Use the "spawn" start method unconditionally

Rather than trying to support both "fork" and "spawn", force "spawn" to
be used no matter what.

Split the monitor process's entry point out into a separate module to
avoid having a global multiprocessing `Queue` object get created when it
is reimported in the spawned process, as otherwise Python 3.8 and 3.9
report that some semaphores are leaked by the child process.

Author: godlygeek

src/pytest_pystack/_monitor.py

@@ -0,0 +1,53 @@
+import shlex
+import subprocess
+import sys
+from queue import Empty
+
+from ._config import PystackConfig
+
+
+def monitor(config: PystackConfig, pid, queue, debug_detected):
+    pystack_cmd = [
+        config.pystack_path,
+        "remote",
+    ]
+    if config.pystack_args:
+        pystack_cmd += shlex.split(config.pystack_args)
+    handled_test_cases = set()
+    while True:
+        testcase = queue.get()
+        if testcase is None:
+            break
+
+        if testcase in handled_test_cases:
+            continue
+        handled_test_cases.add(testcase)
+
+        try:
+            new_testcase = queue.get(timeout=config.threshold)
+            if new_testcase != testcase:
+                print(
+                    f"new test {new_testcase} should not start before previous {testcase} test finished",
+                    file=sys.__stderr__,
+                )
+                raise Exception(
+                    "new test should not start before previous test finished"
+                )
+        except Empty:
+            output = ""
+            output += f"\n\n**** PYSTACK  -- {testcase} ***\n"
+            output += f"Timed out waiting for process {pid} to finish {testcase}:"
+            proc = subprocess.run(
+                [*pystack_cmd, str(pid)],
+                stdout=subprocess.PIPE,
+                text=True,
+            )
+            output += proc.stdout
+            output += "**** PYSTACK  ***\n"
+            is_debug = debug_detected.is_set()
+            debug_detected.clear()
+            if config.print_stderr and not is_debug:
+                print(output, file=sys.__stderr__)
+            if config.output_file:
+                with open(config.output_file, "a") as f:
+                    print(output, file=f)

src/pytest_pystack/_monitor_process.py

@@ -1,11 +1,7 @@
 import os
-import shlex
-import subprocess
-import sys
-from queue import Empty
 
-from ._config import PystackConfig
 from ._debug_detect import debug_detected
+from ._monitor import monitor
 from ._multiprocessing_context import MP_CTX
 
 _queue = MP_CTX.Queue()
@@ -15,7 +11,7 @@
 def start(config):
     global _process
     _process = MP_CTX.Process(
-        target=_run_monitor,
+        target=monitor,
         args=(
             config,
             os.getpid(),
@@ -34,50 +30,3 @@ def stop():
         _process.join(timeout=5)
         if _process.is_alive():
             _process.kill()
-
-
-def _run_monitor(config: PystackConfig, pid, queue, debug_detected):
-    pystack_cmd = [
-        config.pystack_path,
-        "remote",
-    ]
-    if config.pystack_args:
-        pystack_cmd += shlex.split(config.pystack_args)
-    handled_test_cases = set()
-    while True:
-        testcase = queue.get()
-        if testcase is None:
-            break
-
-        if testcase in handled_test_cases:
-            continue
-        handled_test_cases.add(testcase)
-
-        try:
-            new_testcase = queue.get(timeout=config.threshold)
-            if new_testcase != testcase:
-                print(
-                    f"new test {new_testcase} should not start before previous {testcase} test finished",
-                    file=sys.__stderr__,
-                )
-                raise Exception(
-                    "new test should not start before previous test finished"
-                )
-        except Empty:
-            output = ""
-            output += f"\n\n**** PYSTACK  -- {testcase} ***\n"
-            output += f"Timed out waiting for process {pid} to finish {testcase}:"
-            proc = subprocess.run(
-                [*pystack_cmd, str(pid)],
-                stdout=subprocess.PIPE,
-                text=True,
-            )
-            output += proc.stdout
-            output += "**** PYSTACK  ***\n"
-            is_debug = debug_detected.is_set()
-            debug_detected.clear()
-            if config.print_stderr and not is_debug:
-                print(output, file=sys.__stderr__)
-            if config.output_file:
-                with open(config.output_file, "a") as f:
-                    print(output, file=f)

src/pytest_pystack/_multiprocessing_context.py

@@ -1,7 +1,7 @@
 import multiprocessing
 
-# Ensure the "forkserver" spawn method is never used, because we have
-# known incompatibilities with it, while both "fork" and "spawn" work.
-MP_CTX = multiprocessing.get_context(
-    "fork" if multiprocessing.get_start_method() == "fork" else "spawn"
-)
+# We have known incompatibilities with the "forkserver" start method.
+# The "fork" method is expected to work, but can potentially lead to
+# strange failure modes because of things being inherited by our monitor
+# process that shouldn't have been. Use "spawn" unconditionally instead.
+MP_CTX = multiprocessing.get_context("spawn")