Fix sequence creation to set the t_xmin of a sequence's tuple to

FrozenTransactionId, not the XID of the creating transaction.  Without
this it's possible for a reference to a long-gone CLOG record to occur,
per Christian Meunier's bug report of 10-Jan-02.  Worse, the sequence
tuple would become invisible to SELECTs after 2 billion transactions.

Since the fix is applied during sequence creation it does not help
existing databases, unless you drop and recreate every sequence.
However, we intend to force initdb for 7.2RC1 anyway, to fix a pg_proc
error, so I see no need to do more for this problem.

src/backend/commands/sequence.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/commands/sequence.c,v 1.67 2001/11/05 17:46:24 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/commands/sequence.c,v 1.68 2002/01/11 18:16:04 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -43,7 +43,7 @@
 #define SEQ_MINVALUE	(-SEQ_MAXVALUE)
 
 /*
- * We don't want to log each fetching values from sequences,
+ * We don't want to log each fetching of a value from a sequence,
  * so we pre-log a few fetches in advance. In the event of
  * crash we can lose as much as we pre-logged.
  */
@@ -199,17 +199,51 @@ DefineSequence(CreateSeqStmt *seq)
 	/* hack: ensure heap_insert will insert on the just-created page */
 	rel->rd_targblock = 0;
 
-	/* Now - form & insert sequence tuple */
+	/* Now form & insert sequence tuple */
 	tuple = heap_formtuple(tupDesc, value, null);
 	heap_insert(rel, tuple);
 
+	Assert(ItemPointerGetOffsetNumber(&(tuple->t_self)) == FirstOffsetNumber);
+
 	/*
-	 * After crash REDO of heap_insert above would re-init page and our
-	 * magic number would be lost. We have to log sequence creation. This
-	 * means two log records instead of one -:(
+	 * Two special hacks here:
+	 *
+	 * 1. Since VACUUM does not process sequences, we have to force the tuple
+	 * to have xmin = FrozenTransactionId now.  Otherwise it would become
+	 * invisible to SELECTs after 2G transactions.  It is okay to do this
+	 * because if the current transaction aborts, no other xact will ever
+	 * examine the sequence tuple anyway.
+	 *
+	 * 2. Even though heap_insert emitted a WAL log record, we have to emit
+	 * an XLOG_SEQ_LOG record too, since (a) the heap_insert record will
+	 * not have the right xmin, and (b) REDO of the heap_insert record
+	 * would re-init page and sequence magic number would be lost.  This
+	 * means two log records instead of one :-(
 	 */
 	LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 	START_CRIT_SECTION();
+
+	{
+		/*
+		 * Note that the "tuple" structure is still just a local tuple record
+		 * created by heap_formtuple; its t_data pointer doesn't point at the
+		 * disk buffer.  To scribble on the disk buffer we need to fetch the
+		 * item pointer.  But do the same to the local tuple, since that will
+		 * be the source for the WAL log record, below.
+		 */
+		ItemId		itemId;
+		Item		item;
+
+		itemId = PageGetItemId((Page) page, FirstOffsetNumber);
+		item = PageGetItem((Page) page, itemId);
+
+		((HeapTupleHeader) item)->t_xmin = FrozenTransactionId;
+		((HeapTupleHeader) item)->t_infomask |= HEAP_XMIN_COMMITTED;
+
+		tuple->t_data->t_xmin = FrozenTransactionId;
+		tuple->t_data->t_infomask |= HEAP_XMIN_COMMITTED;
+	}
+
 	{
 		xl_seq_rec	xlrec;
 		XLogRecPtr	recptr;
@@ -217,6 +251,7 @@ DefineSequence(CreateSeqStmt *seq)
 		Form_pg_sequence newseq = (Form_pg_sequence) GETSTRUCT(tuple);
 
 		/* We do not log first nextval call, so "advance" sequence here */
+		/* Note we are scribbling on local tuple, not the disk buffer */
 		newseq->is_called = true;
 		newseq->log_cnt = 0;
 
@@ -239,8 +274,8 @@ DefineSequence(CreateSeqStmt *seq)
 	END_CRIT_SECTION();
 
 	LockBuffer(buf, BUFFER_LOCK_UNLOCK);
-	ReleaseBuffer(buf);
-	heap_close(rel, AccessExclusiveLock);
+	WriteBuffer(buf);
+	heap_close(rel, NoLock);
 }
 
 
@@ -628,7 +663,6 @@ read_info(char *caller, SeqTable elm, Buffer *buf)
 	elm->increment = seq->increment_by;
 
 	return seq;
-
 }
 
 
@@ -858,6 +892,8 @@ seq_redo(XLogRecPtr lsn, XLogRecord *record)
 
 	page = (Page) BufferGetPage(buffer);
 
+	/* Always reinit the page and reinstall the magic number */
+	/* See comments in DefineSequence */
 	PageInit((Page) page, BufferGetPageSize(buffer), sizeof(sequence_magic));
 	sm = (sequence_magic *) PageGetSpecialPointer(page);
 	sm->magic = SEQ_MAGIC;