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A pending asynchronous request is handled by process_pending_request(), which previously not only processed an in-progress remote query but performed ExecForeignScan() to produce a tuple to return to the local server asynchronously from the result of the remote query. But that led to a server crash when executing a query or led to an "InstrStartNode called twice in a row" or "InstrEndLoop called on running node" failure when doing EXPLAIN ANALYZE of it, in cases where the plan tree for it contained multiple async-capable nodes accessing the same initplan/subplan that contained multiple async-capable nodes scanning the same foreign tables as for the parent async-capable nodes, as reported by Andrey Lepikhov. The reason is that the second step in process_pending_request() invoked when executing the initplan/subplan for one of the parent async-capable nodes caused recursive execution of the initplan/subplan for another of the parent async-capable nodes. To fix, split process_pending_request() into the two steps and postpone the second step until ForeignAsyncConfigureWait() is called for each of the pending asynchronous requests. Also, in ExecAppendAsyncEventWait() we assumed that FDWs would register at least one wait event in a WaitEventSet created there when they were called from ForeignAsyncConfigureWait() in that function, but allow FDWs to register zero wait events in the WaitEventSet; modify ExecAppendAsyncEventWait() to just return in that case. Oversight in commit 27e1f1456. Back-patch to v14 where that commit went in. Andrey Lepikhov and Etsuro Fujita Discussion: https://postgr.es/m/fe5eaa19-1704-e4a4-76ee-3b9d37ade399@postgrespro.ru
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/.