Fix concurrent sequence drops during sequence synchronization.

A recent BF failure showed that commit 7a485bd641 did not handle the case
where a sequence is dropped concurrently during sequence synchronization
on the subscriber. Previously, pg_get_sequence_data() would ERROR out if
the sequence was dropped concurrently. After 7a485bd641, it instead
returns NULL, which leads to an assertion failure on the subscriber.

To handle this change, update sequence synchronization to skip sequences
for which pg_get_sequence_data() returns NULL.

Author: vignesh C <vignesh21@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CALDaNm0FoGdt+1mzua0t-=wYdup5_zmFrvfNf-L=MGBnj9HAcg@mail.gmail.com

diff --git a/src/backend/replication/logical/sequencesync.c b/src/backend/replication/logical/sequencesync.c
index 509388d1bf4e0cc11838c7992258915c196da1a8..165f909b3ba97b695c36a87c6c4cb8ae7f6b8b8e 100644 (file)
--- a/src/backend/replication/logical/sequencesync.c
+++ b/src/backend/replication/logical/sequencesync.c
@@ -233,6 +233,7 @@ get_and_validate_seq_info(TupleTableSlot *slot, Relation *sequence_rel,
 {
    bool        isnull;
    int         col = 0;
+   Datum       datum;
    Oid         remote_typid;
    int64       remote_start;
    int64       remote_increment;
@@ -251,8 +252,14 @@ get_and_validate_seq_info(TupleTableSlot *slot, Relation *sequence_rel,
    *seqinfo = seqinfo_local =
        (LogicalRepSequenceInfo *) list_nth(seqinfos, *seqidx);
 
-   seqinfo_local->last_value = DatumGetInt64(slot_getattr(slot, ++col, &isnull));
-   Assert(!isnull);
+   /*
+    * last_value can be NULL if the sequence was dropped concurrently (see
+    * pg_get_sequence_data()).
+    */
+   datum = slot_getattr(slot, ++col, &isnull);
+   if (isnull)
+       return COPYSEQ_SKIPPED;
+   seqinfo_local->last_value = DatumGetInt64(datum);
 
    seqinfo_local->is_called = DatumGetBool(slot_getattr(slot, ++col, &isnull));
    Assert(!isnull);
@@ -400,7 +407,7 @@ copy_sequences(WalReceiverConn *conn)
        int         batch_skipped_count = 0;
        int         batch_insuffperm_count = 0;
        int         batch_missing_count;
-       Relation    sequence_rel;
+       Relation    sequence_rel = NULL;
 
        WalRcvExecResult *res;
        TupleTableSlot *slot;
@@ -535,11 +542,21 @@ copy_sequences(WalReceiverConn *conn)
                    batch_insuffperm_count++;
                    break;
                case COPYSEQ_SKIPPED:
-                   ereport(LOG,
-                           errmsg("skip synchronization of sequence \"%s.%s\" because it has been dropped concurrently",
-                                  seqinfo->nspname,
-                                  seqinfo->seqname));
-                   batch_skipped_count++;
+
+                   /*
+                    * Concurrent removal of a sequence on the subscriber is
+                    * treated as success, since the only viable action is to
+                    * skip the corresponding sequence data. Missing sequences
+                    * on the publisher are treated as ERROR.
+                    */
+                   if (seqinfo->found_on_pub)
+                   {
+                       ereport(LOG,
+                               errmsg("skip synchronization of sequence \"%s.%s\" because it has been dropped concurrently",
+                                      seqinfo->nspname,
+                                      seqinfo->seqname));
+                       batch_skipped_count++;
+                   }
                    break;
            }