Resetting the XLOG can't include rewinding it to logical position zero,

because we need page LSNs stored in the main database to be less than
the current XLOG position.  Hence, generate the new XLOG segment at last
old segment number plus one.

contrib/pg_resetxlog/pg_resetxlog.c

@@ -12,8 +12,10 @@
  *		 by scanning the old xlog if necessary.
  *	  3. Modify pg_control to reflect a "shutdown" state with a checkpoint
  *	     record at the start of xlog.
- *	  4. Flush the existing xlog files and write a new segment 0 with
- *	     just a checkpoint record in it.
+ *	  4. Flush the existing xlog files and write a new segment with
+ *	     just a checkpoint record in it.  The new segment is positioned
+ *		 just past the end of the old xlog, so that existing LSNs in
+ *		 data pages will appear to be "in the past".
  * This is all pretty straightforward except for the intuition part of
  * step 2 ...
  *
@@ -21,7 +23,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Header: /cvsroot/pgsql/contrib/pg_resetxlog/Attic/pg_resetxlog.c,v 1.1 2001/03/14 00:57:43 tgl Exp $
+ * $Header: /cvsroot/pgsql/contrib/pg_resetxlog/Attic/pg_resetxlog.c,v 1.2 2001/03/16 05:08:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -108,6 +110,7 @@ static char XLogDir[MAXPGPATH];
 static char ControlFilePath[MAXPGPATH];
 
 static ControlFileData ControlFile;	/* pg_control values */
+static uint32 newXlogId, newXlogSeg; /* ID/Segment of new XLOG segment */
 static bool guessed = false;	/* T if we had to guess at any values */
 
 
@@ -390,6 +393,8 @@ CheckControlVersion0(char *buffer, int len)
 	ControlFile.catalog_version_no = oldfile->catalog_version_no;
 
 	ControlFile.state = oldfile->state;
+	ControlFile.logId = oldfile->logId;
+	ControlFile.logSeg = oldfile->logSeg;
 
 	ControlFile.blcksz = oldfile->blcksz;
 	ControlFile.relseg_size = oldfile->relseg_size;
@@ -671,6 +676,8 @@ PrintControlValues(void)
 	printf("Guessed-at pg_control values:\n\n"
 		   "pg_control version number:            %u\n"
 		   "Catalog version number:               %u\n"
+		   "Current log file id:                  %u\n"
+	       "Next log file segment:                %u\n"
 		   "Latest checkpoint's StartUpID:        %u\n"
 		   "Latest checkpoint's NextXID:          %u\n"
 		   "Latest checkpoint's NextOID:          %u\n"
@@ -681,6 +688,8 @@ PrintControlValues(void)
 
 		   ControlFile.pg_control_version,
 		   ControlFile.catalog_version_no,
+		   ControlFile.logId,
+		   ControlFile.logSeg,
 		   ControlFile.checkPointCopy.ThisStartUpID,
 		   ControlFile.checkPointCopy.nextXid,
 		   ControlFile.checkPointCopy.nextOid,
@@ -701,17 +710,24 @@ RewriteControlFile(void)
 	char		buffer[BLCKSZ];	/* need not be aligned */
 
 	/*
-	 * Adjust fields as needed to force an empty XLOG.
+	 * Adjust fields as needed to force an empty XLOG starting at the
+	 * next available segment.
 	 */
-	ControlFile.checkPointCopy.redo.xlogid = 0;
-	ControlFile.checkPointCopy.redo.xrecoff = SizeOfXLogPHD;
+	newXlogId = ControlFile.logId;
+	newXlogSeg = ControlFile.logSeg;
+	/* be sure we wrap around correctly at end of a logfile */
+	NextLogSeg(newXlogId, newXlogSeg);
+
+	ControlFile.checkPointCopy.redo.xlogid = newXlogId;
+	ControlFile.checkPointCopy.redo.xrecoff =
+		newXlogSeg * XLogSegSize + SizeOfXLogPHD;
 	ControlFile.checkPointCopy.undo = ControlFile.checkPointCopy.redo;
 	ControlFile.checkPointCopy.time = time(NULL);
 
 	ControlFile.state = DB_SHUTDOWNED;
 	ControlFile.time = time(NULL);
-	ControlFile.logId = 0;
-	ControlFile.logSeg = 1;
+	ControlFile.logId = newXlogId;
+	ControlFile.logSeg = newXlogSeg + 1;
 	ControlFile.checkPoint = ControlFile.checkPointCopy.redo;
 	ControlFile.prevCheckPoint.xlogid = 0;
 	ControlFile.prevCheckPoint.xrecoff = 0;
@@ -848,7 +864,7 @@ WriteEmptyXLOG(void)
 	record->xl_crc = crc;
 
 	/* Write the first page */
-	XLogFileName(path, 0, 0);
+	XLogFileName(path, newXlogId, newXlogSeg);
 
 	unlink(path);