wireproto: implement batching on peer executor interface

This is a bit more complicated than non-batch requests because we
need to buffer sends until the last request arrives *and* we need
to support resolving futures as data arrives from the remote.

In a classical concurrent.futures executor model, the future
"starts" as soon as it is submitted. However, we have nothing to
start until the last command is submitted.

If we did nothing, calling result() would deadlock, since the future
hasn't "started." So in the case where we queue the command, we return
a special future type whose result() will trigger sendcommands().
This eliminates the deadlock potential. It also serves as a check
against callers who may be calling result() prematurely, as it will
prevent any subsequent callcommands() from working. This behavior
is slightly annoying and a bit restrictive. But it's the world
that half duplex connections forces on us.

In order to support streaming responses, we were previously using
a generator. But with a futures-based API, we're using futures
and not generators. So in order to get streaming, we need a
background thread to read data from the server.

The approach taken in this patch is to leverage the ThreadPoolExecutor
from concurrent.futures for managing a background thread. We create
an executor and future that resolves when all response data is
processed (or an error occurs). When exiting the context manager,
we wait on that background reading before returning.

I was hoping we could manually spin up a threading.Thread and this
would be simple. But I ran into a few deadlocks when implementing.
After looking at the source code to concurrent.futures, I figured
it would just be easier to use a ThreadPoolExecutor than implement
all the code needed to manually manage a thread.

To prove this works, a use of the batch API in discovery has been
updated.

Differential Revision: https://phab.mercurial-scm.org/D3269

mercurial/setdiscovery.py

@@ -155,11 +155,14 b' def findcommonheads(ui, local, remote,'
     sample = _limitsample(ownheads, initialsamplesize)
     # indices between sample and externalized version must match
     sample = list(sample)
-    batch = remote.iterbatch()
-    batch.heads()
-    batch.known(dag.externalizeall(sample))
-    batch.submit()
-    srvheadhashes, yesno = batch.results()
+
+    with remote.commandexecutor() as e:
+        fheads = e.callcommand('heads', {})
+        fknown = e.callcommand('known', {
+            'nodes': dag.externalizeall(sample),
+        })
+
+    srvheadhashes, yesno = fheads.result(), fknown.result()
 
     if cl.tip() == nullid:
         if srvheadhashes != [nullid]:

mercurial/wireprotov1peer.py

@@ -9,6 +9,7 b' from __future__ import absolute_import'
 
 import hashlib
 import sys
+import weakref
 
 from .i18n import _
 from .node import (
@@ -180,6 +181,26 b' def encodebatchcmds(req):'
 
     return ';'.join(cmds)
 
+class unsentfuture(pycompat.futures.Future):
+    """A Future variation to represent an unsent command.
+
+    Because we buffer commands and don't submit them immediately, calling
+    ``result()`` on an unsent future could deadlock. Futures for buffered
+    commands are represented by this type, which wraps ``result()`` to
+    call ``sendcommands()``.
+    """
+
+    def result(self, timeout=None):
+        if self.done():
+            return pycompat.futures.Future.result(self, timeout)
+
+        self._peerexecutor.sendcommands()
+
+        # This looks like it will infinitely recurse. However,
+        # sendcommands() should modify __class__. This call serves as a check
+        # on that.
+        return self.result(timeout)
+
 @zi.implementer(repository.ipeercommandexecutor)
 class peerexecutor(object):
     def __init__(self, peer):
@@ -187,6 +208,9 b' class peerexecutor(object):'
         self._sent = False
         self._closed = False
         self._calls = []
+        self._futures = weakref.WeakSet()
+        self._responseexecutor = None
+        self._responsef = None
 
     def __enter__(self):
         return self
@@ -214,20 +238,35 b' class peerexecutor(object):'
         # Commands are either batchable or they aren't. If a command
         # isn't batchable, we send it immediately because the executor
         # can no longer accept new commands after a non-batchable command.
-        # If a command is batchable, we queue it for later.
+        # If a command is batchable, we queue it for later. But we have
+        # to account for the case of a non-batchable command arriving after
+        # a batchable one and refuse to service it.
+
+        def addcall():
+            f = pycompat.futures.Future()
+            self._futures.add(f)
+            self._calls.append((command, args, fn, f))
+            return f
 
         if getattr(fn, 'batchable', False):
-            pass
+            f = addcall()
+
+            # But since we don't issue it immediately, we wrap its result()
+            # to trigger sending so we avoid deadlocks.
+            f.__class__ = unsentfuture
+            f._peerexecutor = self
         else:
             if self._calls:
                 raise error.ProgrammingError(
                     '%s is not batchable and cannot be called on a command '
                     'executor along with other commands' % command)
 
-        # We don't support batching yet. So resolve it immediately.
-        f = pycompat.futures.Future()
-        self._calls.append((command, args, fn, f))
+            f = addcall()
+
+            # Non-batchable commands can never coexist with another command
+            # in this executor. So send the command immediately.
         self.sendcommands()
+
         return f
 
     def sendcommands(self):
@@ -239,10 +278,18 b' class peerexecutor(object):'
 
         self._sent = True
 
+        # Unhack any future types so caller seens a clean type and to break
+        # cycle between us and futures.
+        for f in self._futures:
+            if isinstance(f, unsentfuture):
+                f.__class__ = pycompat.futures.Future
+                f._peerexecutor = None
+
         calls = self._calls
         # Mainly to destroy references to futures.
         self._calls = None
 
+        # Simple case of a single command. We call it synchronously.
         if len(calls) == 1:
             command, args, fn, f = calls[0]
 
@@ -259,14 +306,99 b' class peerexecutor(object):'
 
             return
 
-        raise error.ProgrammingError('support for multiple commands not '
-                                     'yet implemented')
+        # Batch commands are a bit harder. First, we have to deal with the
+        # @batchable coroutine. That's a bit annoying. Furthermore, we also
+        # need to preserve streaming. i.e. it should be possible for the
+        # futures to resolve as data is coming in off the wire without having
+        # to wait for the final byte of the final response. We do this by
+        # spinning up a thread to read the responses.
+
+        requests = []
+        states = []
+
+        for command, args, fn, f in calls:
+            # Future was cancelled. Ignore it.
+            if not f.set_running_or_notify_cancel():
+                continue
+
+            try:
+                batchable = fn.batchable(fn.__self__,
+                                         **pycompat.strkwargs(args))
+            except Exception:
+                f.set_exception_info(*sys.exc_info()[1:])
+                return
+
+            # Encoded arguments and future holding remote result.
+            try:
+                encodedargs, fremote = next(batchable)
+            except Exception:
+                f.set_exception_info(*sys.exc_info()[1:])
+                return
+
+            requests.append((command, encodedargs))
+            states.append((command, f, batchable, fremote))
+
+        if not requests:
+            return
+
+        # This will emit responses in order they were executed.
+        wireresults = self._peer._submitbatch(requests)
+
+        # The use of a thread pool executor here is a bit weird for something
+        # that only spins up a single thread. However, thread management is
+        # hard and it is easy to encounter race conditions, deadlocks, etc.
+        # concurrent.futures already solves these problems and its thread pool
+        # executor has minimal overhead. So we use it.
+        self._responseexecutor = pycompat.futures.ThreadPoolExecutor(1)
+        self._responsef = self._responseexecutor.submit(self._readbatchresponse,
+                                                        states, wireresults)
 
     def close(self):
         self.sendcommands()
 
+        if self._closed:
+            return
+
         self._closed = True
 
+        if not self._responsef:
+            return
+
+        # We need to wait on our in-flight response and then shut down the
+        # executor once we have a result.
+        try:
+            self._responsef.result()
+        finally:
+            self._responseexecutor.shutdown(wait=True)
+            self._responsef = None
+            self._responseexecutor = None
+
+            # If any of our futures are still in progress, mark them as
+            # errored. Otherwise a result() could wait indefinitely.
+            for f in self._futures:
+                if not f.done():
+                    f.set_exception(error.ResponseError(
+                        _('unfulfilled batch command response')))
+
+            self._futures = None
+
+    def _readbatchresponse(self, states, wireresults):
+        # Executes in a thread to read data off the wire.
+
+        for command, f, batchable, fremote in states:
+            # Grab raw result off the wire and teach the internal future
+            # about it.
+            remoteresult = next(wireresults)
+            fremote.set(remoteresult)
+
+            # And ask the coroutine to decode that value.
+            try:
+                result = next(batchable)
+            except Exception:
+                f.set_exception_info(*sys.exc_info()[1:])
+            else:
+                f.set_result(result)
+
 class wirepeer(repository.legacypeer):
     """Client-side interface for communicating with a peer repository.