issue #535: core: unicode.encode() may take importer lock on 2.x

Found on Python 2.4, where import happens immediately following connect.

- Main thread executes import statement, triggers request to parent
- Broker thread attempts to deliver request via Router
- Router discovers parent has disconnected, prepares a dead message
- .dead() calls unicode.encode() to format reason string
- .encode() attemptsto import a codec module
- deadlock

----

(gdb) pystack
/usr/local/python2.4.6/lib/python2.4/encodings/__init__.py (69): search_function
<stdin> (733): dead
<stdin> (2717): _maybe_send_dead
<stdin> (2724): _invoke
<stdin> (2749): _async_route
<stdin> (1635): _receive_one
<stdin> (1603): _internal_receive
<stdin> (1613): on_receive
<stdin> (2931): _call
<stdin> (2942): _loop_once
<stdin> (2988): _do_broker_main
<stdin> (545): _profile_hook
<stdin> (3007): _broker_main
/usr/local/python2.4.6/lib/python2.4/threading.py (420): run
/usr/local/python2.4.6/lib/python2.4/threading.py (424): __bootstrap

mitogen/core.py

@@ -103,6 +103,9 @@ IOLOG = logging.getLogger('mitogen.io')
 IOLOG.setLevel(logging.INFO)
 
 LATIN1_CODEC = encodings.latin_1.Codec()
+# str.encode() may take import lock. Deadlock possible if broker calls
+# .encode() on behalf of thread currently waiting for module.
+UTF8_CODEC = encodings.latin_1.Codec()
 
 _v = False
 _vv = False
@@ -271,9 +274,8 @@ class Kwargs(dict):
         def __init__(self, dct):
             for k, v in dct.iteritems():
                 if type(k) is unicode:
-                    self[k.encode()] = v
-                else:
-                    self[k] = v
+                    k, _ = UTF8_CODEC.encode(k)
+                self[k] = v
 
     def __repr__(self):
         return 'Kwargs(%s)' % (dict.__repr__(self),)
@@ -735,7 +737,7 @@ class Message(object):
         """
         Syntax helper to construct a dead message.
         """
-        kwargs['data'] = (reason or u'').encode()
+        kwargs['data'], _ = UTF8_CODEC.encode(reason or u'')
         return cls(reply_to=IS_DEAD, **kwargs)
 
     @classmethod
@@ -1332,7 +1334,7 @@ class Importer(object):
 
         if mod.__package__ and not PY3:
             # 2.x requires __package__ to be exactly a string.
-            mod.__package__ = mod.__package__.encode()
+            mod.__package__, _ = UTF8_CODEC.encode(mod.__package__)
 
         source = self.get_source(fullname)
         try: