Replace implementation of pg_log as a relation accessed through the

buffer manager with 'pg_clog', a specialized access method modeled
on pg_xlog.  This simplifies startup (don't need to play games to
open pg_log; among other things, OverrideTransactionSystem goes away),
should improve performance a little, and opens the door to recycling
commit log space by removing no-longer-needed segments of the commit
log.  Actual recycling is not there yet, but I felt I should commit
this part separately since it'd still be useful if we chose not to
do transaction ID wraparound.

src/include/access/clog.h

@@ -0,0 +1,47 @@
+/*
+ * clog.h
+ *
+ * PostgreSQL transaction-commit-log manager
+ *
+ * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $Id: clog.h,v 1.1 2001/08/25 18:52:42 tgl Exp $
+ */
+#ifndef CLOG_H
+#define CLOG_H
+
+#include "access/xlog.h"
+
+/*
+ * Possible transaction statuses --- note that all-zeroes is the initial
+ * state.
+ */
+typedef int XidStatus;
+
+#define TRANSACTION_STATUS_IN_PROGRESS		0x00
+#define TRANSACTION_STATUS_COMMITTED		0x01
+#define TRANSACTION_STATUS_ABORTED			0x02
+/* 0x03 is available without changing commit log space allocation */
+
+
+extern void TransactionIdSetStatus(TransactionId xid, XidStatus status);
+extern XidStatus TransactionIdGetStatus(TransactionId xid);
+
+extern int	CLOGShmemSize(void);
+extern void CLOGShmemInit(void);
+extern void BootStrapCLOG(void);
+extern void StartupCLOG(void);
+extern void ShutdownCLOG(void);
+extern void CheckPointCLOG(void);
+extern void ExtendCLOG(TransactionId newestXact);
+extern void TruncateCLOG(TransactionId oldestXact);
+
+/* XLOG stuff */
+#define CLOG_ZEROPAGE		0x00
+
+extern void clog_redo(XLogRecPtr lsn, XLogRecord *record);
+extern void clog_undo(XLogRecPtr lsn, XLogRecord *record);
+extern void clog_desc(char *buf, uint8 xl_info, char *rec);
+
+#endif	 /* CLOG_H */