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The original implementation of ConditionVariableBroadcast was, per its self-description, "the dumbest way possible". Thomas Munro found out it was a bit too dumb. An awakened process may immediately re-queue itself, if the specific condition it's waiting for is not yet satisfied. If this happens before ConditionVariableBroadcast is able to see the wait queue as empty, then ConditionVariableBroadcast will re-awaken the same process, repeating the cycle. Given unlucky timing this back-and-forth can repeat indefinitely; loops lasting thousands of seconds have been seen in testing. To fix, add our own process to the end of the wait queue to serve as a sentinel, and exit the broadcast loop once our process is not there anymore. There are various special considerations described in the comments, the principal disadvantage being that wakers can no longer be sure whether they awakened a real waiter or just a sentinel. But in practice nobody pays attention to the result of ConditionVariableSignal or ConditionVariableBroadcast anyway, so that problem seems hypothetical. Back-patch to v10 where condition_variable.c was introduced. Tom Lane and Thomas Munro Discussion: https://postgr.es/m/CAEepm=0NWKehYw7NDoUSf8juuKOPRnCyY3vuaSvhrEWsOTAa3w@mail.gmail.com |
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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/.