gh-142654: show the clear error message when sampling on an unknown PID

Lib/profiling/sampling/__main__.py

@@ -46,6 +46,7 @@
 """
 
 from .cli import main
+from .errors import SamplingUnknownProcessError, SamplingModuleNotFoundError, SamplingScriptNotFoundError
 
 def handle_permission_error():
  """Handle PermissionError by displaying appropriate error message."""
@@ -64,3 +65,9 @@ def handle_permission_error():
  main()
  except PermissionError:
  handle_permission_error()
+ except SamplingUnknownProcessError as err:
+ print(f"Tachyon cannot find the process: {err}", file=sys.stderr)
+ sys.exit(1)
+ except (SamplingModuleNotFoundError, SamplingScriptNotFoundError) as err:
+ print(f"Tachyon cannot find the target: {err}", file=sys.stderr)
+ sys.exit(1)

Lib/profiling/sampling/cli.py

@@ -10,7 +10,8 @@
 import time
 from contextlib import nullcontext
 
-from .sample import sample, sample_live
+from .errors import SamplingUnknownProcessError, SamplingModuleNotFoundError, SamplingScriptNotFoundError
+from .sample import sample, sample_live, _is_process_running
 from .pstats_collector import PstatsCollector
 from .stack_collector import CollapsedStackCollector, FlamegraphCollector
 from .heatmap_collector import HeatmapCollector
@@ -743,6 +744,8 @@ def main():
 
 def _handle_attach(args):
  """Handle the 'attach' command."""
+ if not _is_process_running(args.pid):
+ raise SamplingUnknownProcessError(args.pid)
  # Check if live mode is requested
  if args.live:
  _handle_live_attach(args, args.pid)
@@ -792,13 +795,13 @@ def _handle_run(args):
  added_cwd = True
  try:
  if importlib.util.find_spec(args.target) is None:
-sys.exit(f"Error: Module not found: {args.target}")
+raise SamplingModuleNotFoundError(args.target)
  finally:
  if added_cwd:
  sys.path.remove(cwd)
  else:
  if not os.path.exists(args.target):
-sys.exit(f"Error: Script not found: {args.target}")
+raise SamplingScriptNotFoundError(args.target)
 
  # Check if live mode is requested
  if args.live:

Lib/profiling/sampling/errors.py

@@ -0,0 +1,19 @@
+"""Custom exceptions for the sampling profiler."""
+
+class SamplingProfilerError(Exception):
+ """Base exception for sampling profiler errors."""
+
+class SamplingUnknownProcessError(SamplingProfilerError):
+ def __init__(self, pid):
+ self.pid = pid
+ super().__init__(f"Process with PID '{pid}' does not exist.")
+
+class SamplingScriptNotFoundError(SamplingProfilerError):
+ def __init__(self, script_path):
+ self.script_path = script_path
+ super().__init__(f"Script '{script_path}' not found.")
+
+class SamplingModuleNotFoundError(SamplingProfilerError):
+ def __init__(self, module_name):
+ self.module_name = module_name
+ super().__init__(f"Module '{module_name}' not found.")

Lib/profiling/sampling/sample.py

@@ -34,23 +34,29 @@ def __init__(self, pid, sample_interval_usec, all_threads, *, mode=PROFILING_MOD
  self.all_threads = all_threads
  self.mode = mode # Store mode for later use
  self.collect_stats = collect_stats
+ try:
+ self.unwinder = self._new_unwinder(native, gc, opcodes, skip_non_matching_threads)
+ except RuntimeError as err:
+ raise SystemExit(err) from err
+ # Track sample intervals and total sample count
+ self.sample_intervals = deque(maxlen=100)
+ self.total_samples = 0
+ self.realtime_stats = False
+
+ def _new_unwinder(self, native, gc, opcodes, skip_non_matching_threads):
  if _FREE_THREADED_BUILD:
-self.unwinder = _remote_debugging.RemoteUnwinder(
- self.pid, all_threads=self.all_threads, mode=mode, native=native, gc=gc,
+ unwinder = _remote_debugging.RemoteUnwinder(
+ self.pid, all_threads=self.all_threads, mode=self.mode, native=native, gc=gc,
  opcodes=opcodes, skip_non_matching_threads=skip_non_matching_threads,
- cache_frames=True, stats=collect_stats
+ cache_frames=True, stats=self.collect_stats
  )
  else:
- only_active_threads = bool(self.all_threads)
- self.unwinder = _remote_debugging.RemoteUnwinder(
- self.pid, only_active_thread=only_active_threads, mode=mode, native=native, gc=gc,
+ unwinder = _remote_debugging.RemoteUnwinder(
+ self.pid, only_active_thread=bool(self.all_threads), mode=self.mode, native=native, gc=gc,
  opcodes=opcodes, skip_non_matching_threads=skip_non_matching_threads,
- cache_frames=True, stats=collect_stats
+ cache_frames=True, stats=self.collect_stats
  )
- # Track sample intervals and total sample count
- self.sample_intervals = deque(maxlen=100)
- self.total_samples = 0
- self.realtime_stats = False
+ return unwinder
 
  def sample(self, collector, duration_sec=10, *, async_aware=False):
  sample_interval_sec = self.sample_interval_usec / 1_000_000
@@ -86,7 +92,7 @@ def sample(self, collector, duration_sec=10, *, async_aware=False):
  collector.collect_failed_sample()
  errors += 1
  except Exception as e:
- if not self._is_process_running():
+ if not _is_process_running(self.pid):
  break
  raise e from None
 
@@ -148,22 +154,6 @@ def sample(self, collector, duration_sec=10, *, async_aware=False):
  f"({(expected_samples - num_samples) / expected_samples * 100:.2f}%)"
  )
 
- def _is_process_running(self):
- if sys.platform == "linux" or sys.platform == "darwin":
- try:
- os.kill(self.pid, 0)
- return True
- except ProcessLookupError:
- return False
- elif sys.platform == "win32":
- try:
- _remote_debugging.RemoteUnwinder(self.pid)
- except Exception:
- return False
- return True
- else:
- raise ValueError(f"Unsupported platform: {sys.platform}")
-
  def _print_realtime_stats(self):
  """Print real-time sampling statistics."""
  if len(self.sample_intervals) < 2:
@@ -279,6 +269,28 @@ def _print_unwinder_stats(self):
  print(f" {ANSIColors.YELLOW}Stale cache invalidations: {stale_invalidations}{ANSIColors.RESET}")
 
 
+def _is_process_running(pid):
+ if pid <= 0:
+ return False
+ if os.name == "posix":
+ try:
+ os.kill(pid, 0)
+ return True
+ except ProcessLookupError:
+ return False
+ except PermissionError:
+ # EPERM means process exists but we can't signal it
+ return True
+ elif sys.platform == "win32":
+ try:
+ _remote_debugging.RemoteUnwinder(pid)
+ except Exception:
+ return False
+ return True
+ else:
+ raise ValueError(f"Unsupported platform: {sys.platform}")
+
+
 def sample(
  pid,
  collector,

Lib/test/test_profiling/test_sampling_profiler/test_cli.py

@@ -16,6 +16,7 @@
 from test.support import is_emscripten, requires_remote_subprocess_debugging
 
 from profiling.sampling.cli import main
+from profiling.sampling.errors import SamplingScriptNotFoundError, SamplingModuleNotFoundError, SamplingUnknownProcessError
 
 
 class TestSampleProfilerCLI(unittest.TestCase):
@@ -203,12 +204,12 @@ def test_cli_mutually_exclusive_pid_script(self):
  with (
  mock.patch("sys.argv", test_args),
  mock.patch("sys.stderr", io.StringIO()) as mock_stderr,
- self.assertRaises(SystemExit) as cm,
+ self.assertRaises(SamplingScriptNotFoundError) as cm,
  ):
  main()
 
  # Verify the error is about the non-existent script
- self.assertIn("12345", str(cm.exception.code))
+ self.assertIn("12345", str(cm.exception))
 
  def test_cli_no_target_specified(self):
  # In new CLI, must specify a subcommand
@@ -436,6 +437,7 @@ def test_cli_default_collapsed_filename(self):
 
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  main()
@@ -475,6 +477,7 @@ def test_cli_custom_output_filenames(self):
  for test_args, expected_filename, expected_format in test_cases:
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  main()
@@ -513,6 +516,7 @@ def test_argument_parsing_basic(self):
 
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  main()
@@ -534,6 +538,7 @@ def test_sort_options(self):
 
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  main()
@@ -547,6 +552,7 @@ def test_async_aware_flag_defaults_to_running(self):
 
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  main()
@@ -562,6 +568,7 @@ def test_async_aware_with_async_mode_all(self):
 
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  main()
@@ -576,6 +583,7 @@ def test_async_aware_default_is_none(self):
 
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  main()
@@ -697,14 +705,20 @@ def test_async_aware_incompatible_with_all_threads(self):
  def test_run_nonexistent_script_exits_cleanly(self):
  """Test that running a non-existent script exits with a clean error."""
  with mock.patch("sys.argv", ["profiling.sampling.cli", "run", "/nonexistent/script.py"]):
- with self.assertRaises(SystemExit) as cm:
+ with self.assertRaisesRegex(SamplingScriptNotFoundError, "Script '[\\w/.]+' not found."):
  main()
- self.assertIn("Script not found", str(cm.exception.code))
 
  @unittest.skipIf(is_emscripten, "subprocess not available")
  def test_run_nonexistent_module_exits_cleanly(self):
  """Test that running a non-existent module exits with a clean error."""
  with mock.patch("sys.argv", ["profiling.sampling.cli", "run", "-m", "nonexistent_module_xyz"]):
- with self.assertRaises(SystemExit) as cm:
+ with self.assertRaisesRegex(SamplingModuleNotFoundError, "Module '[\\w/.]+' not found."):
+ main()
+
+ def test_cli_attach_nonexistent_pid(self):
+ fake_pid = "99999"
+ with mock.patch("sys.argv", ["profiling.sampling.cli", "attach", fake_pid]):
+ with self.assertRaises(SamplingUnknownProcessError) as cm:
  main()
- self.assertIn("Module not found", str(cm.exception.code))
+
+ self.assertIn(fake_pid, str(cm.exception))

Lib/test/test_profiling/test_sampling_profiler/test_integration.py

@@ -17,7 +17,7 @@
  import profiling.sampling.sample
  from profiling.sampling.pstats_collector import PstatsCollector
  from profiling.sampling.stack_collector import CollapsedStackCollector
- from profiling.sampling.sample import SampleProfiler
+ from profiling.sampling.sample import SampleProfiler, _is_process_running
 except ImportError:
  raise unittest.SkipTest(
  "Test only runs when _remote_debugging is available"
@@ -602,7 +602,7 @@ def test_sample_target_module(self):
 @requires_remote_subprocess_debugging()
 class TestSampleProfilerErrorHandling(unittest.TestCase):
  def test_invalid_pid(self):
- with self.assertRaises((OSError, RuntimeError)):
+ with self.assertRaises((SystemExit, PermissionError)):
  collector = PstatsCollector(sample_interval_usec=100, skip_idle=False)
  profiling.sampling.sample.sample(-1, collector, duration_sec=1)
 
@@ -638,7 +638,7 @@ def test_is_process_running(self):
  sample_interval_usec=1000,
  all_threads=False,
  )
- self.assertTrue(profiler._is_process_running())
+ self.assertTrue(_is_process_running(profiler.pid))
  self.assertIsNotNone(profiler.unwinder.get_stack_trace())
  subproc.process.kill()
  subproc.process.wait()
@@ -647,7 +647,7 @@ def test_is_process_running(self):
  )
 
  # Exit the context manager to ensure the process is terminated
- self.assertFalse(profiler._is_process_running())
+ self.assertFalse(_is_process_running(profiler.pid))
  self.assertRaises(
  ProcessLookupError, profiler.unwinder.get_stack_trace
  )

Lib/test/test_profiling/test_sampling_profiler/test_modes.py

@@ -252,6 +252,7 @@ def test_gil_mode_validation(self):
 
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  try:
@@ -313,6 +314,7 @@ def test_gil_mode_cli_argument_parsing(self):
 
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  try:
@@ -432,6 +434,7 @@ def test_exception_mode_validation(self):
 
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  try:
@@ -493,6 +496,7 @@ def test_exception_mode_cli_argument_parsing(self):
 
  with (
  mock.patch("sys.argv", test_args),
+ mock.patch("profiling.sampling.cli._is_process_running", return_value=True),
  mock.patch("profiling.sampling.cli.sample") as mock_sample,
  ):
  try:

Misc/NEWS.d/next/Library/2025-12-13-10-34-59.gh-issue-142654.fmm974.rst

@@ -0,0 +1,2 @@
+Show the clearer error message when using ``profiling.sampling`` on an
+unknown PID.