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Fix port script fails when DB has no backfilled events. #8729

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Nov 11, 2020
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Fix port script fails when DB has no backfilled events. #8729

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c646698
Fix port script fails when DB has no backfilled events.
erikjohnston Nov 6, 2020
d3432a3
Newsfile
erikjohnston Nov 6, 2020
bd0be5a
Remove superfluous condition
erikjohnston Nov 11, 2020
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1 change: 1 addition & 0 deletions changelog.d/8729.bugfix
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@@ -0,0 +1 @@
Fix port script fails when DB has no backfilled events. Broke in v1.21.0.
12 changes: 5 additions & 7 deletions scripts/synapse_port_db
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Expand Up @@ -820,14 +820,12 @@ class Porter(object):
"ALTER SEQUENCE events_stream_seq RESTART WITH %s", (next_id,)
)

txn.execute("SELECT -MIN(stream_ordering) FROM events")
txn.execute("SELECT GREATEST(-MIN(stream_ordering), 1) FROM events")
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I assume this does fix things, but what is the purpose of negating the minimum stream_ordering here? And wouldn't using GREATEST and - here mean that we almost always end up with 1 as it's greater?

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stream_ordering is negative for backfilled events, so if e.g. the min is -52, then GREATEST(- -52 1) is 52, while if there are no backfilled events we may end up with a positive min so we get GREATEST(-1, 1) which is 1.

curr_id = txn.fetchone()[0]
if curr_id:
next_id = curr_id + 1
txn.execute(
"ALTER SEQUENCE events_backfill_stream_seq RESTART WITH %s",
(next_id,),
)
next_id = curr_id + 1
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sooo.. if there are no backfilled events, the SELECT will return 1, so we'll set the RESTART value to 2. I mean, it's not the end of the world, but it doesn't sound right.

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I mean, it really doesn't matter? I can change it if you want, but we do the exact same thing from a fresh DB too

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we do? fair enough. Thought you might change the GREATEST to 0. I don't feel strongly.

txn.execute(
"ALTER SEQUENCE events_backfill_stream_seq RESTART WITH %s", (next_id,),
)

return self.postgres_store.db_pool.runInteraction(
"_setup_events_stream_seqs", r
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