Bug 1433905 - [mozprocess] Existence of _handle on Windows doesn't mean the process is still alive. r=gbrown

The assumption that when a handle is present for the process handler
on Windows doesn't mean that the process is still alive. It could
have already been externally killed, crashed, or closed itself.

The patch makes sure to check the process exit code, and run
clean-up steps if the process is already gone.

Depends on D7395

Differential Revision: https://phabricator.services.mozilla.com/D7396

--HG--
extra : moz-landing-system : lando

testing/mozbase/mozprocess/mozprocess/processhandler.py

@@ -202,14 +202,27 @@ class ProcessHandlerMixin(object):
             return self.returncode
 
         def poll(self):
-            """ Popen.poll
-                Check if child process has terminated. Set and return returncode attribute.
-            """
-            # If we have a handle, the process is alive
-            if isWin and getattr(self, '_handle', None):
-                return None
+            """Check if child process has terminated, and set returncode attribute.
 
-            return subprocess.Popen.poll(self)
+            This method overrides the Popen.poll implementation for our custom
+            process handling for Windows.
+            """
+            if isWin:
+                # If we have a handle, check that the process is still alive.
+                if self._handle:
+                    returncode = winprocess.GetExitCodeProcess(self._handle)
+
+                    # If the process doesn't exist anymore run cleanup steps
+                    if returncode != winprocess.STILL_ACTIVE:
+                        self.returncode = returncode
+                        self._cleanup()
+                    else:
+                        self.returncode = None
+
+            else:
+                self.returncode = subprocess.Popen.poll(self)
+
+            return self.returncode
 
         def wait(self):
             """ Popen.wait