Use process uids and usernames on Unix and Windows respectively.

Windows doesn't have EUIDs, so instead we'll check that the usernames
match. This doesn't seem the least bit secure to me, but it's Windows so
they're probably owned anyway. If anyone knows one of the "proper" ways
to determine if another process has the same owner on Windows, I'd love
to know about it.

 * FIXES Issue #58 but I don't have a Windows machine to test so maybe
   it's still broken.
   https://github.com/isislovecruft/python-gnupg/issues/58

gnupg/_meta.py

@@ -75,19 +75,40 @@ class GPGMeta(type):
         instance containing the gpg-agent process' information to
         ``cls._agent_proc``.
 
+        For Unix systems, we check that the effective UID of this
+        ``python-gnupg`` process is also the owner of the gpg-agent
+        process. For Windows, we check that the usernames of the owners are
+        the same. (Sorry Windows users; maybe you should switch to anything
+        else.)
+
         :returns: True if there exists a gpg-agent process running under the
                   same effective user ID as that of this program. Otherwise,
-                  returns None.
+                  returns False.
         """
-        identity = psutil.Process(os.getpid()).uids
+        this_process = psutil.Process(os.getpid())
+        ownership_match = False
+
+        if _util._running_windows:
+            identity = this_process.username()
+        else:
+            identity = this_process.uids
+
         for proc in psutil.process_iter():
             if (proc.name == "gpg-agent") and proc.is_running:
                 log.debug("Found gpg-agent process with pid %d" % proc.pid)
-                if proc.uids == identity:
-                    log.debug(
-                        "Effective UIDs of this process and gpg-agent match")
-                    setattr(cls, '_agent_proc', proc)
-                    return True
+                if _util._running_windows:
+                    if proc.username() == identity:
+                        ownership_match = True
+                else:
+                    if proc.uids == identity:
+                        ownership_match = True
+
+        if ownership_match:
+            log.debug("Effective UIDs of this process and gpg-agent match")
+            setattr(cls, '_agent_proc', proc)
+            return True
+
+        return False
 
 
 class GPGBase(object):

gnupg/_util.py

@@ -56,6 +56,9 @@ try:
 except NameError:
     _py3k = True
 
+_running_windows = False
+if "win" in sys.platform:
+    _running_windows = True
 
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