The code specific to the execution of archive_cleanup_command,
recovery_end_command and restore_command is moved to a new file named
shell_restore.c. The code is split into three functions:
- shell_restore(), that attempts the execution of a shell-based
restore_command.
- shell_archive_cleanup(), for archive_cleanup_command.
- shell_recovery_end(), for recovery_end_command.
This introduces no functional changes, with failure patterns and logs
generated in consequence being the same as before (one case actually
generates one less DEBUG2 message "could not restore" when a restore
command succeeds but the follow-up stat() to check the size fails, but
that only matters with a elevel high enough).
This is preparatory work for allowing recovery modules, a facility
similar to archive modules, with callbacks shaped similarly to the
functions introduced here.
Author: Nathan Bossart
Reviewed-by: Andres Freund, Michael Paquier
Discussion: https://postgr.es/m/20221227192449.GA3672473@nathanxps13
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