Issue with cancel_before_shmem_exit while searching to remove a particular registered exit callbacks

Hi,

I found an issue while executing a backup use case(please see [1] for
queries) on postgres version 12.

Firstly, pg_start_backup registers nonexclusive_base_backup_cleanup as
on_shmem_exit call back which will
add this function to the before_shmem_exit_list array which is
supposed to be removed on pg_stop_backup
so that we can do the pending cleanup and issue a warning for each
pg_start_backup for which we did not call
the pg_stop backup. Now, I executed a query for which JIT is picked
up, then the the llvm compiler inserts it's
own exit callback i.e. llvm_shutdown at the end of
before_shmem_exit_list array. Now, I executed pg_stop_backup
and call to cancel_before_shmem_exit() is made with the expectation
that the nonexclusive_base_backup_cleanup
callback is removed from before_shmem_exit_list array.

Since the cancel_before_shmem_exit() only checks the last entry in the
before_shmem_exit_list array, which is
llvm compiler's exit callback, so we exit the
cancel_before_shmem_exit() without removing the intended
nonexclusive_base_backup_cleanup callback which remains still the
before_shmem_exit_list and gets executed
during the session exit throwing a warning "aborting backup due to
backend exiting before pg_stop_backup was called",
which is unintended.

Attached is the patch that fixes the above problem by making
cancel_before_shmem_exit() to look for the
given function(and for given args) in the entire
before_shmem_exit_list array, not just the last entry, starting
from the last entry.

Request the community take this patch for review for v12.

Having said that, abovementioned problem for backup use case does not
occur for v13 and latest versions of
postgres (please below description[2]), but these kinds of issues can
come, if the cancel_before_shmem_exit()
is left to just look at the last array entry while removing a
registered callback.

There's also a comment in cancel_before_shmem_exit() function
description "For simplicity, only the latest entry
can be removed. (We could work harder but there is no need for current uses.)

Since we start to find use cases now, there is a need to enhance
cancel_before_shmem_exit(), so I also propose
to have the same attached patch for v13 and latest versions.

Thoughts?

[1]
CREATE TABLE t1 (id SERIAL);
INSERT INTO t1 (id) SELECT * FROM generate_series(1, 20000000);
SELECT * FROM pg_start_backup('label', false, false);
/*JIT is enabled in my session and it is being picked by below query*/
EXPLAIN (ANALYZE, VERBOSE, BUFFERS) SELECT COUNT(*) FROM t1;
SELECT * FROM pg_stop_backup(false, true);

[2]
for v13 and latest versions, start_backup first registers do_pg_abort_backup,
and then pg_start_backup_callback, performs startup backup operations
and unregisters only pg_start_backup_callback from before_shmem_exit_list,
retaining do_pg_abort_backup still in the list, which is to be called
on session's exit
JIT compiler inserts it's own exit call back at the end of
before_shmem_exit_list array.
stop_backup registers pg_stop_backup_callback, performs stop operations,
unregisters pg_stop_backup_callback from before_shmem_exit_list, and sets
the sessionBackupState = SESSION_BACKUP_NONE, note that the callback
do_pg_abort_backup registered by start_backup command still exists in the
before_shmem_exit_list and will not be removed by stop_backup. On session exit,
do_pg_abort_backup gets called but returns without performing any operations(not
even throwing a warning), by checking sessionBackupState which was set to
SESSION_BACKUP_NONE by stop_backup.

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com