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In most of the backend code, XLogReaders open a WAL segment file with BasicOpenFile() (xlogutils.c, WAL summarizer and WAL sender). On ERROR, the opened file descriptor could leak while reading WAL segments, piling them over time if a session can spawn XLogReaders at will, leading to EMFILE with various consequences, the worst being a backend-side PANIC. The problem has been reported as reachable with pg_walinspect (EXECUTE privilege revoked from PUBLIC by default), or replication slot functions (REPLICATION privilege required). XLogReaderState gains a memory context reset callback, registered if a segment_open callback has the idea to open a WAL segment with BasicOpenFile(). XLogReaderRegisterResetCallback() can be used by a segment_open callback to ensure the cleanup of any opened file descriptor. No backpatch is done. This breaks the ABI of XLogReaderState (sizeof), and MemoryContextUnregisterResetCallback() is new in v19. There may be an argument for fixing this issue in v19, but the release is close enough that this change does not seem worth taking a risk for. Author: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com> Reviewed-by: Sami Imseih <samimseih.pg@gmail.com> Reviewed-by: Chao Li <li.evan.chao@gmail.com> Reviewed-by: Michael Paquier <michael@paquier.xyz> Discussion: https://postgr.es/m/CALj2ACVwDuOXXDjj2cVdnTKoxsgTSLDin4XoL63AnM6aUgMQaA@mail.gmail.com