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Fix 500 error on /messages when we accumulate more than 5 backward extremities (#11027)

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Found while working on the Gitter backfill script and noticed
it only happened after we sent 7 batches, https://gitlab.com/gitterHQ/webapp/-/merge_requests/2229#note_665906390

When there are more than 5 backward extremities for a given depth,
backfill will throw an error because we sliced the extremity list
to 5 but then try to iterate over the full list. This causes
us to look for state that we never fetched and we get a `KeyError`.

Before when calling `/messages` when there are more than 5 backward extremities:
```
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/synapse/http/server.py", line 258, in _async_render_wrapper
    callback_return = await self._async_render(request)
  File "/usr/local/lib/python3.8/site-packages/synapse/http/server.py", line 446, in _async_render
    callback_return = await raw_callback_return
  File "/usr/local/lib/python3.8/site-packages/synapse/rest/client/room.py", line 580, in on_GET
    msgs = await self.pagination_handler.get_messages(
  File "/usr/local/lib/python3.8/site-packages/synapse/handlers/pagination.py", line 396, in get_messages
    await self.hs.get_federation_handler().maybe_backfill(
  File "/usr/local/lib/python3.8/site-packages/synapse/handlers/federation.py", line 133, in maybe_backfill
    return await self._maybe_backfill_inner(room_id, current_depth, limit)
  File "/usr/local/lib/python3.8/site-packages/synapse/handlers/federation.py", line 386, in _maybe_backfill_inner
    likely_extremeties_domains = get_domains_from_state(states[e_id])
KeyError: '$zpFflMEBtZdgcMQWTakaVItTLMjLFdKcRWUPHbbSZJl'
```
This commit is contained in:
Eric Eastwood 2021-10-14 18:53:45 -05:00 • committed by GitHub
parent efd0074ab7
commit daf498e099
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GPG key ID: 4AEE18F83AFDEB23
4 changed files with 79 additions and 12 deletions
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24
synapse/handlers/federation.py
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@ -238,18 +238,10 @@ class FederationHandler:
)
return False
logger.debug(
"room_id: %s, backfill: current_depth: %s, max_depth: %s, extrems: %s",
room_id,
current_depth,
max_depth,
sorted_extremeties_tuple,
)
# We ignore extremities that have a greater depth than our current depth
# as:
# 1. we don't really care about getting events that have happened
# before our current position; and
# after our current position; and
# 2. we have likely previously tried and failed to backfill from that
# extremity, so to avoid getting "stuck" requesting the same
# backfill repeatedly we drop those extremities.
@ -257,9 +249,19 @@ class FederationHandler:
t for t in sorted_extremeties_tuple if int(t[1]) <= current_depth
]
logger.debug(
"room_id: %s, backfill: current_depth: %s, limit: %s, max_depth: %s, extrems: %s filtered_sorted_extremeties_tuple: %s",
room_id,
current_depth,
limit,
max_depth,
sorted_extremeties_tuple,
filtered_sorted_extremeties_tuple,
)
# However, we need to check that the filtered extremities are non-empty.
# If they are empty then either we can a) bail or b) still attempt to
# backill. We opt to try backfilling anyway just in case we do get
# backfill. We opt to try backfilling anyway just in case we do get
# relevant events.
if filtered_sorted_extremeties_tuple:
sorted_extremeties_tuple = filtered_sorted_extremeties_tuple
@ -389,7 +391,7 @@ class FederationHandler:
for key, state_dict in states.items()
}
for e_id, _ in sorted_extremeties_tuple:
for e_id in event_ids:
likely_extremeties_domains = get_domains_from_state(states[e_id])
success = await try_backfill(