Issue #18999: Make multiprocessing use context objects.

This allows different parts of a program to use different methods for
starting processes without interfering with each other.

Lib/multiprocessing/semaphore_tracker.py

@@ -26,60 +26,70 @@ from . import current_process
 __all__ = ['ensure_running', 'register', 'unregister']
 
 
-_semaphore_tracker_fd = None
-_lock = threading.Lock()
+class SemaphoreTracker(object):
+
+    def __init__(self):
+        self._lock = threading.Lock()
+        self._fd = None
+
+    def getfd(self):
+        self.ensure_running()
+        return self._fd
+
+    def ensure_running(self):
+        '''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 self._lock:
+            if self._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, w = os.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(w)
+                raise
+            else:
+                self._fd = w
+            finally:
+                os.close(r)
+
+    def register(self, name):
+        '''Register name of semaphore with semaphore tracker.'''
+        self._send('REGISTER', name)
+
+    def unregister(self, name):
+        '''Unregister name of semaphore with semaphore tracker.'''
+        self._send('UNREGISTER', name)
+
+    def _send(self, cmd, name):
+        self.ensure_running()
+        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')
+        nbytes = os.write(self._fd, msg)
+        assert nbytes == len(msg)
 
 
-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.'''
-    global _semaphore_tracker_fd
-    with _lock:
-        if _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, w = os.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(w)
-            raise
-        else:
-            _semaphore_tracker_fd = w
-        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')
-    nbytes = os.write(_semaphore_tracker_fd, msg)
-    assert nbytes == len(msg)
+_semaphore_tracker = SemaphoreTracker()
+ensure_running = _semaphore_tracker.ensure_running
+register = _semaphore_tracker.register
+unregister = _semaphore_tracker.unregister
+getfd = _semaphore_tracker.getfd
 
 
 def main(fd):