PYTHON-3186 Avoid SDAM heartbeat timeouts on AWS Lambda (#912)

Poll monitor socket with timeout=0 one last time after timeout expires.
This avoids heartbeat timeouts and connection churn on Lambda and other FaaS envs.

pymongo/network.py

@@ -244,6 +244,7 @@ def wait_for_read(sock_info, deadline):
     # Only Monitor connections can be cancelled.
     if context:
         sock = sock_info.sock
+        timed_out = False
         while True:
             # SSLSocket can have buffered data which won't be caught by select.
             if hasattr(sock, "pending") and sock.pending() > 0:
@@ -252,15 +253,21 @@ def wait_for_read(sock_info, deadline):
                 # Wait up to 500ms for the socket to become readable and then
                 # check for cancellation.
                 if deadline:
-                    timeout = max(min(deadline - time.monotonic(), _POLL_TIMEOUT), 0.001)
+                    remaining = deadline - time.monotonic()
+                    # When the timeout has expired perform one final check to
+                    # see if the socket is readable. This helps avoid spurious
+                    # timeouts on AWS Lambda and other FaaS environments.
+                    if remaining <= 0:
+                        timed_out = True
+                    timeout = max(min(remaining, _POLL_TIMEOUT), 0)
                 else:
                     timeout = _POLL_TIMEOUT
                 readable = sock_info.socket_checker.select(sock, read=True, timeout=timeout)
             if context.cancelled:
                 raise _OperationCancelled("hello cancelled")
             if readable:
                 return
-            if deadline and time.monotonic() > deadline:
+            if timed_out:
                 raise socket.timeout("timed out")
 
 

test/__init__.py

@@ -44,6 +44,7 @@
 from test.version import Version
 from typing import Dict, no_type_check
 from unittest import SkipTest
+from urllib.parse import quote_plus
 
 import pymongo
 import pymongo.errors
@@ -279,6 +280,22 @@ def client_options(self):
             opts["replicaSet"] = self.replica_set_name
         return opts
 
+    @property
+    def uri(self):
+        """Return the MongoClient URI for creating a duplicate client."""
+        opts = client_context.default_client_options.copy()
+        opts_parts = []
+        for opt, val in opts.items():
+            strval = str(val)
+            if isinstance(val, bool):
+                strval = strval.lower()
+            opts_parts.append(f"{opt}={quote_plus(strval)}")
+        opts_part = "&".join(opts_parts)
+        auth_part = ""
+        if client_context.auth_enabled:
+            auth_part = f"{quote_plus(db_user)}:{quote_plus(db_pwd)}@"
+        return f"mongodb://{auth_part}{self.pair}/?{opts_part}"
+
     @property
     def hello(self):
         if not self._hello:
@@ -359,7 +376,7 @@ def _init_client(self):
                     username=db_user,
                     password=db_pwd,
                     replicaSet=self.replica_set_name,
-                    **self.default_client_options
+                    **self.default_client_options,
                 )
 
                 # May not have this if OperationFailure was raised earlier.
@@ -387,7 +404,7 @@ def _init_client(self):
                         username=db_user,
                         password=db_pwd,
                         replicaSet=self.replica_set_name,
-                        **self.default_client_options
+                        **self.default_client_options,
                     )
                 else:
                     self.client = pymongo.MongoClient(
@@ -490,7 +507,7 @@ def _check_user_provided(self):
             username=db_user,
             password=db_pwd,
             serverSelectionTimeoutMS=100,
-            **self.default_client_options
+            **self.default_client_options,
         )
 
         try:

test/sigstop_sigcont.py

@@ -0,0 +1,85 @@
+# Copyright 2022-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Used by test_client.TestClient.test_sigstop_sigcont."""
+
+import logging
+import sys
+
+sys.path[0:0] = [""]
+
+from pymongo import monitoring
+from pymongo.mongo_client import MongoClient
+
+
+class HeartbeatLogger(monitoring.ServerHeartbeatListener):
+    """Log events until the listener is closed."""
+
+    def __init__(self):
+        self.closed = False
+
+    def close(self):
+        self.closed = True
+
+    def started(self, event: monitoring.ServerHeartbeatStartedEvent) -> None:
+        if self.closed:
+            return
+        logging.info("%s", event)
+
+    def succeeded(self, event: monitoring.ServerHeartbeatSucceededEvent) -> None:
+        if self.closed:
+            return
+        logging.info("%s", event)
+
+    def failed(self, event: monitoring.ServerHeartbeatFailedEvent) -> None:
+        if self.closed:
+            return
+        logging.warning("%s", event)
+
+
+def main(uri: str) -> None:
+    heartbeat_logger = HeartbeatLogger()
+    client = MongoClient(
+        uri,
+        event_listeners=[heartbeat_logger],
+        heartbeatFrequencyMS=500,
+        connectTimeoutMS=500,
+    )
+    client.admin.command("ping")
+    logging.info("TEST STARTED")
+    # test_sigstop_sigcont will SIGSTOP and SIGCONT this process in this loop.
+    while True:
+        try:
+            data = input('Type "q" to quit: ')
+        except EOFError:
+            break
+        if data == "q":
+            break
+    client.admin.command("ping")
+    logging.info("TEST COMPLETED")
+    heartbeat_logger.close()
+    client.close()
+
+
+if __name__ == "__main__":
+    if len(sys.argv) != 2:
+        print("unknown or missing options")
+        print(f"usage: python3 {sys.argv[0]} 'mongodb://localhost'")
+        exit(1)
+
+    # Enable logs in this format:
+    # 2022-03-30 12:40:55,582 INFO <ServerHeartbeatStartedEvent ('localhost', 27017)>
+    FORMAT = "%(asctime)s %(levelname)s %(message)s"
+    logging.basicConfig(format=FORMAT, level=logging.INFO)
+    main(sys.argv[1])

test/test_client.py

@@ -23,6 +23,7 @@
 import signal
 import socket
 import struct
+import subprocess
 import sys
 import threading
 import time
@@ -1688,6 +1689,39 @@ def test_srv_max_hosts_kwarg(self):
         )
         self.assertEqual(len(client.topology_description.server_descriptions()), 2)
 
+    @unittest.skipIf(
+        client_context.load_balancer or client_context.serverless,
+        "loadBalanced clients do not run SDAM",
+    )
+    @unittest.skipIf(sys.platform == "win32", "Windows does not support SIGSTOP")
+    def test_sigstop_sigcont(self):
+        test_dir = os.path.dirname(os.path.realpath(__file__))
+        script = os.path.join(test_dir, "sigstop_sigcont.py")
+        p = subprocess.Popen(
+            [sys.executable, script, client_context.uri],
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+        )
+        self.addCleanup(p.wait, timeout=1)
+        self.addCleanup(p.kill)
+        time.sleep(1)
+        # Stop the child, sleep for twice the streaming timeout
+        # (heartbeatFrequencyMS + connectTimeoutMS), and restart.
+        os.kill(p.pid, signal.SIGSTOP)
+        time.sleep(2)
+        os.kill(p.pid, signal.SIGCONT)
+        time.sleep(0.5)
+        # Tell the script to exit gracefully.
+        outs, _ = p.communicate(input=b"q\n", timeout=10)
+        self.assertTrue(outs)
+        log_output = outs.decode("utf-8")
+        self.assertIn("TEST STARTED", log_output)
+        self.assertIn("ServerHeartbeatStartedEvent", log_output)
+        self.assertIn("ServerHeartbeatSucceededEvent", log_output)
+        self.assertIn("TEST COMPLETED", log_output)
+        self.assertNotIn("ServerHeartbeatFailedEvent", log_output)
+
 
 class TestExhaustCursor(IntegrationTest):
     """Test that clients properly handle errors from exhaust cursors."""