Details
Major - P3 Description
The lookaside table cursors are not operating at isolation level WT_ISO_READ_UNCOMMITTED as intended.
Ticket WT-3891 in February of 2018 included [commit f74fd95|https://github.com/wiredtiger/wiredtiger/pull/3920/commits/f74fd951dac3efcd8b866314d734196a07c77f6f].
That commit changed the previous code:
WT_RET(__wt_txn_begin(las_session, NULL));
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las_session->txn.isolation = WT_ISO_READ_UNCOMMITTED;
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to:
static void__las_set_read_uncommitted(
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WT_SESSION_IMPL *session, WT_TXN_ISOLATION *saved_isolationp)
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{
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*saved_isolationp = session->txn.isolation;
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session->txn.isolation = WT_ISO_READ_UNCOMMITTED;
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}
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...
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__las_set_read_uncommitted(las_session, &saved_isolation); WT_ERR(__wt_txn_begin(las_session, NULL));
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However, the __wt_txn_begin() function blindly sets the isolation level:
static inline int
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__wt_txn_begin(WT_SESSION_IMPL *session, const char *cfg[])
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{
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WT_TXN *txn;
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|
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txn = &session->txn;
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txn->isolation = session->isolation;
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...
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so the change had the effect of changing all LAS operations to operate at the isolation of the underlying WT_SESSION handle, not WT_ISO_READ_UNCOMMITTED as intended.
Summary:
Fix a bug where lookaside table cursors were not operating at isolation level WT_ISO_READ_UNCOMMITTED as intended.