Return whether the pool became empty in join.

Useful with a timeout argument.

Fixes #503.

changelog.rst

@@ -19,6 +19,9 @@
 - Nested callbacks that set and clear an Event no longer cause
   ``wait`` to return prematurely. Reported in :issue:`771` by Sergey
   Vasilyev.
+- :class:`~.Group` and :class:`~.Pool` now return whether
+  :meth:`~.Group.join` returned with an empty group. Suggested by Filippo Sironi in
+  :pr:`503`.
 
 1.1.0 (Mar 5, 2016)
 ===================

gevent/pool.py

@@ -540,17 +540,25 @@ class Group(GroupMappingMixin):
             one gets re-raised is not determined. Only greenlets currently
             in the group when this method is called are guaranteed to
             be checked for exceptions.
+
+        :return bool: A value indicating whether this group became empty.
+           If the timeout is specified and the group did not become empty
+           during that timeout, then this will be a false value. Otherwise
+           it will be a true value.
+
+        .. versionchanged:: 1.2a1
+           Add the return value.
         """
-        if raise_error:
-            greenlets = self.greenlets.copy()
-            self._empty_event.wait(timeout=timeout)
-            for greenlet in greenlets:
-                if greenlet.exception is not None:
-                    if hasattr(greenlet, '_raise_exception'):
-                        greenlet._raise_exception()
-                    raise greenlet.exception
-        else:
-            self._empty_event.wait(timeout=timeout)
+        greenlets = list(self.greenlets) if raise_error else ()
+        result = self._empty_event.wait(timeout=timeout)
+
+        for greenlet in greenlets:
+            if greenlet.exception is not None:
+                if hasattr(greenlet, '_raise_exception'):
+                    greenlet._raise_exception()
+                raise greenlet.exception
+
+        return result
 
     def kill(self, exception=GreenletExit, block=True, timeout=None):
         """

greentest/test__pool.py

@@ -465,7 +465,8 @@ class TestJoinEmpty(greentest.TestCase):
 
     def test(self):
         p = pool.Pool()
-        p.join()
+        res = p.join()
+        self.assertTrue(res, "empty should return true")
 
 
 class TestSpawn(greentest.TestCase):
@@ -481,6 +482,16 @@ class TestSpawn(greentest.TestCase):
         gevent.sleep(0.19 if not greentest.RUNNING_ON_APPVEYOR else 0.5)
         self.assertEqual(len(p), 0)
 
+    def testSpawnAndWait(self):
+        p = pool.Pool(1)
+        self.assertEqual(len(p), 0)
+        p.spawn(gevent.sleep, 0.1)
+        self.assertEqual(len(p), 1)
+        res = p.join(0.01)
+        self.assertFalse(res, "waiting on a full pool should return false")
+        res = p.join()
+        self.assertTrue(res, "waiting to finish should be true")
+        self.assertEqual(len(p), 0)
 
 def error_iter():
     yield 1