Prevent crash on pagination.

[rkfg]

Today I've got pagination stuck on scrolling back and this was in the log:

2018-12-05 09:22:58,198 - synapse.http.server - 112 - ERROR - GET-8313- Failed handle request
via <function _async_render at 0x7f7d18ff8de8>: <SynapseRequest at 0x7f7ccb48d680
method=u'GET' uri=u'/_matrix/client/r0/rooms/<...>/messages?from=t361589-2272987_8803529_88_1180453_319160_392_21245_28472_14&
limit=20&dir=b&filter=%7B%22lazy_load_members%22%3Atrue%7D' clientproto=u'HTTP/1.0' site=8009>: Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 654, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1475, in gotResult
    _inlineCallbacks(r, g, status)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 491, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
--- <exception caught here> ---
  File "/usr/lib/python2.7/dist-packages/synapse/http/server.py", line 81, in wrapped_request_handler
    yield h(self, request)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 491, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python2.7/dist-packages/synapse/http/server.py", line 316, in _async_render
    callback_return = yield callback(request, **kwargs)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 491, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python2.7/dist-packages/synapse/rest/client/v1/room.py", line 479, in on_GET
    event_filter=event_filter,
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/usr/lib/python2.7/dist-packages/synapse/handlers/pagination.py", line 266, in get_messages
    events[0].event_id, state_filter=state_filter,
exceptions.IndexError: list index out of range

So I added a check if events actually has something and then pagination worked again. No idea why it broke, I have some ignored users so maybe that's why.

[codecov-io]

Codecov Report

Merging #4263 into develop will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #4263      +/-   ##
===========================================
- Coverage    73.63%   73.62%   -0.01%     
===========================================
  Files          298      298              
  Lines        29776    29776              
  Branches      4875     4875              
===========================================
- Hits         21925    21923       -2     
- Misses        6419     6420       +1     
- Partials      1432     1433       +1

Impacted Files	Coverage Δ
synapse/handlers/pagination.py	91.66% <100%> (ø)	⬆️
synapse/state/v1.py	90.69% <0%> (-1.56%)	⬇️
synapse/state/v2.py	93.29% <0%> (-0.52%)	⬇️
synapse/handlers/user_directory.py	71.03% <0%> (+0.3%)	⬆️

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[erikjohnston]

Thanks! Sorry for taking so long to look at it.

[richvdh]

@erikjohnston please can we not use the word "crash" when we mean "throw an exception". "crash" sounds like the whole process dies.