bpo-33929: multiprocessing: fix handle leak on race condition (GH-7921)

Fix a race condition in Popen of
multiprocessing.popen_spawn_win32. The child process now duplicates
the read end of pipe instead of "stealing" it.

Previously, the read end of pipe was "stolen" by the child process,
but it leaked a handle if the child process had been terminated
before it could steal the handle from the parent process.

1 files changed, 13 insertions, 2 deletions

diff --git a/Lib/multiprocessing/popen_spawn_win32.py b/Lib/multiprocessing/popen_spawn_win32.py
index 3e42e9c..3b92c8a 100644
--- a/Lib/multiprocessing/popen_spawn_win32.py
+++ b/Lib/multiprocessing/popen_spawn_win32.py
@@ -18,6 +18,12 @@ TERMINATE = 0x10000
 WINEXE = (sys.platform == 'win32' and getattr(sys, 'frozen', False))
 WINSERVICE = sys.executable.lower().endswith("pythonservice.exe")
 
+
+def _close_handles(*handles):
+    for handle in handles:
+        _winapi.CloseHandle(handle)
+
+
 #
 # We define a Popen class similar to the one from subprocess, but
 # whose constructor takes a process object as its argument.
@@ -32,8 +38,12 @@ class Popen(object):
     def __init__(self, process_obj):
         prep_data = spawn.get_preparation_data(process_obj._name)
 
-        # read end of pipe will be "stolen" by the child process
+        # read end of pipe will be duplicated by the child process
         # -- see spawn_main() in spawn.py.
+        #
+        # bpo-33929: Previously, the read end of pipe was "stolen" by the child
+        # process, but it leaked a handle if the child process had been
+        # terminated before it could steal the handle from the parent process.
         rhandle, whandle = _winapi.CreatePipe(None, 0)
         wfd = msvcrt.open_osfhandle(whandle, 0)
         cmd = spawn.get_command_line(parent_pid=os.getpid(),
@@ -56,7 +66,8 @@ class Popen(object):
             self.returncode = None
             self._handle = hp
             self.sentinel = int(hp)
-            self.finalizer = util.Finalize(self, _winapi.CloseHandle, (self.sentinel,))
+            self.finalizer = util.Finalize(self, _close_handles,
+                                           (self.sentinel, int(rhandle)))
 
             # send information to child
             set_spawning_popen(self)