Fix portability issues with parsing of recovery_target_xid

michaelpq · michaelpq · commit 6db27037b993 · 2020-12-23T12:51:22.000+09:00
The parsing of this parameter has been using strtoul(), which is not portable across platforms. On most Unix platforms, unsigned long has a size of 64 bits, while on Windows it is 32 bits. It is common in recovery scenarios to rely on the output of txid_current() or even the newer pg_current_xact_id() to get a transaction ID for setting up recovery_target_xid. The value returned by those functions includes the epoch in the computed result, which would cause strtoul() to fail where unsigned long has a size of 32 bits once the epoch is incremented. WAL records and 2PC data include only information about 32-bit XIDs and it is not possible to have XIDs across more than one epoch, so discarding the high bits from the transaction ID set has no impact on recovery. On the contrary, the use of strtoul() prevents a consistent behavior across platforms depending on the size of unsigned long. This commit changes the parsing of recovery_target_xid to use pg_strtouint64() instead, available down to 9.6. There is one TAP test stressing recovery with recovery_target_xid, where a tweak based on pg_reset{xlog,wal} is added to bump the XID epoch so as this change gets tested, as per an idea from Alexander Lakhin. Reported-by: Alexander Lakhin Discussion: https://postgr.es/m/16780-107fd0c0385b1035@postgresql.org Backpatch-through: 9.6
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
@@ -11938,7 +11938,7 @@ check_recovery_target_xid(char **newval, void **extra, GucSource source)
 		TransactionId *myextra;
 
 		errno = 0;
-		xid = (TransactionId) strtoul(*newval, NULL, 0);
+		xid = (TransactionId) pg_strtouint64(*newval, NULL, 0);
 		if (errno == EINVAL || errno == ERANGE)
 			return false;
 
diff --git a/src/test/recovery/t/003_recovery_targets.pl b/src/test/recovery/t/003_recovery_targets.pl
@@ -50,6 +50,10 @@ sub test_recovery_standby
 my $node_primary = get_new_node('primary');
 $node_primary->init(has_archiving => 1, allows_streaming => 1);
 
+# Bump the transaction ID epoch.  This is useful to stress the portability
+# of recovery_target_xid parsing.
+system_or_bail('pg_resetwal', '--epoch', '1', $node_primary->data_dir);
+
 # Start it
 $node_primary->start;
 
