issue #186: parent: implement FORWARD_MODULE.

To support detach, we must be able to preload the target with every
module it will need prior to detachment. This implements the
intermediary part of the process (i.e. the Ansible fork parent) --
receiving LOAD_MODULE/FORWARD_MODULE pairs and ensuring they reach the
child.

docs/howitworks.rst

@@ -445,6 +445,23 @@ also listen on the following handles:
     In this way, the master need never re-send a module it has already sent to
     a direct descendant.
 
+.. currentmodule:: mitogen.core
+.. data:: FORWARD_MODULE
+
+    Receives `(context, fullname)` tuples from its parent and arranges for a
+    :data:`LOAD_MODULE` to be sent towards `context` for the module `fullname`
+    and any related modules. The module must already have been delivered to the
+    current context by its parent in a prior :data:`LOAD_MODULE` message.
+
+    If the receiver is the immediate parent of `context`, then only
+    :data:`LOAD_MODULE` is sent to the child. Otherwise :data:`LOAD_MODULE` is
+    sent to the next closest parent if the module has not previously been sent
+    on that stream, followed by a copy of the :data:`FORWARD_MODULE` message.
+
+    This message is used to recursively preload indirect children with modules,
+    ensuring they are cached and deduplicated at each hop in the chain leading
+    to the target context.
+
 .. currentmodule:: mitogen.core
 .. data:: DETACHING
 

mitogen/core.py

@@ -75,7 +75,8 @@ DEL_ROUTE = 104
 ALLOCATE_ID = 105
 SHUTDOWN = 106
 LOAD_MODULE = 107
-DETACHING = 108
+FORWARD_MODULE = 108
+DETACHING = 109
 IS_DEAD = 999
 
 try:

mitogen/parent.py

@@ -1106,6 +1106,12 @@ class ModuleForwarder(object):
         self.router = router
         self.parent_context = parent_context
         self.importer = importer
+        router.add_handler(
+            fn=self._on_forward_module,
+            handle=mitogen.core.FORWARD_MODULE,
+            persist=True,
+            policy=mitogen.core.has_parent_authority,
+        )
         router.add_handler(
             fn=self._on_get_module,
             handle=mitogen.core.GET_MODULE,
@@ -1116,34 +1122,64 @@ class ModuleForwarder(object):
     def __repr__(self):
         return 'ModuleForwarder(%r)' % (self.router,)
 
+    def _on_forward_module(self, msg):
+        if msg.is_dead:
+            return
+
+        context_id_s, fullname = msg.data.partition('\x00')
+        context_id = int(context_id_s)
+        stream = self.router.stream_by_id(context_id)
+        if stream.remote_id == mitogen.parent_id:
+            LOG.error('%r: dropping FORWARD_MODULE(%d, %r): no route to child',
+                      self, context_id, fullname)
+            return
+
+        LOG.debug('%r._on_forward_module() sending %r to %r via %r',
+                  self, fullname, context_id, stream.remote_id)
+        self._send_module_and_related(stream, fullname)
+        if stream.remote_id != context_id:
+            stream._send(
+                mitogen.core.Message(
+                    dst_id=stream.remote_id,
+                    handle=mitogen.core.FORWARD_MODULE,
+                    data=msg.data,
+                )
+            )
+
     def _on_get_module(self, msg):
         LOG.debug('%r._on_get_module(%r)', self, msg)
         if msg.is_dead:
             return
 
         fullname = msg.data
-        callback = lambda: self._on_cache_callback(msg, fullname)
-        self.importer._request_module(fullname, callback)
-
-    def _send_one_module(self, msg, tup):
-        self.router._async_route(
-            mitogen.core.Message.pickled(
-                tup,
-                dst_id=msg.src_id,
-                handle=mitogen.core.LOAD_MODULE,
-            )
+        self.importer._request_module(fullname,
+            lambda: self._on_cache_callback(msg, fullname)
         )
 
     def _on_cache_callback(self, msg, fullname):
         LOG.debug('%r._on_get_module(): sending %r', self, fullname)
+        stream = self.router.stream_by_id(msg.src_id)
+        self._send_module_and_related(stream, fullname)
+
+    def _send_module_and_related(self, stream, fullname):
         tup = self.importer._cache[fullname]
-        if tup is not None:
         for related in tup[4]:
             rtup = self.importer._cache.get(related)
-                if not rtup:
-                    LOG.debug('%r._on_get_module(): skipping absent %r',
-                               self, related)
-                    continue
-                self._send_one_module(msg, rtup)
+            if rtup:
+                self._send_one_module(stream, rtup)
+            else:
+                LOG.debug('%r._send_module_and_related(%r): absent: %r',
+                           self, fullname, related)
 
-        self._send_one_module(msg, tup)
+        self._send_one_module(stream, tup)
+
+    def _send_one_module(self, stream, tup):
+        if tup[0] not in stream.sent_modules:
+            stream.sent_modules.add(tup[0])
+            self.router._async_route(
+                mitogen.core.Message.pickled(
+                    tup,
+                    dst_id=stream.remote_id,
+                    handle=mitogen.core.LOAD_MODULE,
+                )
+            )