implement limited WorkerPool size

Author: graingert

Diff for: execnet/gateway_base.py

@@ -64,8 +64,29 @@ def reraise(cls, val, tb):
 # def log_extra(*msg):
 #     f.write(" ".join([str(x) for x in msg]) + "\n")
 
+if sys.version_info >= (3, 7):
+    from contextlib import nullcontext
+else:
+    class nullcontext(object):
+        """Context manager that does no additional processing.
+        Used as a stand-in for a normal context manager, when a particular
+        block of code is only sometimes used with a normal context manager:
+        cm = optional_cm if condition else nullcontext()
+        with cm:
+            # Perform operation, using optional_cm if condition is True
+        """
+
+        def __init__(self, enter_result=None):
+            self.enter_result = enter_result
+
+        def __enter__(self):
+            return self.enter_result
 
-class EmptySemaphore:
+        def __exit__(self, *excinfo):
+            pass
+
+
+class EmptySemaphore(nullcontext):
     acquire = release = lambda self: None
 
 
@@ -238,13 +259,14 @@ class WorkerPool(object):
         when the pool received a trigger_shutdown().
     """
 
-    def __init__(self, execmodel, hasprimary=False):
+    def __init__(self, execmodel, hasprimary=False, size=None):
         """ by default allow unlimited number of spawns. """
         self.execmodel = execmodel
         self._running_lock = self.execmodel.Lock()
         self._running = set()
         self._shuttingdown = False
         self._waitall_events = []
+        self._semaphore = self.execmodel.Semaphore(size)
         if hasprimary:
             if self.execmodel.backend != "thread":
                 raise ValueError("hasprimary=True requires thread model")
@@ -307,7 +329,7 @@ def spawn(self, func, *args, **kwargs):
             of the given func(*args, **kwargs).
         """
         reply = Reply((func, args, kwargs), self.execmodel)
-        with self._running_lock:
+        with self._semaphore, self._running_lock:
             if self._shuttingdown:
                 raise ValueError("pool is shutting down")
             self._running.add(reply)

Diff for: testing/test_threadpool.py

@@ -74,7 +74,6 @@ def test_waitfinish_on_reply(pool):
     pytest.raises(ZeroDivisionError, reply.get)
 
 
-@pytest.mark.xfail(reason="WorkerPool does not implement limited size")
 def test_limited_size(execmodel):
     pool = WorkerPool(execmodel, size=1)
     q = execmodel.queue.Queue()