil.Context(t, testutil.WaitShort)
+		mClock := quartz.NewMock(t)
+		trap := mClock.Trap().NewTicker()
+		defer trap.Close()
+
+		parentAlive := true
+		childCtx, cancel := watchParentContext(ctx, mClock, 1234, func(context.Context, int32) (bool, error) {
+			return parentAlive, nil
+		}, testutil.WaitShort)
+		defer cancel()
+
+		// Wait for the ticker to be created
+		trap.MustWait(ctx).MustRelease(ctx)
+
+		// When: we simulate parent death and advance the clock
+		parentAlive = false
+		mClock.AdvanceNext()
+
+		// Then: The context should be canceled
+		_ = testutil.TryReceive(ctx, t, childCtx.Done())
+	})
+
+	t.Run("DoesNotCancelWhenParentAlive", func(t *testing.T) {
+		t.Parallel()
+		ctx := testutil.Context(t, testutil.WaitShort)
+		mClock := quartz.NewMock(t)
+		trap := mClock.Trap().NewTicker()
+		defer trap.Close()
+
+		childCtx, cancel := watchParentContext(ctx, mClock, 1234, func(context.Context, int32) (bool, error) {
+			return true, nil // Parent always alive
+		}, testutil.WaitShort)
+		defer cancel()
+
+		// Wait for the ticker to be created
+		trap.MustWait(ctx).MustRelease(ctx)
+
+		// When: we advance the clock several times with the parent alive
+		for range 3 {
+			mClock.AdvanceNext()
+		}
+
+		// Then: context should not be canceled
+		require.NoError(t, childCtx.Err())
+	})
+
+	t.Run("RespectsParentContext", func(t *testing.T) {
+		t.Parallel()
+		ctx, cancelParent := context.WithCancel(context.Background())
+		mClock := quartz.NewMock(t)
+
+		childCtx, cancel := watchParentContext(ctx, mClock, 1234, func(context.Context, int32) (bool, error) {
+			return true, nil
+		}, testutil.WaitShort)
+		defer cancel()
+
+		// When: we cancel the parent context
+		cancelParent()
+
+		// Then: The context should be canceled
+		require.ErrorIs(t, childCtx.Err(), context.Canceled)
+	})
+
+	t.Run("DoesNotCancelOnError", func(t *testing.T) {
+		t.Parallel()
+		ctx := testutil.Context(t, testutil.WaitShort)
+		mClock := quartz.NewMock(t)
+		trap := mClock.Trap().NewTicker()
+		defer trap.Close()
+
+		// Simulate an error checking parent status (e.g., permission denied).
+		// We should not cancel the context in this case to avoid disrupting
+		// the SSH connection.
+		childCtx, cancel := watchParentContext(ctx, mClock, 1234, func(context.Context, int32) (bool, error) {
+			return false, xerrors.New("permission denied")
+		}, testutil.WaitShort)
+		defer cancel()
+
+		// Wait for the ticker to be created
+		trap.MustWait(ctx).MustRelease(ctx)
+
+		// When: we advance clock several times
+		for range 3 {
+			mClock.AdvanceNext()
+		}
+
+		// Context should NOT be canceled since we got an error (not a definitive "not alive")
+		require.NoError(t, childCtx.Err(), "context was canceled even though pidExists returned an error")
+	})
+}
+
 func (c *fakeCloser) Close() error {
 	*c.closes = append(*c.closes, c)
 	return c.err

cli/ssh_test.go

@@ -1122,6 +1122,97 @@ func TestSSH(t *testing.T) {
 		}
 	})
 
+	// This test ensures that the SSH session exits when the parent process dies.
+	t.Run("StdioExitOnParentDeath", func(t *testing.T) {
+		t.Parallel()
+
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+		defer cancel()
+
+		// sleepStart -> agentReady -> sessionStarted -> sleepKill -> sleepDone -> cmdDone
+		sleepStart := make(chan int)
+		agentReady := make(chan struct{})
+		sessionStarted := make(chan struct{})
+		sleepKill := make(chan struct{})
+		sleepDone := make(chan struct{})
+
+		// Start a sleep process which we will pretend is the parent.
+		go func() {
+			sleepCmd := exec.Command("sleep", "infinity")
+			if !assert.NoError(t, sleepCmd.Start(), "failed to start sleep command") {
+				return
+			}
+			sleepStart <- sleepCmd.Process.Pid
+			defer close(sleepDone)
+			<-sleepKill
+			sleepCmd.Process.Kill()
+			_ = sleepCmd.Wait()
+		}()
+
+		client, workspace, agentToken := setupWorkspaceForAgent(t)
+		go func() {
+			defer close(agentReady)
+			_ = agenttest.New(t, client.URL, agentToken)
+			coderdtest.NewWorkspaceAgentWaiter(t, client, workspace.ID).WaitFor(coderdtest.AgentsReady)
+		}()
+
+		clientOutput, clientInput := io.Pipe()
+		serverOutput, serverInput := io.Pipe()
+		defer func() {
+			for _, c := range []io.Closer{clientOutput, clientInput, serverOutput, serverInput} {
+				_ = c.Close()
+			}
+		}()
+
+		// Start a connection to the agent once it's ready
+		go func() {
+			<-agentReady
+			conn, channels, requests, err := ssh.NewClientConn(&testutil.ReaderWriterConn{
+				Reader: serverOutput,
+				Writer: clientInput,
+			}, "", &ssh.ClientConfig{
+				// #nosec
+				HostKeyCallback: ssh.InsecureIgnoreHostKey(),
+			})
+			if !assert.NoError(t, err, "failed to create SSH client connection") {
+				return
+			}
+			defer conn.Close()
+
+			sshClient := ssh.NewClient(conn, channels, requests)
+			defer sshClient.Close()
+
+			session, err := sshClient.NewSession()
+			if !assert.NoError(t, err, "failed to create SSH session") {
+				return
+			}
+			close(sessionStarted)
+			<-sleepDone
+			assert.NoError(t, session.Close())
+		}()
+
+		// Wait for our "parent" process to start
+		sleepPid := testutil.RequireReceive(ctx, t, sleepStart)
+		// Wait for the agent to be ready
+		testutil.SoftTryReceive(ctx, t, agentReady)
+		inv, root := clitest.New(t, "ssh", "--stdio", workspace.Name, "--test.force-ppid", fmt.Sprintf("%d", sleepPid))
+		clitest.SetupConfig(t, client, root)
+		inv.Stdin = clientOutput
+		inv.Stdout = serverInput
+		inv.Stderr = io.Discard
+
+		// Start the command
+		clitest.Start(t, inv.WithContext(ctx))
+
+		// Wait for a session to be established
+		testutil.SoftTryReceive(ctx, t, sessionStarted)
+		// Now kill the fake "parent"
+		close(sleepKill)
+		// The sleep process should exit
+		testutil.SoftTryReceive(ctx, t, sleepDone)
+		// And then the command should exit. This is tracked by clitest.Start.
+	})
+
 	t.Run("ForwardAgent", func(t *testing.T) {
 		if runtime.GOOS == "windows" {
 			t.Skip("Test not supported on windows")