gh-142654: show the clear error message when sampling on an unknown PID in tachyon (#142655)

Co-authored-by: Pablo Galindo Salgado <pablogsal@gmail.com>

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

@@ -34,24 +34,30 @@ 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
-        if _FREE_THREADED_BUILD:
-            self.unwinder = _remote_debugging.RemoteUnwinder(
-                self.pid, all_threads=self.all_threads, mode=mode, native=native, gc=gc,
-                opcodes=opcodes, skip_non_matching_threads=skip_non_matching_threads,
-                cache_frames=True, stats=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,
-                opcodes=opcodes, skip_non_matching_threads=skip_non_matching_threads,
-                cache_frames=True, 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:
+            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=self.collect_stats
+            )
+        else:
+            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=self.collect_stats
+            )
+        return unwinder
+
     def sample(self, collector, duration_sec=10, *, async_aware=False):
         sample_interval_sec = self.sample_interval_usec / 1_000_000
         running_time = 0
@@ -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,