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pg_signal_dispatch_thread() responded to the client (signal sender) and disconnected the pipe before actually setting the shared variables that make the signal visible to the backend process's main thread. In the worst case, it seems, effective delivery of the signal could be postponed for as long as the machine has any other work to do. To fix, just move the pg_queue_signal() call so that we do it before responding to the client. This essentially makes pgkill() synchronous, which is a stronger guarantee than we have on Unix. That may be overkill, but on the other hand we have not seen comparable timing bugs on any Unix platform. While at it, add some comments to this sadly underdocumented code. Problem diagnosis and fix by Amit Kapila; I just added the comments. Back-patch to all supported versions, as it appears that this can cause visible NOTIFY timing oddities on all of them, and there might be other misbehavior due to slow delivery of other signals. Discussion: https://postgr.es/m/32745.1575303812@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/.