wireproto: buffer output frames when in half duplex mode

Previously, when told that a response was ready, the server reactor
would instruct the caller to send frames immediately. This was OK
as an initial implementation. But it would not work for half-duplex
connections where the sender can't receive until all data has been
transmitted - such as httplib based clients.

In this commit, we teach the reactor that output frames should
be buffered until end of input is seen. This required a new
event to inform the reactor of end of input. The result from that
event will instruct the consumer to send all buffered frames.

The HTTP server is buffered by default.

This change effectively hides the complexity of buffering within
the reactor so that transports need not be concerned about it.
This helps keep the transports "dumb" and will make implementing
multiple requests-responses per atomic exchange (like an HTTP
request) much simpler.

Differential Revision: https://phab.mercurial-scm.org/D2860

mercurial/wireprotoframing.py

@@ -308,10 +308,24 class serverreactor(object):
     wantframe
        Indicates that nothing of interest happened and the server is waiting on
        more frames from the client before anything interesting can be done.
+
+    noop
+       Indicates no additional action is required.
     """
 
-    def __init__(self):
+    def __init__(self, deferoutput=False):
+        """Construct a new server reactor.
+
+        ``deferoutput`` can be used to indicate that no output frames should be
+        instructed to be sent until input has been exhausted. In this mode,
+        events that would normally generate output frames (such as a command
+        response being ready) will instead defer instructing the consumer to
+        send those frames. This is useful for half-duplex transports where the
+        sender cannot receive until all data has been transmitted.
+        """
+        self._deferoutput = deferoutput
         self._state = 'idle'
+        self._bufferedframegens = []
         self._activecommand = None
         self._activeargs = None
         self._activedata = None
@@ -344,8 +358,33 class serverreactor(object):
 
         The raw bytes response is passed as an argument.
         """
+        framegen = createbytesresponseframesfrombytes(data)
+
+        if self._deferoutput:
+            self._bufferedframegens.append(framegen)
+            return 'noop', {}
+        else:
+            return 'sendframes', {
+                'framegen': framegen,
+            }
+
+    def oninputeof(self):
+        """Signals that end of input has been received.
+
+        No more frames will be received. All pending activity should be
+        completed.
+        """
+        if not self._deferoutput or not self._bufferedframegens:
+            return 'noop', {}
+
+        # If we buffered all our responses, emit those.
+        def makegen():
+            for gen in self._bufferedframegens:
+                for frame in gen:
+                    yield frame
+
         return 'sendframes', {
-            'framegen': createbytesresponseframesfrombytes(data),
+            'framegen': makegen(),
         }
 
     def onapplicationerror(self, msg):

mercurial/wireprotoserver.py

@@ -401,6 +401,10 def _processhttpv2reflectrequest(ui, rep
         states.append(json.dumps((action, meta), sort_keys=True,
                                  separators=(', ', ': ')))
 
+    action, meta = reactor.oninputeof()
+    meta['action'] = action
+    states.append(json.dumps(meta, sort_keys=True, separators=(', ',': ')))
+
     res.status = b'200 OK'
     res.headers[b'Content-Type'] = b'text/plain'
     res.setbodybytes(b'\n'.join(states))
@@ -411,7 +415,10 def _processhttpv2request(ui, repo, req,
     Called when the HTTP request contains unified frame-based protocol
     frames for evaluation.
     """
-    reactor = wireprotoframing.serverreactor()
+    # TODO Some HTTP clients are full duplex and can receive data before
+    # the entire request is transmitted. Figure out a way to indicate support
+    # for that so we can opt into full duplex mode.
+    reactor = wireprotoframing.serverreactor(deferoutput=True)
     seencommand = False
 
     while True:
@@ -448,6 +455,19 def _processhttpv2request(ui, repo, req,
             raise error.ProgrammingError(
                 'unhandled action from frame processor: %s' % action)
 
+    action, meta = reactor.oninputeof()
+    if action == 'sendframes':
+        # We assume we haven't started sending the response yet. If we're
+        # wrong, the response type will raise an exception.
+        res.status = b'200 OK'
+        res.headers[b'Content-Type'] = FRAMINGTYPE
+        res.setbodygen(meta['framegen'])
+    elif action == 'noop':
+        pass
+    else:
+        raise error.ProgrammingError('unhandled action from frame processor: %s'
+                                     % action)
+
 def _httpv2runcommand(ui, repo, req, res, authedperm, reqcommand, reactor,
                       command):
     """Dispatch a wire protocol command made from HTTPv2 requests.
@@ -504,6 +524,8 def _httpv2runcommand(ui, repo, req, res
 
     if action == 'sendframes':
         res.setbodygen(meta['framegen'])
+    elif action == 'noop':
+        pass
     else:
         raise error.ProgrammingError('unhandled event from reactor: %s' %
                                      action)

tests/test-http-api-httpv2.t

@@ -401,7 +401,7 Command frames can be reflected via debu
   s>     Server: testing stub value\r\n
   s>     Date: $HTTP_DATE$\r\n
   s>     Content-Type: text/plain\r\n
-  s>     Content-Length: 291\r\n
+  s>     Content-Length: 310\r\n
   s>     \r\n
   s>     received: 1 2 command1\n
   s>     ["wantframe", {"state": "command-receiving-args"}]\n
@@ -409,6 +409,7 Command frames can be reflected via debu
   s>     ["wantframe", {"state": "command-receiving-args"}]\n
   s>     received: 2 2 \x04\x00\x03\x00bar1val\n
   s>     ["runcommand", {"args": {"bar1": "val", "foo": "val1"}, "command": "command1", "data": null}]\n
-  s>     received: <no frame>
+  s>     received: <no frame>\n
+  s>     {"action": "noop"}
 
   $ cat error.log

tests/test-wireproto-serverreactor.py

@@ -9,8 +9,8 from mercurial import (
 
 ffs = framing.makeframefromhumanstring
 
-def makereactor():
-    return framing.serverreactor()
+def makereactor(deferoutput=False):
+    return framing.serverreactor(deferoutput=deferoutput)
 
 def sendframes(reactor, gen):
     """Send a generator of frame bytearray to a reactor.
@@ -95,6 +95,9 class ServerReactorTests(unittest.TestCa
             'data': None,
         })
 
+        result = reactor.oninputeof()
+        self.assertaction(result, 'noop')
+
     def test1argument(self):
         reactor = makereactor()
         results = list(sendcommandframes(reactor, b'mycommand',
@@ -310,6 +313,37 class ServerReactorTests(unittest.TestCa
             b'error-response application some message',
         ])
 
+    def test1commanddeferresponse(self):
+        """Responses when in deferred output mode are delayed until EOF."""
+        reactor = makereactor(deferoutput=True)
+        results = list(sendcommandframes(reactor, b'mycommand', {}))
+        self.assertEqual(len(results), 1)
+        self.assertaction(results[0], 'runcommand')
+
+        result = reactor.onbytesresponseready(b'response')
+        self.assertaction(result, 'noop')
+        result = reactor.oninputeof()
+        self.assertaction(result, 'sendframes')
+        self.assertframesequal(result[1]['framegen'], [
+            b'bytes-response eos response',
+        ])
+
+    def testmultiplecommanddeferresponse(self):
+        reactor = makereactor(deferoutput=True)
+        list(sendcommandframes(reactor, b'command1', {}))
+        list(sendcommandframes(reactor, b'command2', {}))
+
+        result = reactor.onbytesresponseready(b'response1')
+        self.assertaction(result, 'noop')
+        result = reactor.onbytesresponseready(b'response2')
+        self.assertaction(result, 'noop')
+        result = reactor.oninputeof()
+        self.assertaction(result, 'sendframes')
+        self.assertframesequal(result[1]['framegen'], [
+            b'bytes-response eos response1',
+            b'bytes-response eos response2'
+        ])
+
 if __name__ == '__main__':
     import silenttestrunner
     silenttestrunner.main(__name__)