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Test and fix find_first_stream_ordering_after_ts

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It seemed to suffer from a bunch of off-by-one errors.
This commit is contained in:
Richard van der Hoff 2018-03-05 11:47:48 +00:00
parent 06a14876e5
commit c818fcab11
2 changed files with 119 additions and 14 deletions
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66
synapse/storage/event_push_actions.py
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@ -493,15 +493,15 @@ class EventPushActionsWorkerStore(SQLBaseStore):
"""Gets the stream ordering corresponding to a given timestamp.
Specifically, finds the stream_ordering of the first event that was
received after the timestamp. This is done by a binary search on the
events table, since there is no index on received_ts, so is
received on or after the timestamp. This is done by a binary search on
the events table, since there is no index on received_ts, so is
relatively slow.
Args:
ts (int): timestamp in millis
Returns:
Deferred[int]: stream ordering of the first event received after
Deferred[int]: stream ordering of the first event received on/after
the timestamp
"""
return self.runInteraction(
@ -510,16 +510,24 @@ class EventPushActionsWorkerStore(SQLBaseStore):
ts,
)
def _find_first_stream_ordering_after_ts_txn(self, txn, ts):
@staticmethod
def _find_first_stream_ordering_after_ts_txn(txn, ts):
"""
Find the stream_ordering of the first event that was received after
a given timestamp. This is relatively slow as there is no index on
received_ts but we can then use this to delete push actions before
Find the stream_ordering of the first event that was received on or
after a given timestamp. This is relatively slow as there is no index
on received_ts but we can then use this to delete push actions before
this.
received_ts must necessarily be in the same order as stream_ordering
and stream_ordering is indexed, so we manually binary search using
stream_ordering
Args:
txn (twisted.enterprise.adbapi.Transaction):
ts (int): timestamp to search for
Returns:
int: stream ordering
"""
txn.execute("SELECT MAX(stream_ordering) FROM events")
max_stream_ordering = txn.fetchone()[0]
@ -527,23 +535,53 @@ class EventPushActionsWorkerStore(SQLBaseStore):
if max_stream_ordering is None:
return 0
# We want the first stream_ordering in which received_ts is greater
# than or equal to ts. Call this point X.
#
# We maintain the invariants:
#
# range_start <= X <= range_end
#
range_start = 0
range_end = max_stream_ordering
range_end = max_stream_ordering + 1
# Given a stream_ordering, look up the timestamp at that
# stream_ordering.
#
# The array may be sparse (we may be missing some stream_orderings).
# We treat the gaps as the same as having the same value as the
# preceding entry, because we will pick the lowest stream_ordering
# which satisfies our requirement of received_ts >= ts.
#
# For example, if our array of events indexed by stream_ordering is
# [10, <none>, 20], we should treat this as being equivalent to
# [10, 10, 20].
#
sql = (
"SELECT received_ts FROM events"
" WHERE stream_ordering > ?"
" ORDER BY stream_ordering"
" WHERE stream_ordering <= ?"
" ORDER BY stream_ordering DESC"
" LIMIT 1"
)
while range_end - range_start > 1:
middle = int((range_end + range_start) / 2)
while range_end - range_start > 0:
middle = (range_end + range_start) // 2
txn.execute(sql, (middle,))
middle_ts = txn.fetchone()[0]
row = txn.fetchone()
if row is None:
# no rows with stream_ordering<=middle
range_start = middle + 1
continue
middle_ts = row[0]
if ts > middle_ts:
range_start = middle
# we got a timestamp lower than the one we were looking for.
# definitely need to look higher: X > middle.
range_start = middle + 1
else:
# we got a timestamp higher than (or the same as) the one we
# were looking for. We aren't yet sure about the point we
# looked up, but we can be sure that X <= middle.
range_end = middle
return range_end