Rework shutdown callback of archiver modules

As currently designed, with a callback registered in a ERROR_CLEANUP
block, the shutdown callback would get called twice when updating
archive_library on SIGHUP, which is something that we want to avoid to
ease the life of extension writers.

Anyway, an ERROR in the archiver process is treated as a FATAL, stopping
it immediately, hence there is no need for a ERROR_CLEANUP block.
Instead of that, the shutdown callback is not called upon
before_shmem_exit(), giving to the modules the opportunity to do any
cleanup actions before the server shuts down its subsystems.

While on it, this commit adds some testing coverage for the shutdown
callback.  Neither shell_archive nor basic_archive have been using it,
and one is added to shell_archive, whose trigger is checked in a TAP
test through a shutdown sequence.

Author: Nathan Bossart, Bharath Rupireddy
Reviewed-by: Kyotaro Horiguchi, Michael Paquier
Discussion: https://postgr.es/m/20221015221328.GB1821022@nathanxps13
Backpatch-through: 15

1 files changed, 8 insertions, 0 deletions

diff --git a/src/backend/postmaster/shell_archive.c b/src/backend/postmaster/shell_archive.c
index 8a54d02e7bb..71d567065a2 100644
--- a/src/backend/postmaster/shell_archive.c
+++ b/src/backend/postmaster/shell_archive.c
@@ -23,6 +23,7 @@
 
 static bool shell_archive_configured(void);
 static bool shell_archive_file(const char *file, const char *path);
+static void shell_archive_shutdown(void);
 
 void
 shell_archive_init(ArchiveModuleCallbacks *cb)
@@ -31,6 +32,7 @@ shell_archive_init(ArchiveModuleCallbacks *cb)
 
 	cb->check_configured_cb = shell_archive_configured;
 	cb->archive_file_cb = shell_archive_file;
+	cb->shutdown_cb = shell_archive_shutdown;
 }
 
 static bool
@@ -156,3 +158,9 @@ shell_archive_file(const char *file, const char *path)
 	elog(DEBUG1, "archived write-ahead log file \"%s\"", file);
 	return true;
 }
+
+static void
+shell_archive_shutdown(void)
+{
+	elog(DEBUG1, "archiver process shutting down");
+}