Fix propagation of errors in multiprocessing.util.spawnv_passfds

The error return pipe, `errpipe_write`, was not included in the "FDs to
keep" that were passed to `_posixsubprocess.fork_exec`. This resulted in
the pipe being closed in `_posixsubprocess.child_exec`, which in turn
meant errors from `os.execv`/`os.execve` could not be written to it,
preventing propagation.

We saw this manifest when our env vars became too large. `strace`
showed:
```
[pid   114] execve("/default-pegasus-venv/bin/python", ["/default-pegasus-venv/bin/python", "-B", "-c", "from multiprocessing.resource_tr"...], 0x62a996e37620 /* 311 vars */) = -1 E2BIG (Argument list too long)
[pid   114] write(31, "OSError:", 8)    = -1 EBADF (Bad file descriptor)
[pid   114] write(31, "7", 1)           = -1 EBADF (Bad file descriptor)
[pid   114] write(31, ":", 1)           = -1 EBADF (Bad file descriptor)
[pid   114] write(31, "", 0)            = -1 EBADF (Bad file descriptor)
```

This change fixes this, matching the behaviour in
`subprocess.Popen._execute_child`:
https://github.com/python/cpython/blob/138ed6db9f89171983dc32af4e7ad2e73d46a940/Lib/subprocess.py#L1897-L1901

Author: lwfitzgerald

Lib/multiprocessing/util.py

@@ -513,8 +513,9 @@ def _flush_std_streams():
 
 def spawnv_passfds(path, args, passfds):
     import _posixsubprocess
-    passfds = tuple(sorted(map(int, passfds)))
     errpipe_read, errpipe_write = os.pipe()
+    passfds = tuple(
+        sorted(itertools.chain(map(int, passfds), (errpipe_write,))))
     try:
         return _posixsubprocess.fork_exec(
             args, [path], True, passfds, None, None,

Lib/test/_test_multiprocessing.py

@@ -53,6 +53,7 @@
 import multiprocessing.managers
 import multiprocessing.pool
 import multiprocessing.queues
+import multiprocessing.spawn
 from multiprocessing.connection import wait
 
 from multiprocessing import util
@@ -982,6 +983,28 @@ def test_forkserver_without_auth_fails(self):
         proc.start()
         proc.join()
 
+    @only_run_in_spawn_testsuite("spawn-specific test")
+    def test_spawn_errors_propagated(self):
+        """
+        Test that errors starting the subprocess using spawning are propagated
+
+        This covers a bug seen where `multiprocessing.util.spawnv_passfds`
+        did not correctly include the error return pipe in the FDs to keep
+        when calling `_posixsubprocess.fork_exec`.
+        """
+        if self.TYPE == "threads":
+            self.skipTest(f"test not appropriate for {self.TYPE}")
+        # Temporarily set the spawn executable to a directory to force failure
+        original_exe = multiprocessing.spawn.get_executable()
+        try:
+            with tempfile.TemporaryDirectory() as temp_dir:
+                multiprocessing.spawn.set_executable(temp_dir)
+                p = self.Process()
+                with self.assertRaises(OSError):
+                    p.start()
+        finally:
+            multiprocessing.spawn.set_executable(original_exe)
+
 #
 #
 #