forked from MirrorHub/synapse
Make sure that we close cursors before returning from a query (#6408)
There are lots of words in the comment as to why this is a good idea. Fixes #6403.
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changelog.d/6408.bugfix
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changelog.d/6408.bugfix
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@ -0,0 +1 @@
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Fix an intermittent exception when handling read-receipts.
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@ -409,16 +409,15 @@ class SQLBaseStore(object):
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i = 0
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N = 5
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while True:
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cursor = LoggingTransaction(
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conn.cursor(),
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name,
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self.database_engine,
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after_callbacks,
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exception_callbacks,
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)
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try:
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txn = conn.cursor()
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txn = LoggingTransaction(
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txn,
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name,
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self.database_engine,
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after_callbacks,
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exception_callbacks,
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)
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r = func(txn, *args, **kwargs)
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r = func(cursor, *args, **kwargs)
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conn.commit()
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return r
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except self.database_engine.module.OperationalError as e:
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@ -456,6 +455,40 @@ class SQLBaseStore(object):
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)
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continue
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raise
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finally:
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# we're either about to retry with a new cursor, or we're about to
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# release the connection. Once we release the connection, it could
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# get used for another query, which might do a conn.rollback().
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#
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# In the latter case, even though that probably wouldn't affect the
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# results of this transaction, python's sqlite will reset all
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# statements on the connection [1], which will make our cursor
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# invalid [2].
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#
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# In any case, continuing to read rows after commit()ing seems
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# dubious from the PoV of ACID transactional semantics
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# (sqlite explicitly says that once you commit, you may see rows
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# from subsequent updates.)
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#
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# In psycopg2, cursors are essentially a client-side fabrication -
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# all the data is transferred to the client side when the statement
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# finishes executing - so in theory we could go on streaming results
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# from the cursor, but attempting to do so would make us
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# incompatible with sqlite, so let's make sure we're not doing that
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# by closing the cursor.
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#
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# (*named* cursors in psycopg2 are different and are proper server-
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# side things, but (a) we don't use them and (b) they are implicitly
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# closed by ending the transaction anyway.)
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#
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# In short, if we haven't finished with the cursor yet, that's a
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# problem waiting to bite us.
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#
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# TL;DR: we're done with the cursor, so we can close it.
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#
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# [1]: https://github.com/python/cpython/blob/v3.8.0/Modules/_sqlite/connection.c#L465
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# [2]: https://github.com/python/cpython/blob/v3.8.0/Modules/_sqlite/cursor.c#L236
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cursor.close()
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except Exception as e:
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logger.debug("[TXN FAIL] {%s} %s", name, e)
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raise
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@ -280,7 +280,7 @@ class ReceiptsWorkerStore(SQLBaseStore):
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args.append(limit)
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txn.execute(sql, args)
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return (r[0:5] + (json.loads(r[5]),) for r in txn)
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return list(r[0:5] + (json.loads(r[5]),) for r in txn)
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return self.runInteraction(
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"get_all_updated_receipts", get_all_updated_receipts_txn
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