Fix ALTER SEQUENCE locking

In 1753b1b027035029c2a2a1649065762fafbf63f3, the pg_sequence system
catalog was introduced.  This made sequence metadata changes
transactional, while the actual sequence values are still behaving
nontransactionally.  This requires some refinement in how ALTER
SEQUENCE, which operates on both, locks the sequence and the catalog.

The main problems were:

- Concurrent ALTER SEQUENCE causes "tuple concurrently updated" error,
  caused by updates to pg_sequence catalog.

- Sequence WAL writes and catalog updates are not protected by same
  lock, which could lead to inconsistent recovery order.

- nextval() disregarding uncommitted ALTER SEQUENCE changes.

To fix, nextval() and friends now lock the sequence using
RowExclusiveLock instead of AccessShareLock.  ALTER SEQUENCE locks the
sequence using ShareRowExclusiveLock.  This means that nextval() and
ALTER SEQUENCE block each other, and ALTER SEQUENCE on the same sequence
blocks itself.  (This was already the case previously for the OWNER TO,
RENAME, and SET SCHEMA variants.)  Also, rearrange some code so that the
entire AlterSequence is protected by the lock on the sequence.

As an exception, use reduced locking for ALTER SEQUENCE ... RESTART.
Since that is basically a setval(), it does not require the full locking
of other ALTER SEQUENCE actions.  So check whether we are only running a
RESTART and run with less locking if so.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>
Reported-by: Jason Petersen <jason@citusdata.com>
Reported-by: Andres Freund <andres@anarazel.de>

1 files changed, 48 insertions, 21 deletions

diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
index 4a56926b530..0f7cf1dce8a 100644
--- a/src/backend/commands/sequence.c
+++ b/src/backend/commands/sequence.c
@@ -93,11 +93,12 @@ static HTAB *seqhashtab = NULL; /* hash table for SeqTable items */
 static SeqTableData *last_used_seq = NULL;
 
 static void fill_seq_with_data(Relation rel, HeapTuple tuple);
-static Relation open_share_lock(SeqTable seq);
+static Relation lock_and_open_sequence(SeqTable seq);
 static void create_seq_hashtable(void);
 static void init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel);
 static Form_pg_sequence_data read_seq_tuple(Relation rel,
 			   Buffer *buf, HeapTuple seqdatatuple);
+static LOCKMODE alter_sequence_get_lock_level(List *options);
 static void init_params(ParseState *pstate, List *options, bool for_identity,
 						bool isInit,
 						Form_pg_sequence seqform,
@@ -427,7 +428,9 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
 	HeapTuple	tuple;
 
 	/* Open and lock sequence. */
-	relid = RangeVarGetRelid(stmt->sequence, AccessShareLock, stmt->missing_ok);
+	relid = RangeVarGetRelid(stmt->sequence,
+							 alter_sequence_get_lock_level(stmt->options),
+							 stmt->missing_ok);
 	if (relid == InvalidOid)
 	{
 		ereport(NOTICE,
@@ -443,12 +446,6 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
 		aclcheck_error(ACLCHECK_NOT_OWNER, ACL_KIND_CLASS,
 					   stmt->sequence->relname);
 
-	/* lock page' buffer and read tuple into new sequence structure */
-	seqdata = read_seq_tuple(seqrel, &buf, &seqdatatuple);
-
-	/* Copy old sequence data into workspace */
-	memcpy(&newseqdata, seqdata, sizeof(FormData_pg_sequence_data));
-
 	rel = heap_open(SequenceRelationId, RowExclusiveLock);
 	tuple = SearchSysCacheCopy1(SEQRELID,
 								ObjectIdGetDatum(relid));
@@ -458,6 +455,12 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
 
 	seqform = (Form_pg_sequence) GETSTRUCT(tuple);
 
+	/* lock page's buffer and read tuple into new sequence structure */
+	seqdata = read_seq_tuple(seqrel, &buf, &seqdatatuple);
+
+	/* Copy old sequence data into workspace */
+	memcpy(&newseqdata, seqdata, sizeof(FormData_pg_sequence_data));
+
 	/* Check and set new values */
 	init_params(pstate, stmt->options, stmt->for_identity, false, seqform, &changed_seqform, &newseqdata, &owned_by);
 
@@ -508,12 +511,12 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
 
 	ObjectAddressSet(address, RelationRelationId, relid);
 
-	relation_close(seqrel, NoLock);
-
 	if (changed_seqform)
 		CatalogTupleUpdate(rel, &tuple->t_self, tuple);
 	heap_close(rel, RowExclusiveLock);
 
+	relation_close(seqrel, NoLock);
+
 	return address;
 }
 
@@ -594,7 +597,7 @@ nextval_internal(Oid relid, bool check_permissions)
 	bool		cycle;
 	bool		logit = false;
 
-	/* open and AccessShareLock sequence */
+	/* open and lock sequence */
 	init_sequence(relid, &elm, &seqrel);
 
 	if (check_permissions &&
@@ -829,7 +832,7 @@ currval_oid(PG_FUNCTION_ARGS)
 	SeqTable	elm;
 	Relation	seqrel;
 
-	/* open and AccessShareLock sequence */
+	/* open and lock sequence */
 	init_sequence(relid, &elm, &seqrel);
 
 	if (pg_class_aclcheck(elm->relid, GetUserId(),
@@ -869,7 +872,7 @@ lastval(PG_FUNCTION_ARGS)
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("lastval is not yet defined in this session")));
 
-	seqrel = open_share_lock(last_used_seq);
+	seqrel = lock_and_open_sequence(last_used_seq);
 
 	/* nextval() must have already been called for this sequence */
 	Assert(last_used_seq->last_valid);
@@ -913,7 +916,7 @@ do_setval(Oid relid, int64 next, bool iscalled)
 	int64		maxv,
 				minv;
 
-	/* open and AccessShareLock sequence */
+	/* open and lock sequence */
 	init_sequence(relid, &elm, &seqrel);
 
 	if (pg_class_aclcheck(elm->relid, GetUserId(), ACL_UPDATE) != ACLCHECK_OK)
@@ -1042,15 +1045,15 @@ setval3_oid(PG_FUNCTION_ARGS)
 
 
 /*
- * Open the sequence and acquire AccessShareLock if needed
+ * Open the sequence and acquire lock if needed
  *
  * If we haven't touched the sequence already in this transaction,
- * we need to acquire AccessShareLock.  We arrange for the lock to
+ * we need to acquire a lock.  We arrange for the lock to
  * be owned by the top transaction, so that we don't need to do it
  * more than once per xact.
  */
 static Relation
-open_share_lock(SeqTable seq)
+lock_and_open_sequence(SeqTable seq)
 {
 	LocalTransactionId thislxid = MyProc->lxid;
 
@@ -1063,7 +1066,7 @@ open_share_lock(SeqTable seq)
 		PG_TRY();
 		{
 			CurrentResourceOwner = TopTransactionResourceOwner;
-			LockRelationOid(seq->relid, AccessShareLock);
+			LockRelationOid(seq->relid, RowExclusiveLock);
 		}
 		PG_CATCH();
 		{
@@ -1078,7 +1081,7 @@ open_share_lock(SeqTable seq)
 		seq->lxid = thislxid;
 	}
 
-	/* We now know we have AccessShareLock, and can safely open the rel */
+	/* We now know we have the lock, and can safely open the rel */
 	return relation_open(seq->relid, NoLock);
 }
 
@@ -1135,7 +1138,7 @@ init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel)
 	/*
 	 * Open the sequence relation.
 	 */
-	seqrel = open_share_lock(elm);
+	seqrel = lock_and_open_sequence(elm);
 
 	if (seqrel->rd_rel->relkind != RELKIND_SEQUENCE)
 		ereport(ERROR,
@@ -1217,6 +1220,30 @@ read_seq_tuple(Relation rel, Buffer *buf, HeapTuple seqdatatuple)
 }
 
 /*
+ * Check the sequence options list and return the appropriate lock level for
+ * ALTER SEQUENCE.
+ *
+ * Most sequence option changes require a self-exclusive lock and should block
+ * concurrent nextval() et al.  But RESTART does not, because it's not
+ * transactional.  Also take a lower lock if no option at all is present.
+ */
+static LOCKMODE
+alter_sequence_get_lock_level(List *options)
+{
+	ListCell   *option;
+
+	foreach(option, options)
+	{
+		DefElem    *defel = (DefElem *) lfirst(option);
+
+		if (strcmp(defel->defname, "restart") != 0)
+			return ShareRowExclusiveLock;
+	}
+
+	return RowExclusiveLock;
+}
+
+/*
  * init_params: process the options list of CREATE or ALTER SEQUENCE, and
  * store the values into appropriate fields of seqform, for changes that go
  * into the pg_sequence catalog, and seqdataform for changes to the sequence
@@ -1850,7 +1877,7 @@ pg_sequence_last_value(PG_FUNCTION_ARGS)
 	bool		is_called;
 	int64		result;
 
-	/* open and AccessShareLock sequence */
+	/* open and lock sequence */
 	init_sequence(relid, &elm, &seqrel);
 
 	if (pg_class_aclcheck(relid, GetUserId(), ACL_SELECT | ACL_USAGE) != ACLCHECK_OK)