tests/test-newercgi.t

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

#require no-msys # MSYS will translate web paths as if they were file paths

This is a rudimentary test of the CGI files as of d74fc8dec2b4.

  $ hg init test

  $ cat >hgweb.cgi <<HGWEB
  > #!$PYTHON
  > #
  > # An example CGI script to use hgweb, edit as necessary
  > 
  > import cgitb
  > cgitb.enable()
  > 
  > from mercurial import demandimport; demandimport.enable()
  > from mercurial.hgweb import hgweb
  > from mercurial.hgweb import wsgicgi
  > 
  > application = hgweb("test", "Empty test repository")
  > wsgicgi.launch(application)
  > HGWEB

  $ chmod 755 hgweb.cgi

  $ cat >hgweb.config <<HGWEBDIRCONF
  > [paths]
  > test = test
  > HGWEBDIRCONF

  $ cat >hgwebdir.cgi <<HGWEBDIR
  > #!$PYTHON
  > #
  > # An example CGI script to export multiple hgweb repos, edit as necessary
  > 
  > import cgitb
  > cgitb.enable()
  > 
  > from mercurial import demandimport; demandimport.enable()
  > from mercurial.hgweb import hgwebdir
  > from mercurial.hgweb import wsgicgi
  > 
  > application = hgwebdir("hgweb.config")
  > wsgicgi.launch(application)
  > HGWEBDIR

  $ chmod 755 hgwebdir.cgi

  $ . "$TESTDIR/cgienv"
  $ $PYTHON hgweb.cgi > page1
  $ $PYTHON hgwebdir.cgi > page2

  $ PATH_INFO="/test/"
  $ PATH_TRANSLATED="/var/something/test.cgi"
  $ REQUEST_URI="/test/test/"
  $ SCRIPT_URI="http://hg.omnifarious.org/test/test/"
  $ SCRIPT_URL="/test/test/"
  $ $PYTHON hgwebdir.cgi > page3

  $ grep -i error page1 page2 page3
  [1]