Make identity sequence management more robust

Some code could get confused when certain catalog state involving both
identity and serial sequences was present, perhaps during an attempt
to upgrade the latter to the former.  Specifically, dropping the
default of a serial column maintains the ownership of the sequence by
the column, and so it would then be possible to afterwards make the
column an identity column that would now own two sequences.  This
causes the code that looks up the identity sequence to error out,
making the new identity column inoperable until the ownership of the
previous sequence is released.

To fix this, make the identity sequence lookup only consider sequences
with the appropriate dependency type for an identity sequence, so it
only ever finds one (unless something else is broken).  In the above
example, the old serial sequence would then be ignored.  Reorganize
the various owned-sequence-lookup functions a bit to make this
clearer.
Reported-by: Laurenz Albe <laurenz.albe@cybertec.at>
Discussion: https://www.postgresql.org/message-id/flat/470c54fc8590be4de0f41b0d295fd6390d5e8a6c.camel@cybertec.at

src/backend/catalog/pg_depend.c

@@ -705,10 +705,11 @@ sequenceIsOwned(Oid seqId, char deptype, Oid *tableId, int32 *colId)
 
 /*
  * Collect a list of OIDs of all sequences owned by the specified relation,
- * and column if specified.
+ * and column if specified.  If deptype is not zero, then only find sequences
+ * with the specified dependency type.
  */
-List *
-getOwnedSequences(Oid relid, AttrNumber attnum)
+static List *
+getOwnedSequences_internal(Oid relid, AttrNumber attnum, char deptype)
 {
 	List	   *result = NIL;
 	Relation	depRel;
@@ -750,7 +751,8 @@ getOwnedSequences(Oid relid, AttrNumber attnum)
 			(deprec->deptype == DEPENDENCY_AUTO || deprec->deptype == DEPENDENCY_INTERNAL) &&
 			get_rel_relkind(deprec->objid) == RELKIND_SEQUENCE)
 		{
-			result = lappend_oid(result, deprec->objid);
+			if (!deptype || deprec->deptype == deptype)
+				result = lappend_oid(result, deprec->objid);
 		}
 	}
 
@@ -762,17 +764,32 @@ getOwnedSequences(Oid relid, AttrNumber attnum)
 }
 
 /*
- * Get owned sequence, error if not exactly one.
+ * Collect a list of OIDs of all sequences owned (identity or serial) by the
+ * specified relation.
+ */
+List *
+getOwnedSequences(Oid relid)
+{
+	return getOwnedSequences_internal(relid, 0, 0);
+}
+
+/*
+ * Get owned identity sequence, error if not exactly one.
  */
 Oid
-getOwnedSequence(Oid relid, AttrNumber attnum)
+getIdentitySequence(Oid relid, AttrNumber attnum, bool missing_ok)
 {
-	List	   *seqlist = getOwnedSequences(relid, attnum);
+	List	   *seqlist = getOwnedSequences_internal(relid, attnum, DEPENDENCY_INTERNAL);
 
 	if (list_length(seqlist) > 1)
 		elog(ERROR, "more than one owned sequence found");
 	else if (list_length(seqlist) < 1)
-		elog(ERROR, "no owned sequence found");
+	{
+		if (missing_ok)
+			return InvalidOid;
+		else
+			elog(ERROR, "no owned sequence found");
+	}
 
 	return linitial_oid(seqlist);
 }

src/backend/commands/tablecmds.c

@@ -1689,7 +1689,7 @@ ExecuteTruncateGuts(List *explicit_rels, List *relids, List *relids_logged,
 		foreach(cell, rels)
 		{
 			Relation	rel = (Relation) lfirst(cell);
-			List	   *seqlist = getOwnedSequences(RelationGetRelid(rel), 0);
+			List	   *seqlist = getOwnedSequences(RelationGetRelid(rel));
 			ListCell   *seqcell;
 
 			foreach(seqcell, seqlist)
@@ -6635,7 +6635,7 @@ ATExecDropIdentity(Relation rel, const char *colName, bool missing_ok, LOCKMODE
 	table_close(attrelation, RowExclusiveLock);
 
 	/* drop the internal sequence */
-	seqid = getOwnedSequence(RelationGetRelid(rel), attnum);
+	seqid = getIdentitySequence(RelationGetRelid(rel), attnum, false);
 	deleteDependencyRecordsForClass(RelationRelationId, seqid,
 									RelationRelationId, DEPENDENCY_INTERNAL);
 	CommandCounterIncrement();

src/backend/parser/parse_utilcmd.c

@@ -1083,7 +1083,7 @@ transformTableLikeClause(CreateStmtContext *cxt, TableLikeClause *table_like_cla
 			 * find sequence owned by old column; extract sequence parameters;
 			 * build new create sequence command
 			 */
-			seq_relid = getOwnedSequence(RelationGetRelid(relation), attribute->attnum);
+			seq_relid = getIdentitySequence(RelationGetRelid(relation), attribute->attnum, false);
 			seq_options = sequence_options(seq_relid);
 			generateSerialExtraStmts(cxt, def,
 									 InvalidOid, seq_options, true,
@@ -3165,7 +3165,7 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
 					if (attnum != InvalidAttrNumber &&
 						TupleDescAttr(tupdesc, attnum - 1)->attidentity)
 					{
-						Oid			seq_relid = getOwnedSequence(relid, attnum);
+						Oid			seq_relid = getIdentitySequence(relid, attnum, false);
 						Oid			typeOid = typenameTypeId(pstate, def->typeName);
 						AlterSeqStmt *altseqstmt = makeNode(AlterSeqStmt);
 
@@ -3216,7 +3216,6 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
 					ListCell   *lc;
 					List	   *newseqopts = NIL;
 					List	   *newdef = NIL;
-					List	   *seqlist;
 					AttrNumber	attnum;
 
 					/*
@@ -3237,14 +3236,13 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
 
 					if (attnum)
 					{
-						seqlist = getOwnedSequences(relid, attnum);
-						if (seqlist)
+						Oid			seq_relid = getIdentitySequence(relid, attnum, true);
+
+						if (seq_relid)
 						{
 							AlterSeqStmt *seqstmt;
-							Oid			seq_relid;
 
 							seqstmt = makeNode(AlterSeqStmt);
-							seq_relid = linitial_oid(seqlist);
 							seqstmt->sequence = makeRangeVar(get_namespace_name(get_rel_namespace(seq_relid)),
 															 get_rel_name(seq_relid), -1);
 							seqstmt->options = newseqopts;

src/backend/rewrite/rewriteHandler.c

@@ -1130,7 +1130,7 @@ build_column_default(Relation rel, int attrno)
 	{
 		NextValueExpr *nve = makeNode(NextValueExpr);
 
-		nve->seqid = getOwnedSequence(RelationGetRelid(rel), attrno);
+		nve->seqid = getIdentitySequence(RelationGetRelid(rel), attrno, false);
 		nve->typeId = att_tup->atttypid;
 
 		return (Node *) nve;

src/include/catalog/dependency.h

@@ -209,8 +209,8 @@ extern long changeDependenciesOn(Oid refClassId, Oid oldRefObjectId,
 extern Oid	getExtensionOfObject(Oid classId, Oid objectId);
 
 extern bool sequenceIsOwned(Oid seqId, char deptype, Oid *tableId, int32 *colId);
-extern List *getOwnedSequences(Oid relid, AttrNumber attnum);
-extern Oid	getOwnedSequence(Oid relid, AttrNumber attnum);
+extern List *getOwnedSequences(Oid relid);
+extern Oid	getIdentitySequence(Oid relid, AttrNumber attnum, bool missing_ok);
 
 extern Oid	get_constraint_index(Oid constraintId);
 

src/test/regress/expected/identity.out

@@ -399,3 +399,8 @@ CREATE TABLE itest_child PARTITION OF itest_parent (
 ) FOR VALUES FROM ('2016-07-01') TO ('2016-08-01'); -- error
 ERROR:  identity columns are not supported on partitions
 DROP TABLE itest_parent;
+-- test that sequence of half-dropped serial column is properly ignored
+CREATE TABLE itest14 (id serial);
+ALTER TABLE itest14 ALTER id DROP DEFAULT;
+ALTER TABLE itest14 ALTER id ADD GENERATED BY DEFAULT AS IDENTITY;
+INSERT INTO itest14 (id) VALUES (DEFAULT);

src/test/regress/sql/identity.sql

@@ -254,3 +254,11 @@ CREATE TABLE itest_child PARTITION OF itest_parent (
     f3 WITH OPTIONS GENERATED ALWAYS AS IDENTITY
 ) FOR VALUES FROM ('2016-07-01') TO ('2016-08-01'); -- error
 DROP TABLE itest_parent;
+
+
+-- test that sequence of half-dropped serial column is properly ignored
+
+CREATE TABLE itest14 (id serial);
+ALTER TABLE itest14 ALTER id DROP DEFAULT;
+ALTER TABLE itest14 ALTER id ADD GENERATED BY DEFAULT AS IDENTITY;
+INSERT INTO itest14 (id) VALUES (DEFAULT);