D3269: wireproto: implement batching on peer executor interface

[indygreg (Gregory Szorc)]

indygreg created this revision.
Herald added a subscriber: mercurial-devel.
Herald added a reviewer: hg-reviewers.

REVISION SUMMARY
  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. So that makes things
  a bit weird.
  
  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.

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D3269

AFFECTED FILES
  mercurial/setdiscovery.py
  mercurial/wireprotov1peer.py

CHANGE DETAILS

diff --git a/mercurial/wireprotov1peer.py b/mercurial/wireprotov1peer.py
--- a/mercurial/wireprotov1peer.py
+++ b/mercurial/wireprotov1peer.py
@@ -10,6 +10,7 @@
 import contextlib
 import hashlib
 import sys
+import weakref
 
 from .i18n import _
 from .node import (
@@ -188,6 +189,9 @@
         self._sent = False
         self._closed = False
         self._calls = []
+        self._futures = weakref.WeakSet()
+        self._responseexecutor = None
+        self._responsef = None
 
     def callcommand(self, command, args):
         if self._sent:
@@ -217,6 +221,7 @@
                     'non-batchable command')
 
         f = pycompat.futures.Future()
+        self._futures.add(f)
 
         self._calls.append((command, args, fn, f))
 
@@ -231,6 +236,7 @@
 
         self._sent = True
 
+        # A single command is easy. We call it synchronously.
         if len(self._calls) == 1:
             command, args, fn, f = self._calls[0]
 
@@ -247,14 +253,99 @@
 
             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 self._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.
 
diff --git a/mercurial/setdiscovery.py b/mercurial/setdiscovery.py
--- a/mercurial/setdiscovery.py
+++ b/mercurial/setdiscovery.py
@@ -155,11 +155,14 @@
     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]:



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