Handle EINTR from poll with zero timeout (#1399)

Closes #1396

CONTRIBUTORS.txt

@@ -259,5 +259,8 @@ In chronological order:
   * Improve contribution guide
   * Add ``HTTPResponse.geturl`` method to provide ``urllib2.urlopen().geturl()`` behavior
 
+* Bruce Merry <http://www.brucemerry.org.za>
+  * Fix leaking exceptions when system calls are interrupted with zero timeout
+
 * [Your name or handle] <[email or website]>
   * [Brief summary of your changes]

test/test_wait.py

@@ -137,6 +137,40 @@ def handler(sig, frame):
     assert interrupt_count[0] > 0
 
 
+@pytest.mark.skipif(
+    not hasattr(signal, "setitimer"),
+    reason="need setitimer() support"
+)
+@pytest.mark.parametrize("wfs", variants)
+def test_eintr_zero_timeout(wfs, spair):
+    a, b = spair
+    interrupt_count = [0]
+
+    def handler(sig, frame):
+        assert sig == signal.SIGALRM
+        interrupt_count[0] += 1
+
+    old_handler = signal.signal(signal.SIGALRM, handler)
+    try:
+        assert not wfs(a, read=True, timeout=0)
+        try:
+            # Start delivering SIGALRM 1000 times per second,
+            # to trigger race conditions such as
+            # https://github.com/urllib3/urllib3/issues/1396.
+            signal.setitimer(signal.ITIMER_REAL, 0.001, 0.001)
+            # Hammer the system call for a while to trigger the
+            # race.
+            for i in range(100000):
+                wfs(a, read=True, timeout=0)
+        finally:
+            # Stop delivering SIGALRM
+            signal.setitimer(signal.ITIMER_REAL, 0)
+    finally:
+        signal.signal(signal.SIGALRM, old_handler)
+
+    assert interrupt_count[0] > 0
+
+
 @pytest.mark.skipif(
     not hasattr(signal, "setitimer"),
     reason="need setitimer() support"

urllib3/util/wait.py

@@ -43,9 +43,6 @@ def _retry_on_intr(fn, timeout):
 else:
     # Old and broken Pythons.
     def _retry_on_intr(fn, timeout):
-        if timeout is not None and timeout <= 0:
-            return fn(timeout)
-
         if timeout is None:
             deadline = float("inf")
         else:
@@ -117,7 +114,7 @@ def _have_working_poll():
     # from libraries like eventlet/greenlet.
     try:
         poll_obj = select.poll()
-        poll_obj.poll(0)
+        _retry_on_intr(poll_obj.poll, 0)
     except (AttributeError, OSError):
         return False
     else: