convenience: skip SO_REUSEPORT for bind on random port (0)

https://github.com/eventlet/eventlet/issues/411

eventlet/convenience.py

@@ -1,6 +1,6 @@
 import sys
+import warnings
 
-from eventlet import greenio
 from eventlet import greenpool
 from eventlet import greenthread
 from eventlet.green import socket
@@ -22,7 +22,11 @@ def connect(addr, family=socket.AF_INET, bind=None):
     return sock
 
 
-def listen(addr, family=socket.AF_INET, backlog=50):
+class ReuseRandomPortWarning(Warning):
+    pass
+
+
+def listen(addr, family=socket.AF_INET, backlog=50, reuse_addr=True, reuse_port=None):
     """Convenience function for opening server sockets.  This
     socket can be used in :func:`~eventlet.serve` or a custom ``accept()`` loop.
 
@@ -38,9 +42,18 @@ def listen(addr, family=socket.AF_INET, backlog=50):
     :return: The listening green socket object.
     """
     sock = socket.socket(family, socket.SOCK_STREAM)
-    if sys.platform[:3] != "win":
+    if reuse_addr and sys.platform[:3] != 'win':
         sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
-    if hasattr(socket, 'SO_REUSEPORT'):
+    if family in (socket.AF_INET, socket.AF_INET6) and addr[1] == 0:
+        if reuse_port:
+            warnings.warn(
+                '''listen on random port (0) with SO_REUSEPORT is dangerous.
+                Double check your intent.
+                Example problem: https://github.com/eventlet/eventlet/issues/411''',
+                ReuseRandomPortWarning, stacklevel=3)
+    elif reuse_port is None:
+        reuse_port = True
+    if reuse_port and hasattr(socket, 'SO_REUSEPORT'):
         # NOTE(zhengwei): linux kernel >= 3.9
         sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
     sock.bind(addr)

tests/convenience_test.py

@@ -1,7 +1,8 @@
 import os
+import warnings
 
 import eventlet
-from eventlet import debug, event
+from eventlet import convenience, debug
 from eventlet.green import socket
 from eventlet.support import six
 import tests
@@ -90,7 +91,7 @@ class TestServe(tests.LimitedTestCase):
         gt.wait()
 
     def test_concurrency(self):
-        evt = event.Event()
+        evt = eventlet.Event()
 
         def waiter(sock, addr):
             sock.sendall(b'hi')
@@ -128,18 +129,41 @@ class TestServe(tests.LimitedTestCase):
         client.sendall(b"echo")
         self.assertEqual(b"echo", client.recv(1024))
 
-    def test_socket_reuse(self):
+
+def test_socket_reuse():
+    # pick a free port with bind to 0 - without SO_REUSEPORT
+    # then close it and try to bind to same port with SO_REUSEPORT
+    # loop helps in case something else used the chosen port before second bind
+    addr = None
+    errors = []
+    for _ in range(5):
         lsock1 = eventlet.listen(('localhost', 0))
-        port = lsock1.getsockname()[1]
-
-        if hasattr(socket, 'SO_REUSEPORT'):
-            lsock2 = eventlet.listen(('localhost', port))
-        else:
-            try:
-                lsock2 = eventlet.listen(('localhost', port))
-                assert lsock2
-                lsock2.close()
-            except socket.error:
-                pass
-
+        addr = lsock1.getsockname()
         lsock1.close()
+        try:
+            lsock1 = eventlet.listen(addr)
+        except socket.error as e:
+            errors.append(e)
+            continue
+        break
+    else:
+        assert False, errors
+
+    if hasattr(socket, 'SO_REUSEPORT'):
+        lsock2 = eventlet.listen(addr)
+    else:
+        try:
+            lsock2 = eventlet.listen(addr)
+            assert lsock2
+            lsock2.close()
+        except socket.error:
+            pass
+
+    lsock1.close()
+
+
+def test_reuse_random_port_warning():
+    with warnings.catch_warnings(record=True) as w:
+        eventlet.listen(('localhost', 0), reuse_port=True).close()
+        assert len(w) == 1
+        assert issubclass(w[0].category, convenience.ReuseRandomPortWarning)