PYTHON-2234 Only rely on standard library in daemon.py (#597)

pymongo/daemon.py

@@ -22,13 +22,14 @@ client-side field level encryption is enabled. See
 import os
 import subprocess
 import sys
+import warnings
 
-from pymongo.errors import PyMongoError
 
 # The maximum amount of time to wait for the intermediate subprocess.
 _WAIT_TIMEOUT = 10
 _THIS_FILE = os.path.realpath(__file__)
 
+
 def _popen_wait(popen, timeout):
     """Implement wait timeout support for Python 3."""
     try:
@@ -47,7 +48,9 @@ def _silence_resource_warning(popen):
     # "ResourceWarning: subprocess XXX is still running".
     # See https://bugs.python.org/issue38890 and
     # https://bugs.python.org/issue26741.
-    popen.returncode = 0
+    # popen is None when mongocryptd spawning fails
+    if popen is not None:
+        popen.returncode = 0
 
 
 if sys.platform == 'win32':
@@ -64,8 +67,9 @@ if sys.platform == 'win32':
                     stdin=devnull, stderr=devnull, stdout=devnull)
                 _silence_resource_warning(popen)
         except FileNotFoundError as exc:
-            raise PyMongoError(
-                f'Failed to start {args[0]}: is it on your $PATH?\nOriginal exception: {exc}')
+            warnings.warn(f'Failed to start {args[0]}: is it on your $PATH?\n'
+                          f'Original exception: {exc}', RuntimeWarning,
+                          stacklevel=2)
 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
@@ -87,8 +91,10 @@ else:
                     close_fds=True,
                     stdin=devnull, stderr=devnull, stdout=devnull)
         except FileNotFoundError as exc:
-            raise PyMongoError(
-                f'Failed to start {args[0]}: is it on your $PATH?\nOriginal exception: {exc}')
+            warnings.warn(f'Failed to start {args[0]}: is it on your $PATH?\n'
+                          f'Original exception: {exc}', RuntimeWarning,
+                          stacklevel=2)
+
 
     def _spawn_daemon_double_popen(args):
         """Spawn a daemon process using a double subprocess.Popen."""