[gentoo-portage-dev] [PATCH] _get_lock_fn: support multiprocessing spawn start method (bug 758230)

[Zac Medico <zmedico@gentoo.org>]

Ensure that _get_lock_fn arguments to multiprocessing.Process will
successfully pickle, as required by the spawn start method, which
is the default for macOS since Python 3.8.

Since file descriptors are not inherited unless the fork start
method is used, the subprocess should only try to close an
inherited file descriptor for the fork start method.

Bug: https://bugs.gentoo.org/758230
Signed-off-by: Zac Medico <zmedico@gentoo.org>
---
 lib/portage/locks.py | 36 +++++++++++++++++++++++-------------
 1 file changed, 23 insertions(+), 13 deletions(-)

diff --git a/lib/portage/locks.py b/lib/portage/locks.py
index 1073343be..193045c03 100644
--- a/lib/portage/locks.py
+++ b/lib/portage/locks.py
@@ -44,18 +44,7 @@ def _get_lock_fn():
 	if _lock_fn is not None:
 		return _lock_fn
 
-	def _test_lock(fd, lock_path):
-		os.close(fd)
-		try:
-			with open(lock_path, 'a') as f:
-				fcntl.lockf(f.fileno(), fcntl.LOCK_EX|fcntl.LOCK_NB)
-		except EnvironmentError as e:
-			if e.errno == errno.EAGAIN:
-				# Parent process holds lock, as expected.
-				sys.exit(0)
 
-		# Something went wrong.
-		sys.exit(1)
 
 	fd, lock_path = tempfile.mkstemp()
 	try:
@@ -64,8 +53,16 @@ def _get_lock_fn():
 		except EnvironmentError:
 			pass
 		else:
-			proc = multiprocessing.Process(target=_test_lock,
-				args=(fd, lock_path))
+			proc = multiprocessing.Process(
+				target=_subprocess_test_lock,
+				args=(
+					# Since file descriptors are not inherited unless the fork start
+					# method is used, the subprocess should only try to close an
+					# inherited file descriptor for the fork start method.
+					fd if multiprocessing.get_start_method() == "fork" else None,
+					lock_path,
+				),
+			)
 			proc.start()
 			proc.join()
 			if proc.exitcode == os.EX_OK:
@@ -80,6 +77,19 @@ def _get_lock_fn():
 	_lock_fn = fcntl.flock
 	return _lock_fn
 
+def _subprocess_test_lock(fd, lock_path):
+	if fd is not None:
+		os.close(fd)
+	try:
+		with open(lock_path, 'a') as f:
+			fcntl.lockf(f.fileno(), fcntl.LOCK_EX|fcntl.LOCK_NB)
+	except EnvironmentError as e:
+		if e.errno == errno.EAGAIN:
+			# Parent process holds lock, as expected.
+			sys.exit(0)
+
+	# Something went wrong.
+	sys.exit(1)
 
 _open_fds = {}
 _open_inodes = {}
-- 
2.26.2