PYTHON-2043 Spawn mongocryptd as a daemon process and silence resource warnings

Author: ShaneHarvey

pymongo/daemon.py

@@ -0,0 +1,144 @@
+# Copyright 2019-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.
+
+"""Support for spawning a daemon process.
+
+PyMongo only attempts to spawn the mongocryptd daemon process when automatic
+client-side field level encryption is enabled. See
+:ref:`automatic-client-side-encryption` for more info.
+"""
+
+import os
+import subprocess
+import sys
+import time
+
+# The maximum amount of time to wait for the intermediate subprocess.
+_WAIT_TIMEOUT = 10
+_THIS_FILE = os.path.realpath(__file__)
+
+if sys.version_info[0] < 3:
+    def _popen_wait(popen, timeout):
+        """Implement wait timeout support for Python 2."""
+        from pymongo.monotonic import time as _time
+        deadline = _time() + timeout
+        # Initial delay of 1ms
+        delay = .0005
+        while True:
+            returncode = popen.poll()
+            if returncode is not None:
+                return returncode
+
+            remaining = deadline - _time()
+            if remaining <= 0:
+                # Just return None instead of raising an error.
+                return None
+            delay = min(delay * 2, remaining, .5)
+            time.sleep(delay)
+
+else:
+    def _popen_wait(popen, timeout):
+        """Implement wait timeout support for Python 3."""
+        try:
+            return popen.wait(timeout=timeout)
+        except subprocess.TimeoutExpired:
+            # Silence TimeoutExpired errors.
+            return None
+
+
+def _silence_resource_warning(popen):
+    """Silence Popen's ResourceWarning.
+
+    Note this should only be used if the process was created as a daemon.
+    """
+    # Set the returncode to avoid this warning when popen is garbage collected:
+    # "ResourceWarning: subprocess XXX is still running".
+    # See https://bugs.python.org/issue38890 and
+    # https://bugs.python.org/issue26741.
+    popen.returncode = 0
+
+
+if sys.platform == 'win32':
+    # On Windows we spawn the daemon process simply by using DETACHED_PROCESS.
+    _DETACHED_PROCESS = getattr(subprocess, 'DETACHED_PROCESS', 0x00000008)
+
+    def _spawn_daemon(args):
+        """Spawn a daemon process (Windows)."""
+        with open(os.devnull, 'r+b') as devnull:
+            popen = subprocess.Popen(
+                args,
+                creationflags=_DETACHED_PROCESS,
+                stdin=devnull, stderr=devnull, stdout=devnull)
+            _silence_resource_warning(popen)
+else:
+    # On Unix we spawn the daemon process with a double Popen.
+    # 1) The first Popen runs this file as a Python script using the current
+    #    interpreter.
+    # 2) The script then decouples itself and performs the second Popen to
+    #    spawn the daemon process.
+    # 3) The original process waits up to 10 seconds for the script to exit.
+    #
+    # Note that we do not call fork() directly because we want this procedure
+    # to be safe to call from any thread. Using Popen instead of fork also
+    # avoids triggering the application's os.register_at_fork() callbacks when
+    # we spawn the mongocryptd daemon process.
+    def _spawn(args):
+        """Spawn the process and silence stdout/stderr."""
+        with open(os.devnull, 'r+b') as devnull:
+            return subprocess.Popen(
+                args,
+                close_fds=True,
+                stdin=devnull, stderr=devnull, stdout=devnull)
+
+
+    def _spawn_daemon_double_popen(args):
+        """Spawn a daemon process using a double subprocess.Popen."""
+        spawner_args = [sys.executable, _THIS_FILE]
+        spawner_args.extend(args)
+        temp_proc = subprocess.Popen(spawner_args, close_fds=True)
+        # Reap the intermediate child process to avoid creating zombie
+        # processes.
+        _popen_wait(temp_proc, _WAIT_TIMEOUT)
+
+
+    def _spawn_daemon(args):
+        """Spawn a daemon process (Unix)."""
+        # "If Python is unable to retrieve the real path to its executable,
+        # sys.executable will be an empty string or None".
+        if sys.executable:
+            _spawn_daemon_double_popen(args)
+        else:
+            # Fallback to spawn a non-daemon process without silencing the
+            # resource warning. We do not use fork here because it is not
+            # safe to call from a thread on all systems.
+            # Unfortunately, this means that:
+            # 1) If the parent application is killed via Ctrl-C, the
+            #    non-daemon process will also be killed.
+            # 2) Each non-daemon process will hang around as a zombie process
+            #    until the main application exits.
+            _spawn(args)
+
+
+    if __name__ == '__main__':
+        # Attempt to start a new session to decouple from the parent.
+        if hasattr(os, 'setsid'):
+            try:
+                os.setsid()
+            except OSError:
+                pass
+
+        # We are performing a double fork (Popen) to spawn the process as a
+        # daemon so it is safe to ignore the resource warning.
+        _silence_resource_warning(_spawn(sys.argv[1:]))
+        os._exit(0)

pymongo/encryption.py

@@ -52,6 +52,7 @@
 from pymongo.ssl_support import get_ssl_context
 from pymongo.uri_parser import parse_host
 from pymongo.write_concern import WriteConcern
+from pymongo.daemon import _spawn_daemon
 
 
 _HTTPS_PORT = 443
@@ -146,9 +147,7 @@ def spawn(self):
         self._spawned = True
         args = [self.opts._mongocryptd_spawn_path or 'mongocryptd']
         args.extend(self.opts._mongocryptd_spawn_args)
-        # Silence mongocryptd output, users should pass --logpath.
-        with open(os.devnull, 'wb') as devnull:
-            subprocess.Popen(args, stdout=devnull, stderr=devnull)
+        _spawn_daemon(args)
 
     def mark_command(self, database, cmd):
         """Mark a command for encryption.