Fix recently added incorrect assertion

Commit df3737a651 added an incorrect assertion about the preconditions
for invoking the backup cleanup callback: it misfires at session end in
case a backup completes successfully.  Fix it, using coding from Michaël
Paquier.  Also add some tests for the various cases.

Reported by Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20221021.161038.1277961198945653224.horikyota.ntt@gmail.com

src/backend/access/transam/xlog.c

@@ -8841,9 +8841,8 @@ do_pg_abort_backup(int code, Datum arg)
 {
 	bool		during_backup_start = DatumGetBool(arg);
 
-	/* Only one of these conditions can be true */
-	Assert(during_backup_start ^
-		   (sessionBackupState == SESSION_BACKUP_RUNNING));
+	/* If called during backup start, there shouldn't be one already running */
+	Assert(!during_backup_start || sessionBackupState == SESSION_BACKUP_NONE);
 
 	if (during_backup_start || sessionBackupState != SESSION_BACKUP_NONE)
 	{

src/test/recovery/t/020_archive_status.pl

@@ -248,4 +248,18 @@ my $logfile = slurp_file($standby2->logfile, $log_location);
 ok( $logfile =~ qr/archiver process shutting down/,
 	'check shutdown callback of shell archive module');
 
+# Test that we can enter and leave backup mode without crashes
+my ($stderr, $cmdret);
+$cmdret = $primary->psql(
+	'postgres',
+	"SELECT pg_backup_start('onebackup'); "
+	  . "SELECT pg_backup_stop();"
+	  . "SELECT pg_backup_start(repeat('x', 1026))",
+	stderr => \$stderr);
+is($cmdret, 3, "psql fails correctly");
+like($stderr, qr/backup label too long/, "pg_backup_start fails gracefully");
+$primary->safe_psql('postgres',
+	"SELECT pg_backup_start('onebackup'); SELECT pg_backup_stop();");
+$primary->safe_psql('postgres', "SELECT pg_backup_start('twobackup')");
+
 done_testing();