iwait is tolerant of having the greenlets switch while it yields. Fixes #467.

changelog.rst

@@ -42,6 +42,9 @@ Unreleased
   module no longer leaks file descriptors. Reported in :pr:`374` by 陈小玉.
 - The example ``echoserver.py`` no longer binds to the standard X11
   TCP port. Reported in :issue:`485` by minusf.
+- ``gevent.iwait`` no longer throws ``LoopExit`` if the caller
+  switches greenlets between return values. Reported and initial patch
+  in :pr:`467` by Alexey Borzenkov.
 
 1.1a1 (Jun 29, 2015)
 ====================

gevent/hub.py

@@ -598,6 +598,41 @@ class Waiter(object):
     # and unwraps it in wait() thus checking that switch() was indeed called
 
 
+class _MultipleWaiter(Waiter):
+    """
+    An internal extension of Waiter that can be used if multiple objects
+    must be waited on, and there is a chance that in between waits greenlets
+    might be switched out. All greenlets that switch to this waiter
+    will have their value returned.
+
+    This does not handle exceptions or throw methods.
+    """
+    _DEQUE = None
+    __slots__ = ['_values']
+
+    def __init__(self, *args, **kwargs):
+        Waiter.__init__(self, *args, **kwargs)
+        self._values = self._deque()
+
+    @classmethod
+    def _deque(cls):
+        if cls._DEQUE is None:
+            from collections import deque
+            cls._DEQUE = deque
+        return cls._DEQUE()
+
+    def switch(self, value):
+        self._values.append(value)
+        Waiter.switch(self, True)
+
+    def get(self):
+        if not self._values:
+            Waiter.get(self)
+            Waiter.clear(self)
+
+        return self._values.popleft()
+
+
 def iwait(objects, timeout=None, count=None):
     """
     Yield objects as they are ready, until all (or `count`) are ready or `timeout` expired.
@@ -612,14 +647,13 @@ def iwait(objects, timeout=None, count=None):
         yield get_hub().join(timeout=timeout)
         return
 
-    waiter = Waiter()
+    count = len(objects) if count is None else min(count, len(objects))
+    waiter = _MultipleWaiter()
     switch = waiter.switch
 
     if timeout is not None:
         timer = get_hub().loop.timer(timeout, priority=-1)
-        timer.start(waiter.switch, _NONE)
-
-    count = len(objects) if count is None else min(count, len(objects))
+        timer.start(switch, _NONE)
 
     try:
         for obj in objects:

greentest/test__execmodules.py

@@ -12,9 +12,8 @@ def make_exec_test(path, module):
 
     def test(self):
         #sys.stderr.write('%s %s\n' % (module, path))
-        f = open(path)
-        src = f.read()
-        f.close()
+        with open(path, 'rb') as f:
+            src = f.read()
         six.exec_(src, {})
 
     name = "test_" + module.replace(".", "_")

greentest/test__issue467.py

+import gevent
+#import socket # on windows
+
+# iwait should not raise `LoopExit: This operation would block forever`
+# or `AssertionError: Invalid switch into ...`
+# if the caller of iwait causes greenlets to switch in between
+# return values
+
+
+def worker(i):
+    # Have one of them raise an exception to test that case
+    if i == 2:
+        raise ValueError(i)
+    return i
+
+
+def main():
+    finished = 0
+    # Wait on a group that includes one that will already be
+    # done, plus some that will finish as we watch
+    done_worker = gevent.spawn(worker, "done")
+    gevent.joinall((done_worker,))
+
+    workers = [gevent.spawn(worker, i) for i in range(3)]
+    workers.append(done_worker)
+    for g in gevent.iwait(workers):
+        finished += 1
+        # Simulate doing something that causes greenlets to switch;
+        # a non-zero timeout is crucial
+        gevent.sleep(0.01)
+
+    assert finished == 4
+
+if __name__ == '__main__':
+    main()