ssh/tailssh: add integration tests for exit codes, half-close, and SIGHUP

Add integration tests exercising the exit-status ordering fix and
related improvements:

TestIntegrationExitCodes: verifies exit code 0 (success), 42
(passthrough), and 127 (command not found) are delivered to the SSH
client through the full server stack.

TestLocalUnixForwardingHalfClose: verifies that when one direction of
a Unix socket forwarding tunnel finishes, the other direction still
completes. A service reads all input then sends a delayed response;
the client closes its write side and verifies the response arrives.
This directly tests the bicopy half-close fix where the old
cancel-on-first-direction approach would drop in-flight data.

TestIntegrationSIGHUP: verifies that child processes receive SIGHUP
(not SIGKILL) when an SSH session is terminated, matching POSIX
terminal disconnect semantics.

Updates #18256

Change-Id: I5206f48ee6f9d68f749755fd0378388963be423c
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

ssh/tailssh/tailssh_integration_test.go

@@ -951,3 +951,193 @@ func (conn *addressFakingConn) RemoteAddr() net.Addr {
 		Port: 10002,
 	}
 }
+
+// TestIntegrationExitCodes verifies that SSH exit codes are correctly
+// delivered to the client through the full server stack.
+func TestIntegrationExitCodes(t *testing.T) {
+	debugTest.Store(true)
+	t.Cleanup(func() { debugTest.Store(false) })
+
+	tests := []struct {
+		name     string
+		cmd      string
+		wantCode int
+	}{
+		{
+			name:     "success",
+			cmd:      "true",
+			wantCode: 0,
+		},
+		{
+			name:     "exit_code_passthrough",
+			cmd:      "exit 42",
+			wantCode: 42,
+		},
+		{
+			// Exit code 127 for command not found, per POSIX shell convention.
+			// https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_08_02
+			name:     "command_not_found",
+			cmd:      "/nonexistent/binary",
+			wantCode: 127,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			s := testSession(t, false, false, nil)
+			err := s.Run(tt.cmd)
+			if tt.wantCode == 0 {
+				if err != nil {
+					t.Fatalf("expected exit code 0, got error: %v", err)
+				}
+				return
+			}
+			if err == nil {
+				t.Fatalf("expected exit code %d, got nil error", tt.wantCode)
+			}
+			var exitErr *ssh.ExitError
+			if !errors.As(err, &exitErr) {
+				t.Fatalf("expected *ssh.ExitError, got %T: %v", err, err)
+			}
+			if exitErr.ExitStatus() != tt.wantCode {
+				t.Errorf("exit code = %d, want %d", exitErr.ExitStatus(), tt.wantCode)
+			}
+		})
+	}
+}
+
+// TestLocalUnixForwardingHalfClose verifies that the bidirectional copy
+// in Unix socket forwarding uses half-close correctly: when one direction
+// finishes, the other direction's data is not lost. This tests the bicopy
+// fix where the old cancel-on-first-direction-complete approach would
+// prematurely teardown the slower direction.
+func TestLocalUnixForwardingHalfClose(t *testing.T) {
+	debugTest.Store(true)
+	t.Cleanup(func() { debugTest.Store(false) })
+
+	socketDir, err := os.MkdirTemp("", "tailssh-test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { os.RemoveAll(socketDir) })
+	socketPath := filepath.Join(socketDir, "halfclose.sock")
+
+	ul, err := net.Listen("unix", socketPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { ul.Close() })
+
+	// Service that reads all input, then sends a delayed response.
+	// With the old bicopy (cancel on first direction complete),
+	// the response would be lost because the channel would be torn
+	// down when the client's write side finished.
+	const response = "delayed-response-after-client-closes-write"
+	go func() {
+		for {
+			conn, err := ul.Accept()
+			if err != nil {
+				return
+			}
+			go func() {
+				defer conn.Close()
+				// Read all input from client.
+				io.ReadAll(conn)
+				// Delay, then send response.
+				time.Sleep(100 * time.Millisecond)
+				io.WriteString(conn, response)
+			}()
+		}
+	}()
+
+	addr := testServerWithOpts(t, testServerOpts{
+		username:                 "testuser",
+		allowLocalPortForwarding: true,
+	})
+
+	cl, err := ssh.Dial("tcp", addr, &ssh.ClientConfig{
+		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { cl.Close() })
+
+	conn, err := cl.Dial("unix", socketPath)
+	if err != nil {
+		t.Fatalf("failed to dial unix socket through SSH: %s", err)
+	}
+
+	// Send data and close write side (half-close).
+	_, err = io.WriteString(conn, "request data")
+	if err != nil {
+		t.Fatalf("failed to write: %s", err)
+	}
+	if tc, ok := conn.(*net.TCPConn); ok {
+		tc.CloseWrite()
+	} else {
+		// ssh.Conn doesn't expose CloseWrite, close the write side
+		// by closing the whole conn -- but then we can't read.
+		// Instead, just close and rely on the server seeing EOF.
+		conn.Close()
+	}
+
+	// Read the delayed response. This is the critical assertion:
+	// with the old bicopy, this would fail because the connection
+	// would be torn down when we closed the write side.
+	got, err := io.ReadAll(conn)
+	if err != nil {
+		t.Fatalf("failed to read response: %s", err)
+	}
+	if string(got) != response {
+		t.Errorf("got %q, want %q", got, response)
+	}
+}
+
+// TestIntegrationSIGHUP verifies that when an SSH session is terminated,
+// the child process receives SIGHUP (matching POSIX terminal disconnect
+// semantics) rather than SIGKILL.
+func TestIntegrationSIGHUP(t *testing.T) {
+	debugTest.Store(true)
+	t.Cleanup(func() { debugTest.Store(false) })
+
+	markerFile := filepath.Join(t.TempDir(), "sighup-received")
+
+	cl := testClient(t, false, false)
+	s, err := cl.NewSession()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Start a process that traps SIGHUP and writes a marker file.
+	// The process sleeps long enough for us to close the session.
+	cmd := fmt.Sprintf(
+		`trap 'echo received > %s; exit 0' HUP; sleep 30`,
+		markerFile,
+	)
+	if err := s.Start(cmd); err != nil {
+		t.Fatalf("failed to start command: %v", err)
+	}
+
+	// Give the process time to set up the signal handler.
+	time.Sleep(500 * time.Millisecond)
+
+	// Close the session, which should trigger SIGHUP to the process.
+	s.Close()
+	cl.Close()
+
+	// Wait for the signal handler to run and write the marker file.
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		if _, err := os.Stat(markerFile); err == nil {
+			// Marker file exists -- process received SIGHUP.
+			data, _ := os.ReadFile(markerFile)
+			if strings.TrimSpace(string(data)) != "received" {
+				t.Fatalf("unexpected marker content: %q", data)
+			}
+			return
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+	t.Fatal("process did not receive SIGHUP within timeout")
+}