[PATCH 1 of 4] client: fail gracefully on unexpected prompts (issue5516)

[Gábor Stefanik]

# HG changeset patch
# User Gábor Stefanik <gabor.stefanik at nng.com>
# Date 1503326357 -7200
#      Mon Aug 21 16:39:17 2017 +0200
# Node ID 242d719e85ce451e61ab262eec443668bb4792ce
# Parent  8e959ad6a25c9fe6ab490e1a4cbd5f7445d9bcb1
client: fail gracefully on unexpected prompts (issue5516)

Right now, if hglib encounters an unexpected prompt, it fails with a rather
opaque "unexpected data on required channel 'L'" error message. Furthermore,
if subsequently another command is called on the same client instance, both
the client and server processes lock up hard (at least on Windows), and the
server process rapidly leaks ~2GB of memory.

Fix this by responding with an empty string to any unexpected prompt. This
will trigger an "abort: response expected" exception from the server, which
is easily handled as a CommandError, and subsequent commands sent from the
same client work as expected.

This doesn't completely resolve bug 5516, as unexpected requests on another
required channel (e.g. I) can still cause a lockup.
However, it does fix the most common case of an unexpected password prompt.

diff -r 8e959ad6a25c -r 242d719e85ce hglib/client.py
--- a/hglib/client.py   Mon Apr 03 16:02:08 2017 -0500
+++ b/hglib/client.py   Mon Aug 21 16:39:17 2017 +0200
@@ -235,6 +235,8 @@
                 reply = prompt(size, out.getvalue())
                 return reply
             inchannels[b('L')] = func
+        else:
+            inchannels[b('L')] = lambda _: ''
         if input is not None:
             inchannels[b('I')] = input

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