sql/schemachanger: clean up SequenceOwner elements during restore

Previously, when restoring a backup taken in middle of a DROP COLUMN,
where a column had a sequence owner assigned, it was possible for the
backup to be unrestorable. This would happen because the sequence
reference would have been dropped in the plan, but the seqeunce owner
element was still within the state. To address this, this test updates
the rewrite logic to clean up any SequenceOwner elements which have the
referenced sequence already removed.

Fixes: cockroachdb#130778

Release note (bug fix): Addressed a rare bug that could prevent backups
taken during a DROP COLUMN where the column was sequence owner. The
restore would fail with:  "rewriting descriptor ids: missing rewrite for
<id> in SequenceOwner..."

pkg/sql/catalog/rewrite/rewrite.go

@@ -767,6 +767,23 @@ func rewriteSchemaChangerState(
 			data.SchemaID = rewrite.ParentSchemaID
 			continue
 		}
+
+		// maybeRemoveMissingReference potentially removes and element based on
+		// missing references, when it is safe to do so.
+		maybeRemoveMissingReference := func(element scpb.Element, missingID descpb.ID) (removed bool) {
+			switch e := element.(type) {
+			case *scpb.SequenceOwner:
+				if e.SequenceID == missingID {
+					state.Targets = append(state.Targets[:i], state.Targets[i+1:]...)
+					state.CurrentStatuses = append(state.CurrentStatuses[:i], state.CurrentStatuses[i+1:]...)
+					state.TargetRanks = append(state.TargetRanks[:i], state.TargetRanks[i+1:]...)
+					i--
+					return true
+				}
+			}
+			return false
+		}
+
 		if err := screl.WalkDescIDs(t.Element(), func(id *descpb.ID) error {
 			if *id == descpb.InvalidID {
 				// Some descriptor ID fields in elements may be deliberately unset.
@@ -775,6 +792,13 @@ func rewriteSchemaChangerState(
 			}
 			rewrite, ok := descriptorRewrites[*id]
 			if !ok {
+				// If the reference was removed, no rewrite is needed
+				// and we are done.
+				if maybeRemoveMissingReference(t.Element(), *id) {
+					return nil
+				}
+				// Otherwise, something is gravely wrong and the state cannot
+				// be repaired.
 				return errors.Errorf("missing rewrite for id %d in %s", *id, screl.ElementString(t.Element()))
 			}
 			*id = rewrite.ID

pkg/sql/schemachanger/testdata/end_to_end/drop_column_sequence_owner/drop_column_sequence_owner.definition

@@ -0,0 +1,49 @@
+setup
+CREATE TABLE t (i INT PRIMARY KEY, j INT, k int);
+CREATE SEQUENCE sq1 OWNED BY t.j;
+COMMENT ON TABLE t IS 't has a comment';
+COMMENT ON COLUMN t.j IS 'j has a comment';
+INSERT INTO t VALUES(-1);
+INSERT INTO t VALUES(-2);
+INSERT INTO t VALUES(-3);
+----
+
+stage-exec phase=PostCommitPhase stage=:
+INSERT INTO t VALUES($stageKey);
+INSERT INTO t VALUES($stageKey + 1);
+UPDATE t SET k=$stageKey;
+UPDATE t SET k=i;
+DELETE FROM t WHERE i=-1;
+DELETE FROM t WHERE i=$stageKey;
+INSERT INTO t VALUES($stageKey);
+INSERT INTO t VALUES(-1);
+----
+
+# Each insert will be injected twice per stage, plus 3 injected
+# at the start.
+stage-query phase=PostCommitPhase stage=:
+SELECT count(*)=($successfulStageCount*2)+3 FROM t;
+----
+true
+
+stage-exec phase=PostCommitNonRevertiblePhase stage=:
+INSERT INTO t VALUES($stageKey);
+INSERT INTO t VALUES($stageKey + 1);
+UPDATE t SET k=$stageKey;
+UPDATE t SET k=i;
+DELETE FROM t WHERE i=-1;
+DELETE FROM t WHERE i=$stageKey;
+INSERT INTO t VALUES($stageKey);
+INSERT INTO t VALUES(-1);
+----
+
+# Each insert will be injected twice per stage, plus 3 injected
+# at the start.
+stage-query phase=PostCommitNonRevertiblePhase stage=:
+SELECT count(*)=($successfulStageCount*2)+3 FROM t;
+----
+true
+
+test
+ALTER TABLE t DROP COLUMN j
+----