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We have repeatedly seen the buildfarm reach the Assert(false) in SyncRepGetSyncStandbysPriority. This apparently is due to failing to consider the possibility that the sync_standby_priority values in shared memory might be inconsistent; but they will be whenever only some of the walsenders have updated their values after a change in the synchronous_standby_names setting. That function is vastly too complex for what it does, anyway, so rewriting it seems better than trying to apply a band-aid fix. Furthermore, the API of SyncRepGetSyncStandbys is broken by design: it returns a list of WalSnd array indexes, but there is nothing guaranteeing that the contents of the WalSnd array remain stable. Thus, if some walsender exits and then a new walsender process takes over that WalSnd array slot, a caller might make use of WAL position data that it should not, potentially leading to incorrect decisions about whether to release transactions that are waiting for synchronous commit. To fix, replace SyncRepGetSyncStandbys with a new function SyncRepGetCandidateStandbys that copies all the required data from shared memory while holding the relevant mutexes. If the associated walsender process then exits, this data is still safe to make release decisions with, since we know that that much WAL *was* sent to a valid standby server. This incidentally means that we no longer need to treat sync_standby_priority as protected by the SyncRepLock rather than the per-walsender mutex. SyncRepGetSyncStandbys is no longer used by the core code, so remove it entirely in HEAD. However, it seems possible that external code is relying on that function, so do not remove it from the back branches. Instead, just remove the known-incorrect Assert. When the bug occurs, the function will return a too-short list, which callers should treat as meaning there are not enough sync standbys, which seems like a reasonably safe fallback until the inconsistent state is resolved. Moreover it's bug-compatible with what has been happening in non-assert builds. We cannot do anything about the walsender-replacement race condition without an API/ABI break. The bogus assertion exists back to 9.6, but 9.6 is sufficiently different from the later branches that the patch doesn't apply at all. I chose to just remove the bogus assertion in 9.6, feeling that the probability of a bad outcome from the walsender-replacement race condition is too low to justify rewriting the whole patch for 9.6. Discussion: https://postgr.es/m/21519.1585272409@sss.pgh.pa.us
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. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ 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 may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.