issue #76: reduce Context duplication during unpickling

When unpickling a context, arrange for there to be a single instance
representing that context, managed by the corresponding router. This
context_by_id() was already in use by parent.py, it just needs to move
down.

This to eventually reach the point where a single Context exists that
needs 'disconnect' fired on it, so all sleeping receivers are definitely
woken.

mitogen/core.py

@@ -1279,7 +1279,7 @@ def _unpickle_context(router, context_id, name):
                 (isinstance(name, UnicodeType) and len(name) < 100))
             ):
         raise TypeError('cannot unpickle Context: bad input')
-    return router.context_class(router, context_id, name)
+    return router.context_by_id(context_id, name=name)
 
 
 class Poller(object):
@@ -1732,6 +1732,15 @@ class Router(object):
             _, (_, func, _) = self._handle_map.popitem()
             func(Message.dead())
 
+    def context_by_id(self, context_id, via_id=None, create=True, name=None):
+        context = self._context_by_id.get(context_id)
+        if create and not context:
+            context = self.context_class(self, context_id, name=name)
+            if via_id is not None:
+                context.via = self.context_by_id(via_id)
+            self._context_by_id[context_id] = context
+        return context
+
     def register(self, context, stream):
         _v and LOG.debug('register(%r, %r)', context, stream)
         self._stream_by_id[context.context_id] = stream

mitogen/parent.py

@@ -1581,15 +1581,6 @@ class Router(mitogen.core.Router):
     def allocate_id(self):
         return self.id_allocator.allocate()
 
-    def context_by_id(self, context_id, via_id=None, create=True):
-        context = self._context_by_id.get(context_id)
-        if create and not context:
-            context = self.context_class(self, context_id)
-            if via_id is not None:
-                context.via = self.context_by_id(via_id)
-            self._context_by_id[context_id] = context
-        return context
-
     connection_timeout_msg = u"Connection timed out."
 
     def _connect(self, klass, name=None, **kwargs):