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commit    26c4e896869093f07e9acb982f71bb3c4dae5901
Author:   Masahiko Sawada <msawada@postgresql.org>
Date:     Sat Nov 16 01:06:00 2024 +0000

    Fix a possibility of logical replication slot's restart_lsn going backwards.
    
    Previously LogicalIncreaseRestartDecodingForSlot() accidentally
    accepted any LSN as the candidate_lsn and candidate_valid after the
    restart_lsn of the replication slot was updated, so it potentially
    caused the restart_lsn to move backwards.
    
    A scenario where this could happen in logical replication is: after a
    logical replication restart, based on previous candidate_lsn and
    candidate_valid values in memory, the restart_lsn advances upon
    receiving a subscriber acknowledgment. Then, logical decoding restarts
    from an older point, setting candidate_lsn and candidate_valid based
    on an old RUNNING_XACTS record. Subsequent subscriber acknowledgments
    then update the restart_lsn to an LSN older than the current value.
    
    In the reported case, after WAL files were removed by a checkpoint,
    the retreated restart_lsn prevented logical replication from
    restarting due to missing WAL segments.
    
    This change essentially modifies the 'if' condition to 'else if'
    condition within the function. The previous code had an asymmetry in
    this regard compared to LogicalIncreaseXminForSlot(), which does
    almost the same thing for different fields.
    
    The WAL removal issue was reported by Hubert Depesz Lubaczewski.
    
    Backpatch to all supported versions, since the bug exists since 9.4
    where logical decoding was introduced.
    
    Reviewed-by: Tomas Vondra, Ashutosh Bapat, Amit Kapila
    Discussion: https://postgr.es/m/Yz2hivgyjS1RfMKs%40depesz.com
    Discussion: https://postgr.es/m/85fff40e-148b-4e86-b921-b4b846289132%40vondra.me
    Backpatch-through: 13


 src/backend/replication/logical/logical.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index 2f7b2c85d9b..2d76164de53 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -1645,6 +1645,7 @@ LogicalIncreaseRestartDecodingForSlot(XLogRecPtr current_lsn, XLogRecPtr restart
 	/* don't overwrite if have a newer restart lsn */
 	if (restart_lsn <= slot->data.restart_lsn)
 	{
+		SpinLockRelease(&slot->mutex);
 	}
 
 	/*
@@ -1655,6 +1656,7 @@ LogicalIncreaseRestartDecodingForSlot(XLogRecPtr current_lsn, XLogRecPtr restart
 	{
 		slot->candidate_restart_valid = current_lsn;
 		slot->candidate_restart_lsn = restart_lsn;
+		SpinLockRelease(&slot->mutex);
 
 		/* our candidate can directly be used */
 		updated_lsn = true;
@@ -1665,7 +1667,7 @@ LogicalIncreaseRestartDecodingForSlot(XLogRecPtr current_lsn, XLogRecPtr restart
 	 * might never end up updating if the receiver acks too slowly. A missed
 	 * value here will just cause some extra effort after reconnecting.
 	 */
-	if (slot->candidate_restart_valid == InvalidXLogRecPtr)
+	else if (slot->candidate_restart_valid == InvalidXLogRecPtr)
 	{
 		slot->candidate_restart_valid = current_lsn;
 		slot->candidate_restart_lsn = restart_lsn;


[parent: 2bdd3b248924]