issue #140: prevent duplicate watcher thread creation

When a Broker() is running with install_watcher=True, arrange for only
one watcher thread to exist for each target thread, and to reset the
mapping of watchers to targets after process fork.

This is probably the last change I want to make to the watcher feature
before deciding to rip it out, it may be more trouble than it is worth.

mitogen/master.py

@@ -117,16 +117,46 @@ def scan_code_imports(co):
                        co.co_consts[arg2] or ())
 
 
+_join_lock = threading.Lock()
+_join_process_id = None
+_join_callbacks_by_target = {}
+_join_thread_by_target = {}
+
+
+def _join_thread_reset():
+    """If we have forked since the watch dictionaries were initialized, all
+    that has is garbage, so clear it."""
+    global _join_process_id
+
+    if os.getpid() != _join_process_id:
+        _join_process_id = os.getpid()
+        _join_callbacks_by_target.clear()
+        _join_thread_by_target.clear()
+
+
 def join_thread_async(target_thread, on_join):
     """Start a thread that waits for another thread to shutdown, before
     invoking `on_join()`. In CPython it seems possible to use this method to
     ensure a non-main thread is signalled when the main thread has exitted,
     using yet another thread as a proxy."""
+
     def _watch():
         target_thread.join()
+        for on_join in _join_callbacks_by_target[target_thread]:
             on_join()
-    thread = threading.Thread(target=_watch)
-    thread.start()
+
+    _join_lock.acquire()
+    try:
+        _join_thread_reset()
+        _join_callbacks_by_target.setdefault(target_thread, []).append(on_join)
+        if target_thread not in _join_thread_by_target:
+            _join_thread_by_target[target_thread] = threading.Thread(
+                name='mitogen.master.join_thread_async',
+                target=_watch,
+            )
+            _join_thread_by_target[target_thread].start()
+    finally:
+        _join_lock.release()
 
 
 class SelectError(mitogen.core.Error):