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51865a0a01
Trading a little too heavily on letting the code path be the same whether we were creating shared data structures or only attaching to them, InitPredicateLocks() inserted the "scratch" PredicateLockTargetHash entry unconditionally. This is just wrong if we're in a postmaster child, which would only reach this code in EXEC_BACKEND builds. Most of the time, the hash_search(HASH_ENTER) call would simply report that the entry already existed, causing no visible effect since the code did not bother to check for that possibility. However, if this happened while some other backend had transiently removed the "scratch" entry, then that other backend's eventual RestoreScratchTarget would suffer an assert failure; this appears to be the explanation for a recent failure on buildfarm member culicidae. In non-assert builds, there would be no visible consequences there either. But nonetheless this is a pretty bad bug for EXEC_BACKEND builds, for two reasons: 1. Each new backend would perform the hash_search(HASH_ENTER) call without holding any lock that would prevent concurrent access to the PredicateLockTargetHash hash table. This creates a low but certainly nonzero risk of corruption of that hash table. 2. In the event that the race condition occurred, by reinserting the scratch entry too soon, we were defeating the entire purpose of the scratch entry, namely to guarantee that transaction commit could move hash table entries around with no risk of out-of-memory failure. The odds of an actual OOM failure are quite low, but not zero, and if it did happen it would again result in corruption of the hash table. The user-visible symptoms of such corruption are a little hard to predict, but would presumably amount to misbehavior of SERIALIZABLE transactions that'd require a crash or postmaster restart to fix. To fix, just skip the hash insertion if IsUnderPostmaster. I also inserted a bunch of assertions that the expected things happen depending on whether IsUnderPostmaster is true. That might be overkill, since most comparable code in other functions isn't quite that paranoid, but once burnt twice shy. In passing, also move a couple of lines to places where they seemed to make more sense. Diagnosis of problem by Thomas Munro, patch by me. Back-patch to all supported branches. Discussion: https://postgr.es/m/10593.1500670709@sss.pgh.pa.us |
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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/.