Make TRUNCATE ... RESTART IDENTITY restart sequences transactionally.

In the previous coding, we simply issued ALTER SEQUENCE RESTART commands,
which do not roll back on error.  This meant that an error between
truncating and committing left the sequences out of sync with the table
contents, with potentially bad consequences as were noted in a Warning on
the TRUNCATE man page.

To fix, create a new storage file (relfilenode) for a sequence that is to
be reset due to RESTART IDENTITY.  If the transaction aborts, we'll
automatically revert to the old storage file.  This acts just like a
rewriting ALTER TABLE operation.  A penalty is that we have to take
exclusive lock on the sequence, but since we've already got exclusive lock
on its owning table, that seems unlikely to be much of a problem.

The interaction of this with usual nontransactional behaviors of sequence
operations is a bit weird, but it's hard to see what would be completely
consistent.  Our choice is to discard cached-but-unissued sequence values
both when the RESTART is executed, and at rollback if any; but to not touch
the currval() state either time.

In passing, move the sequence reset operations to happen before not after
any AFTER TRUNCATE triggers are fired.  The previous ordering was not
logically sensible, but was forced by the need to minimize inconsistency
if the triggers caused an error.  Transactional rollback is a much better
solution to that.

Patch by Steve Singer, rather heavily adjusted by me.

1 files changed, 117 insertions, 35 deletions

diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
index 62d1fbfb0eb..bb8ebce25a0 100644
--- a/src/backend/commands/sequence.c
+++ b/src/backend/commands/sequence.c
@@ -68,6 +68,7 @@ typedef struct SeqTableData
 {
 	struct SeqTableData *next;	/* link to next SeqTable object */
 	Oid			relid;			/* pg_class OID of this sequence */
+	Oid			filenode;		/* last seen relfilenode of this sequence */
 	LocalTransactionId lxid;	/* xact in which we last did a seq op */
 	bool		last_valid;		/* do we have a valid "last" value? */
 	int64		last;			/* value last returned by nextval */
@@ -87,6 +88,7 @@ static SeqTable seqtab = NULL;	/* Head of list of SeqTable items */
  */
 static SeqTableData *last_used_seq = NULL;
 
+static void fill_seq_with_data(Relation rel, HeapTuple tuple);
 static int64 nextval_internal(Oid relid);
 static Relation open_share_lock(SeqTable seq);
 static void init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel);
@@ -109,9 +111,6 @@ DefineSequence(CreateSeqStmt *seq)
 	CreateStmt *stmt = makeNode(CreateStmt);
 	Oid			seqoid;
 	Relation	rel;
-	Buffer		buf;
-	Page		page;
-	sequence_magic *sm;
 	HeapTuple	tuple;
 	TupleDesc	tupDesc;
 	Datum		value[SEQ_COL_LASTCOL];
@@ -211,6 +210,100 @@ DefineSequence(CreateSeqStmt *seq)
 	rel = heap_open(seqoid, AccessExclusiveLock);
 	tupDesc = RelationGetDescr(rel);
 
+	/* now initialize the sequence's data */
+	tuple = heap_form_tuple(tupDesc, value, null);
+	fill_seq_with_data(rel, tuple);
+
+	/* process OWNED BY if given */
+	if (owned_by)
+		process_owned_by(rel, owned_by);
+
+	heap_close(rel, NoLock);
+}
+
+/*
+ * Reset a sequence to its initial value.
+ *
+ * The change is made transactionally, so that on failure of the current
+ * transaction, the sequence will be restored to its previous state.
+ * We do that by creating a whole new relfilenode for the sequence; so this
+ * works much like the rewriting forms of ALTER TABLE.
+ *
+ * Caller is assumed to have acquired AccessExclusiveLock on the sequence,
+ * which must not be released until end of transaction.  Caller is also
+ * responsible for permissions checking.
+ */
+void
+ResetSequence(Oid seq_relid)
+{
+	Relation	seq_rel;
+	SeqTable	elm;
+	Form_pg_sequence seq;
+	Buffer		buf;
+	Page		page;
+	HeapTuple	tuple;
+	HeapTupleData tupledata;
+	ItemId		lp;
+
+	/*
+	 * Read the old sequence.  This does a bit more work than really
+	 * necessary, but it's simple, and we do want to double-check that it's
+	 * indeed a sequence.
+	 */
+	init_sequence(seq_relid, &elm, &seq_rel);
+	seq = read_info(elm, seq_rel, &buf);
+
+	/*
+	 * Copy the existing sequence tuple.
+	 */
+	page = BufferGetPage(buf);
+	lp = PageGetItemId(page, FirstOffsetNumber);
+	Assert(ItemIdIsNormal(lp));
+
+	tupledata.t_data = (HeapTupleHeader) PageGetItem(page, lp);
+	tupledata.t_len = ItemIdGetLength(lp);
+	tuple = heap_copytuple(&tupledata);
+
+	/* Now we're done with the old page */
+	UnlockReleaseBuffer(buf);
+
+	/*
+	 * Modify the copied tuple to execute the restart (compare the RESTART
+	 * action in AlterSequence)
+	 */
+	seq = (Form_pg_sequence) GETSTRUCT(tuple);
+	seq->last_value = seq->start_value;
+	seq->is_called = false;
+	seq->log_cnt = 1;
+
+	/*
+	 * Create a new storage file for the sequence.  We want to keep the
+	 * sequence's relfrozenxid at 0, since it won't contain any unfrozen XIDs.
+	 */
+	RelationSetNewRelfilenode(seq_rel, InvalidTransactionId);
+
+	/*
+	 * Insert the modified tuple into the new storage file.
+	 */
+	fill_seq_with_data(seq_rel, tuple);
+
+	/* Clear local cache so that we don't think we have cached numbers */
+	/* Note that we do not change the currval() state */
+	elm->cached = elm->last;
+
+	relation_close(seq_rel, NoLock);
+}
+
+/*
+ * Initialize a sequence's relation with the specified tuple as content
+ */
+static void
+fill_seq_with_data(Relation rel, HeapTuple tuple)
+{
+	Buffer		buf;
+	Page		page;
+	sequence_magic *sm;
+
 	/* Initialize first page of relation with special magic number */
 
 	buf = ReadBuffer(rel, P_NEW);
@@ -225,8 +318,7 @@ DefineSequence(CreateSeqStmt *seq)
 	/* hack: ensure heap_insert will insert on the just-created page */
 	RelationSetTargetBlock(rel, 0);
 
-	/* Now form & insert sequence tuple */
-	tuple = heap_form_tuple(tupDesc, value, null);
+	/* Now insert sequence tuple */
 	simple_heap_insert(rel, tuple);
 
 	Assert(ItemPointerGetOffsetNumber(&(tuple->t_self)) == FirstOffsetNumber);
@@ -306,12 +398,6 @@ DefineSequence(CreateSeqStmt *seq)
 	END_CRIT_SECTION();
 
 	UnlockReleaseBuffer(buf);
-
-	/* process OWNED BY if given */
-	if (owned_by)
-		process_owned_by(rel, owned_by);
-
-	heap_close(rel, NoLock);
 }
 
 /*
@@ -323,29 +409,6 @@ void
 AlterSequence(AlterSeqStmt *stmt)
 {
 	Oid			relid;
-
-	/* find sequence */
-	relid = RangeVarGetRelid(stmt->sequence, false);
-
-	/* allow ALTER to sequence owner only */
-	/* if you change this, see also callers of AlterSequenceInternal! */
-	if (!pg_class_ownercheck(relid, GetUserId()))
-		aclcheck_error(ACLCHECK_NOT_OWNER, ACL_KIND_CLASS,
-					   stmt->sequence->relname);
-
-	/* do the work */
-	AlterSequenceInternal(relid, stmt->options);
-}
-
-/*
- * AlterSequenceInternal
- *
- * Same as AlterSequence except that the sequence is specified by OID
- * and we assume the caller already checked permissions.
- */
-void
-AlterSequenceInternal(Oid relid, List *options)
-{
 	SeqTable	elm;
 	Relation	seqrel;
 	Buffer		buf;
@@ -355,8 +418,14 @@ AlterSequenceInternal(Oid relid, List *options)
 	List	   *owned_by;
 
 	/* open and AccessShareLock sequence */
+	relid = RangeVarGetRelid(stmt->sequence, false);
 	init_sequence(relid, &elm, &seqrel);
 
+	/* allow ALTER to sequence owner only */
+	if (!pg_class_ownercheck(relid, GetUserId()))
+		aclcheck_error(ACLCHECK_NOT_OWNER, ACL_KIND_CLASS,
+					   stmt->sequence->relname);
+
 	/* lock page' buffer and read tuple into new sequence structure */
 	seq = read_info(elm, seqrel, &buf);
 	page = BufferGetPage(buf);
@@ -365,7 +434,7 @@ AlterSequenceInternal(Oid relid, List *options)
 	memcpy(&new, seq, sizeof(FormData_pg_sequence));
 
 	/* Check and set new values */
-	init_params(options, false, &new, &owned_by);
+	init_params(stmt->options, false, &new, &owned_by);
 
 	/* Clear local cache so that we don't think we have cached numbers */
 	/* Note that we do not change the currval() state */
@@ -937,6 +1006,7 @@ init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel)
 					(errcode(ERRCODE_OUT_OF_MEMORY),
 					 errmsg("out of memory")));
 		elm->relid = relid;
+		elm->filenode = InvalidOid;
 		elm->lxid = InvalidLocalTransactionId;
 		elm->last_valid = false;
 		elm->last = elm->cached = elm->increment = 0;
@@ -955,6 +1025,18 @@ init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel)
 				 errmsg("\"%s\" is not a sequence",
 						RelationGetRelationName(seqrel))));
 
+	/*
+	 * If the sequence has been transactionally replaced since we last saw it,
+	 * discard any cached-but-unissued values.  We do not touch the currval()
+	 * state, however.
+	 */
+	if (seqrel->rd_rel->relfilenode != elm->filenode)
+	{
+		elm->filenode = seqrel->rd_rel->relfilenode;
+		elm->cached = elm->last;
+	}
+
+	/* Return results */
 	*p_elm = elm;
 	*p_rel = seqrel;
 }