Issue #8713: Support alternative start methods in multiprocessing on Unix.

See http://hg.python.org/sandbox/sbt#spawn

Lib/multiprocessing/semaphore_tracker.py

@@ -0,0 +1,135 @@
+#
+# On Unix we run a server process which keeps track of unlinked
+# semaphores. The server ignores SIGINT and SIGTERM and reads from a
+# pipe.  Every other process of the program has a copy of the writable
+# end of the pipe, so we get EOF when all other processes have exited.
+# Then the server process unlinks any remaining semaphore names.
+#
+# This is important because the system only supports a limited number
+# of named semaphores, and they will not be automatically removed till
+# the next reboot.  Without this semaphore tracker process, "killall
+# python" would probably leave unlinked semaphores.
+#
+
+import errno
+import os
+import signal
+import sys
+import threading
+import warnings
+import _multiprocessing
+
+from . import spawn
+from . import util
+from . import current_process
+
+__all__ = ['ensure_running', 'register', 'unregister']
+
+
+_lock = threading.Lock()
+
+
+def ensure_running():
+    '''Make sure that semaphore tracker process is running.
+
+    This can be run from any process.  Usually a child process will use
+    the semaphore created by its parent.'''
+    with _lock:
+        config = current_process()._config
+        if config.get('semaphore_tracker_fd') is not None:
+            return
+        fds_to_pass = []
+        try:
+            fds_to_pass.append(sys.stderr.fileno())
+        except Exception:
+            pass
+        cmd = 'from multiprocessing.semaphore_tracker import main; main(%d)'
+        r, semaphore_tracker_fd = util.pipe()
+        try:
+            fds_to_pass.append(r)
+            # process will out live us, so no need to wait on pid
+            exe = spawn.get_executable()
+            args = [exe] + util._args_from_interpreter_flags()
+            args += ['-c', cmd % r]
+            util.spawnv_passfds(exe, args, fds_to_pass)
+        except:
+            os.close(semaphore_tracker_fd)
+            raise
+        else:
+            config['semaphore_tracker_fd'] = semaphore_tracker_fd
+        finally:
+            os.close(r)
+
+
+def register(name):
+    '''Register name of semaphore with semaphore tracker.'''
+    _send('REGISTER', name)
+
+
+def unregister(name):
+    '''Unregister name of semaphore with semaphore tracker.'''
+    _send('UNREGISTER', name)
+
+
+def _send(cmd, name):
+    msg = '{0}:{1}\n'.format(cmd, name).encode('ascii')
+    if len(name) > 512:
+        # posix guarantees that writes to a pipe of less than PIPE_BUF
+        # bytes are atomic, and that PIPE_BUF >= 512
+        raise ValueError('name too long')
+    fd = current_process()._config['semaphore_tracker_fd']
+    nbytes = os.write(fd, msg)
+    assert nbytes == len(msg)
+
+
+def main(fd):
+    '''Run semaphore tracker.'''
+    # protect the process from ^C and "killall python" etc
+    signal.signal(signal.SIGINT, signal.SIG_IGN)
+    signal.signal(signal.SIGTERM, signal.SIG_IGN)
+
+    for f in (sys.stdin, sys.stdout):
+        try:
+            f.close()
+        except Exception:
+            pass
+
+    cache = set()
+    try:
+        # keep track of registered/unregistered semaphores
+        with open(fd, 'rb') as f:
+            for line in f:
+                try:
+                    cmd, name = line.strip().split(b':')
+                    if cmd == b'REGISTER':
+                        cache.add(name)
+                    elif cmd == b'UNREGISTER':
+                        cache.remove(name)
+                    else:
+                        raise RuntimeError('unrecognized command %r' % cmd)
+                except Exception:
+                    try:
+                        sys.excepthook(*sys.exc_info())
+                    except:
+                        pass
+    finally:
+        # all processes have terminated; cleanup any remaining semaphores
+        if cache:
+            try:
+                warnings.warn('semaphore_tracker: There appear to be %d '
+                              'leaked semaphores to clean up at shutdown' %
+                              len(cache))
+            except Exception:
+                pass
+        for name in cache:
+            # For some reason the process which created and registered this
+            # semaphore has failed to unregister it. Presumably it has died.
+            # We therefore unlink it.
+            try:
+                name = name.decode('ascii')
+                try:
+                    _multiprocessing.sem_unlink(name)
+                except Exception as e:
+                    warnings.warn('semaphore_tracker: %r: %s' % (name, e))
+            finally:
+                pass