Prevent underflow in KeepLogSeg().

The call to XLogGetReplicationSlotMinimumLSN() might return a
greater LSN than the one given to the function.  Subsequent segment
number calculations might then underflow, which could result in
unexpected behavior when removing or recyling WAL files.  This was
introduced with max_slot_wal_keep_size in c655077639.  To fix, skip
the block of code for replication slots if the LSN is greater.

Reported-by: Xu Xingwang
Author: Kyotaro Horiguchi
Reviewed-by: Junwang Zhao
Discussion: https://postgr.es/m/17903-4288d439dee856c6%40postgresql.org
Backpatch-through: 13

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 32c6bb5d288a14c4a91703df626bff9fee8a0082..691b9622dad5ea959a1dcd65618b1a60fe7a3d8a 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7298,7 +7298,7 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
     * max_slot_wal_keep_size.
     */
    keep = XLogGetReplicationSlotMinimumLSN();
-   if (keep != InvalidXLogRecPtr)
+   if (keep != InvalidXLogRecPtr && keep < recptr)
    {
        XLByteToSeg(keep, segno, wal_segment_size);