gh-86354: Fix child processes not reusing forkserver created by parent

Lib/multiprocessing/forkserver.py

@@ -135,12 +135,27 @@ def ensure_running(self):
                 if not pid:
                     # still alive
                     return
-                # dead, launch it again
-                os.close(self._forkserver_alive_fd)
-                self._forkserver_authkey = None
-                self._forkserver_address = None
-                self._forkserver_alive_fd = None
-                self._forkserver_pid = None
+            if self._forkserver_alive_fd is not None:
+                # forkserver may be inherited from parent, is it still running?
+                try:
+                    # check by writing a probe
+                    os.write(self._forkserver_alive_fd, b'P')
+                except OSError:
+                    pass
+                else:
+                    # still alive
+                    return
+
+            # dead, launch it again
+            if self._forkserver_alive_fd is not None:
+                try:
+                    os.close(self._forkserver_alive_fd)
+                except OSError:
+                    pass
+            self._forkserver_authkey = None
+            self._forkserver_address = None
+            self._forkserver_alive_fd = None
+            self._forkserver_pid = None
 
             cmd = ('from multiprocessing.forkserver import main; ' +
                    'main(%d, %d, %r, **%r)')
@@ -207,6 +222,7 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None,
             os.close(authkey_r)
     else:
         authkey = b''
+    _forkserver._forkserver_authkey = authkey
 
     if preload:
         if sys_path is not None:
@@ -268,9 +284,16 @@ def sigchld_handler(*_unused):
                         break
 
                 if alive_r in rfds:
-                    # EOF because no more client processes left
-                    assert os.read(alive_r, 1) == b'', "Not at EOF?"
-                    raise SystemExit
+                    data = os.read(alive_r, 1)
+                    if data == b'P':
+                        # probe from client
+                        pass
+                    elif data == b'':
+                        # EOF because no more client processes left
+                        raise SystemExit
+                    else:
+                        raise RuntimeError("Unknown data received in alive fd")
+
 
                 if sig_r in rfds:
                     # Got SIGCHLD

Lib/test/_test_multiprocessing.py

@@ -1002,6 +1002,42 @@ def test_forkserver_without_auth_fails(self):
         proc.start()
         proc.join()
 
+    @classmethod
+    def _create_child_and_get_ppid(cls, queue, remaining):
+        queue.put(os.getppid())
+        remaining -= 1
+        if remaining > 0:
+            proc = cls.Process(target=cls._create_child_and_get_ppid, args=(queue, remaining))
+            proc.start()
+            proc.join()
+
+
+    def test_forkserver_reuse(self):
+        # existing forkserver spawned by parent should be reused by children
+        if self.TYPE == "threads":
+            self.skipTest(f"test not appropriate for {self.TYPE}")
+        if multiprocessing.get_start_method() != "forkserver":
+            self.skipTest("forkserver start method specific")
+
+        forkserver = multiprocessing.forkserver._forkserver
+        forkserver.ensure_running()
+        self.assertTrue(forkserver._forkserver_pid)
+        forkserver_pid = forkserver._forkserver_pid
+
+        # start children recursively
+        ppid_queue = self.Queue()
+        proc = self.Process(target=self._create_child_and_get_ppid, args=(ppid_queue, 3))
+        proc.start()
+        proc.join()
+        for i in range(3):
+            # all descendants should have the same ppid (forkserver)
+            ppid = ppid_queue.get()
+            assert ppid == forkserver_pid
+
+        # forkserver should live after descendants ends
+        forkserver.ensure_running()
+        assert forkserver._forkserver_pid == forkserver_pid
+
 #
 #
 #

Misc/NEWS.d/next/Library/2025-10-02-23-05-42.gh-issue-86354.0RGEbU.rst

@@ -0,0 +1 @@
+Fix child processes not reusing forkserver created by parent