Add a notion of a 'catalog version number' that can indicate

when an initdb-forcing change has been applied within a development cycle.
PG_VERSION serves this purpose for official releases, but we can't bump
the PG_VERSION number every time we make a change to the catalogs during
development.  Instead, increase the catalog version number to warn other
developers that you've made an incompatible change.  See my mail to
pghackers for more info.

src/backend/access/transam/Makefile

@@ -4,7 +4,7 @@
 #    Makefile for access/transam
 #
 # IDENTIFICATION
-#    $Header: /cvsroot/pgsql/src/backend/access/transam/Makefile,v 1.7 1999/09/27 15:47:37 vadim Exp $
+#    $Header: /cvsroot/pgsql/src/backend/access/transam/Makefile,v 1.8 1999/10/24 20:42:27 tgl Exp $
 #
 #-------------------------------------------------------------------------
 
@@ -26,6 +26,10 @@ depend dep:
 clean: 
 	rm -f SUBSYS.o $(OBJS)
 
+# ensure that version checks in xlog.c get recompiled when config.h or
+# catversion.h changes, even if "make depend" hasn't been done.
+xlog.o: xlog.c $(SRCDIR)/include/config.h $(SRCDIR)/include/catalog/catversion.h
+
 ifeq (depend,$(wildcard depend))
 include depend
 endif

src/backend/access/transam/xlog.c

@@ -1,3 +1,14 @@
+/*------------------------------------------------------------------------- 
+ *
+ * xlog.c
+ *
+ *
+ * Copyright (c) 1994, Regents of the University of California
+ *
+ * $Header: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v 1.6 1999/10/24 20:42:27 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
 #include <fcntl.h>
 #include <unistd.h>
 #include <errno.h>
@@ -5,8 +16,10 @@
 #include <sys/time.h>
 
 #include "postgres.h"
+
 #include "access/xlog.h"
 #include "access/xact.h"
+#include "catalog/catversion.h"
 #include "storage/sinval.h"
 #include "storage/proc.h"
 #include "storage/spin.h"
@@ -99,12 +112,15 @@ typedef struct ControlFileData
 	DBState			state;			/* */
 
 	/*
-	 * following data used to make sure that configurations for this DB
-	 * do not conflict with the backend
+	 * this data is used to make sure that configuration of this DB
+	 * is compatible with the current backend
 	 */
 	uint32			blcksz;			/* block size for this DB */
-	uint32			relseg_size;		/* segmented file's block number */
-	/* MORE DATA FOLLOWS AT THE END OF THIS STRUCTURE
+	uint32			relseg_size;	/* blocks per segment of large relation */
+	uint32			catalog_version_no;	/* internal version number */
+
+	/*
+	 * MORE DATA FOLLOWS AT THE END OF THIS STRUCTURE
 	 * - locations of data dirs 
 	 */
 } ControlFileData;
@@ -1171,6 +1187,7 @@ BootStrapXLOG()
 	ControlFile->state = DB_SHUTDOWNED;
 	ControlFile->blcksz = BLCKSZ;
 	ControlFile->relseg_size = RELSEG_SIZE;
+	ControlFile->catalog_version_no = CATALOG_VERSION_NO;
 
 	if (write(fd, buffer, BLCKSZ) != BLCKSZ)
 		elog(STOP, "BootStrapXLOG failed to write control file: %d", errno);
@@ -1179,9 +1196,6 @@ BootStrapXLOG()
 		elog(STOP, "BootStrapXLOG failed to fsync control file: %d", errno);
 
 	close(fd);
-
-	return;
-
 }
 
 static char*
@@ -1258,11 +1272,16 @@ tryAgain:
 		!XRecOffIsValid(ControlFile->checkPoint.xrecoff))
 		elog(STOP, "Control file context is broken");
 
+	/* Check for incompatible database */
 	if (ControlFile->blcksz != BLCKSZ)
-		elog(STOP, "database was initialized in BLCKSZ(%d), but the backend was compiled in BLCKSZ(%d)",ControlFile->blcksz,BLCKSZ);
-
+		elog(STOP, "database was initialized with BLCKSZ %d,\n\tbut the backend was compiled with BLCKSZ %d.\n\tlooks like you need to initdb.",
+			 ControlFile->blcksz, BLCKSZ);
 	if (ControlFile->relseg_size != RELSEG_SIZE)
-		elog(STOP, "database was initialized in RELSEG_SIZE(%d), but the backend was compiled in RELSEG_SIZE(%d)",ControlFile->relseg_size, RELSEG_SIZE);
+		elog(STOP, "database was initialized with RELSEG_SIZE %d,\n\tbut the backend was compiled with RELSEG_SIZE %d.\n\tlooks like you need to initdb.",
+			 ControlFile->relseg_size, RELSEG_SIZE);
+	if (ControlFile->catalog_version_no != CATALOG_VERSION_NO)
+		elog(STOP, "database was initialized with CATALOG_VERSION_NO %d,\n\tbut the backend was compiled with CATALOG_VERSION_NO %d.\n\tlooks like you need to initdb.",
+			 ControlFile->catalog_version_no, CATALOG_VERSION_NO);
 
 	if (ControlFile->state == DB_SHUTDOWNED)
 		elog(LOG, "Data Base System was shutdowned at %s",

src/include/catalog/catversion.h

@@ -0,0 +1,56 @@
+/*------------------------------------------------------------------------- 
+ *
+ * catversion.h
+ *	  "Catalog version number" for Postgres.
+ *
+ * The catalog version number is used to flag incompatible changes in
+ * the Postgres system catalogs.  Whenever anyone changes the format of
+ * a system catalog relation, or adds, deletes, or modifies standard
+ * catalog entries in such a way that an updated backend wouldn't work
+ * with an old database (or vice versa), the catalog version number
+ * should be changed.  The version number stored in pg_control by initdb
+ * is checked against the version number compiled into the backend at
+ * startup time, so that a backend can refuse to run in an incompatible
+ * database.
+ *
+ * The point of this feature is to provide a finer grain of compatibility
+ * checking than is possible from looking at the major version number
+ * stored in PG_VERSION.  It shouldn't matter to end users, but during
+ * development cycles we usually make quite a few incompatible changes
+ * to the contents of the system catalogs, and we don't want to bump the
+ * major version number for each one.  What we can do instead is bump
+ * this internal version number.  This should save some grief for
+ * developers who might otherwise waste time tracking down "bugs" that
+ * are really just code-vs-database incompatibilities.
+ *
+ * The rule for developers is: if you commit a change that requires
+ * an initdb, you should update the catalog version number (as well as
+ * notifying the pghackers mailing list, which has been the informal
+ * practice for a long time).
+ *
+ * The catalog version number is placed here since modifying files in
+ * include/catalog is the most common kind of initdb-forcing change.
+ * But it could be used to protect any kind of incompatible change in
+ * database contents or layout, such as altering tuple headers.
+ *
+ *
+ * Copyright (c) 1994, Regents of the University of California
+ *
+ * $Id: catversion.h,v 1.1 1999/10/24 20:42:26 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef CATVERSION_H
+#define CATVERSION_H
+
+/*
+ * We could use anything we wanted for version numbers, but I recommend
+ * following the "YYYYMMDDN" style often used for DNS zone serial numbers.
+ * YYYYMMDD are the date of the change, and N is the number of the change
+ * on that day.  (Hopefully we'll never commit ten independent sets of
+ * catalog changes on the same day...)
+ */
+
+#define CATALOG_VERSION_NO	199910241
+
+#endif