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The apply worker needs to update the state of the subscription tables to 'READY' during the synchronization phase which requires locking the corresponding subscription. The apply worker also waits for the subscription tables to reach the 'SYNCDONE' state after holding the locks on the subscription and the wait is done using WaitLatch. The 'SYNCDONE' state is changed by tablesync workers again by locking the corresponding subscription. Both the state updates use AccessShareLock mode to lock the subscription, so they can't block each other. However, a backend can simultaneously try to acquire a lock on the same subscription using AccessExclusiveLock mode to alter the subscription. Now, the backend's wait on a lock can sneak in between the apply worker and table sync worker causing deadlock. In other words, apply_worker waits for tablesync worker which waits for backend, and backend waits for apply worker. This is not detected by the deadlock detector because apply worker uses WaitLatch. The fix is to release existing locks in apply worker before it starts to wait for tablesync worker to change the state. Reported-by: Tomas Vondra Author: Shlok Kyal Reviewed-by: Amit Kapila, Peter Smith Backpatch-through: 12 Discussion: https://postgr.es/m/d291bb50-12c4-e8af-2af2-7bb9bb4d8e3e@enterprisedb.com
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.