PYTHON-2234: When mongocryptd spawn fails, the driver does not indicate what it tried to spawn (#596)

Co-authored-by: William Zhou <william.zhou@mongodb.com>

Author: prashantmital

pymongo/daemon.py

@@ -23,11 +23,14 @@
 import subprocess
 import sys
 import time
+import warnings
+
 
 # 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."""
@@ -66,7 +69,9 @@ 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':
@@ -75,12 +80,17 @@ def _silence_resource_warning(popen):
 
     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)
+        try:
+            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)
+        except FileNotFoundError as exc:
+            warnings.warn('Failed to start %s: is it on your $PATH?\n'
+                          'Original exception: %s' % (args[0], 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
@@ -95,12 +105,16 @@ def _spawn_daemon(args):
     # 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)
-
+        try:
+            with open(os.devnull, 'r+b') as devnull:
+                return subprocess.Popen(
+                    args,
+                    close_fds=True,
+                    stdin=devnull, stderr=devnull, stdout=devnull)
+        except FileNotFoundError as exc:
+            warnings.warn('Failed to start %s: is it on your $PATH?\n'
+                          'Original exception: %s' % (args[0], exc),
+                          RuntimeWarning, stacklevel=2)
 
     def _spawn_daemon_double_popen(args):
         """Spawn a daemon process using a double subprocess.Popen."""