wireproto: in batch queries, support queries with immediate responses

listkeys and pushkey return without querying the remote when the
remote doesn't support such queries. Before this change, the batching
code didn't handle this convention, resulting in this kind of error:

  $ hg pull ssh://user@dummy/repo1 -r tip -B a
  pulling from ssh://user@dummy/repo1
  remote: ** Unknown exception encountered with possibly-broken third-party extension disable-lookup
  remote: ** which supports versions unknown of Mercurial.
  remote: ** Please disable disable-lookup and try your action again.
  remote: ** If that fixes the bug please report it to the extension author.
  remote: ** Python 2.7.15+ (default, Oct  2 2018, 22:12:08) [GCC 8.2.0]
  remote: ** Mercurial Distributed SCM (version 4.8.1+586-ef54bd33b476+20181224)
  remote: ** Extensions loaded: disable-lookup
  remote: Traceback (most recent call last):
  remote:   File "/tmp/hgtests.i66Npc/install/bin/hg", line 43, in <module>
  remote:     dispatch.run()
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/dispatch.py", line 99, in run
  remote:     status = dispatch(req)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/dispatch.py", line 225, in dispatch
  remote:     ret = _runcatch(req) or 0
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/dispatch.py", line 376, in _runcatch
  remote:     return _callcatch(ui, _runcatchfunc)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/dispatch.py", line 384, in _callcatch
  remote:     return scmutil.callcatch(ui, func)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/scmutil.py", line 166, in callcatch
  remote:     return func()
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/dispatch.py", line 367, in _runcatchfunc
  remote:     return _dispatch(req)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/dispatch.py", line 1021, in _dispatch
  remote:     cmdpats, cmdoptions)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/dispatch.py", line 756, in runcommand
  remote:     ret = _runcommand(ui, options, cmd, d)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/dispatch.py", line 1030, in _runcommand
  remote:     return cmdfunc()
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/dispatch.py", line 1018, in <lambda>
  remote:     d = lambda: util.checksignature(func)(ui, *args, **strcmdopt)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/util.py", line 1670, in check
  remote:     return func(*args, **kwargs)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/commands.py", line 5257, in serve
  remote:     s.serve_forever()
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/wireprotoserver.py", line 797, in serve_forever
  remote:     self.serveuntil(threading.Event())
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/wireprotoserver.py", line 804, in serveuntil
  remote:     _runsshserver(self._ui, self._repo, self._fin, self._fout, ev)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/wireprotoserver.py", line 656, in _runsshserver
  remote:     rsp = wireprotov1server.dispatch(repo, proto, request)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/wireprotov1server.py", line 74, in dispatch
  remote:     return func(repo, proto, *args)
  remote:   File "/tmp/hgtests.i66Npc/install/lib/python/mercurial/wireprotov1server.py", line 195, in batch
  remote:     data[k] = vals[k]
  remote: KeyError: 'namespace'
  abort: unexpected response: empty string
  [255]

Differential Revision: https://phab.mercurial-scm.org/D5482

tests/test-missing-capability.t

@@ -0,0 +1,46 b''
+Checking how hg behaves when one side of a pull/push doesn't support
+some capability (because it's running an older hg version, usually).
+
+  $ hg init repo1
+  $ cd repo1
+  $ echo a > a; hg add -q a; hg commit -q -m a
+  $ hg bookmark a
+  $ hg clone -q . ../repo2
+  $ cd ../repo2
+
+  $ touch $TESTTMP/disable-lookup.py
+  $ disable_cap() {
+  >   rm -f $TESTTMP/disable-lookup.pyc # pyc caching is buggy
+  >   cat <<EOF > $TESTTMP/disable-lookup.py
+  > from mercurial import extensions, wireprotov1server
+  > def wcapabilities(orig, *args, **kwargs):
+  >   cap = orig(*args, **kwargs)
+  >   cap.remove('$1')
+  >   return cap
+  > extensions.wrapfunction(wireprotov1server, '_capabilities', wcapabilities)
+  > EOF
+  > }
+  $ cat >> ../repo1/.hg/hgrc <<EOF
+  > [extensions]
+  > disable-lookup = $TESTTMP/disable-lookup.py
+  > EOF
+  $ cat >> .hg/hgrc <<EOF
+  > [ui]
+  > ssh = "$PYTHON" "$TESTDIR/dummyssh"
+  > EOF
+
+  $ hg pull ssh://user@dummy/repo1 -r tip -B a
+  pulling from ssh://user@dummy/repo1
+  no changes found
+
+  $ disable_cap lookup
+  $ hg pull ssh://user@dummy/repo1 -r tip -B a
+  pulling from ssh://user@dummy/repo1
+  abort: other repository doesn't support revision lookup, so a rev cannot be specified.
+  [255]
+
+  $ disable_cap pushkey
+  $ hg pull ssh://user@dummy/repo1 -r tip -B a
+  pulling from ssh://user@dummy/repo1
+  abort: remote bookmark a not found!
+  [255]

mercurial/wireprotov1peer.py

@@ -240,12 +240,15 b' class peerexecutor(object):'
 
             # Encoded arguments and future holding remote result.
             try:
-                encodedargs, fremote = next(batchable)
+                encargsorres, fremote = next(batchable)
             except Exception:
                 pycompat.future_set_exception_info(f, sys.exc_info()[1:])
                 return
 
-            requests.append((command, encodedargs))
+            if not fremote:
+                f.set_result(encargsorres)
+            else:
+                requests.append((command, encargsorres))
             states.append((command, f, batchable, fremote))
 
         if not requests: