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When two (or more) transactions are waiting for transaction T1 to release a tuple-level lock, and transaction T1 upgrades its lock to a higher level, a spurious deadlock can be reported among the waiting transactions when T1 finishes. The simplest example case seems to be: T1: select id from job where name = 'a' for key share; Y: select id from job where name = 'a' for update; -- starts waiting for X Z: select id from job where name = 'a' for key share; T1: update job set name = 'b' where id = 1; Z: update job set name = 'c' where id = 1; -- starts waiting for X T1: rollback; At this point, transaction Y is rolled back on account of a deadlock: Y holds the heavyweight tuple lock and is waiting for the Xmax to be released, while Z holds part of the multixact and tries to acquire the heavyweight lock (per protocol) and goes to sleep; once X releases its part of the multixact, Z is awakened only to be put back to sleep on the heavyweight lock that Y is holding while sleeping. Kaboom. This can be avoided by having Z skip the heavyweight lock acquisition. As far as I can see, the biggest downside is that if there are multiple Z transactions, the order in which they resume after X finishes is not guaranteed. Backpatch to 9.6. The patch applies cleanly on 9.5, but the new tests don't work there (because isolationtester is not smart enough), so I'm not going to risk it. Author: Oleksii Kliukin Discussion: https://postgr.es/m/B9C9D7CD-EB94-4635-91B6-E558ACEC0EC3@hintbits.com |
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| authentication | ||
| examples | ||
| isolation | ||
| kerberos | ||
| ldap | ||
| locale | ||
| mb | ||
| modules | ||
| perl | ||
| recovery | ||
| regress | ||
| ssl | ||
| subscription | ||
| thread | ||
| Makefile | ||
| README | ||
PostgreSQL tests ================ This directory contains a variety of test infrastructure as well as some of the tests in PostgreSQL. Not all tests are here -- in particular, there are more in individual contrib/ modules and in src/bin. Not all these tests get run by "make check". Check src/test/Makefile to see which tests get run automatically. authentication/ Tests for authentication examples/ Demonstration programs for libpq that double as regression tests via "make check" isolation/ Tests for concurrent behavior at the SQL level locale/ Sanity checks for locale data, encodings, etc mb/ Tests for multibyte encoding (UTF-8) support modules/ Extensions used only or mainly for test purposes, generally not suitable for installing in production databases perl/ Infrastructure for Perl-based TAP tests recovery/ Test suite for recovery and replication regress/ PostgreSQL's main regression test suite, pg_regress ssl/ Tests to exercise and verify SSL certificate handling subscription/ Tests for logical replication thread/ A thread-safety-testing utility used by configure