trx0sys.c, srv0start.c, ibuf0ibuf.c, trx0sys.h, srv0srv.h:

  DO NOT MERGE TO 4.1: add code for automatic downgrade 4.1.1 -> 4.0 if the user has not created multiple tablespaces yet


innobase/include/srv0srv.h:
  DO NOT MERGE TO 4.1: add code for automatic downgrade 4.1.1 -> 4.0 if the user has not created multiple tablespaces yet
innobase/include/trx0sys.h:
  DO NOT MERGE TO 4.1: add code for automatic downgrade 4.1.1 -> 4.0 if the user has not created multiple tablespaces yet
innobase/ibuf/ibuf0ibuf.c:
  DO NOT MERGE TO 4.1: add code for automatic downgrade 4.1.1 -> 4.0 if the user has not created multiple tablespaces yet
innobase/srv/srv0start.c:
  DO NOT MERGE TO 4.1: add code for automatic downgrade 4.1.1 -> 4.0 if the user has not created multiple tablespaces yet
innobase/trx/trx0sys.c:
  DO NOT MERGE TO 4.1: add code for automatic downgrade 4.1.1 -> 4.0 if the user has not created multiple tablespaces yet

innobase/ibuf/ibuf0ibuf.c

@@ -2622,6 +2622,14 @@ ibuf_merge_or_delete_for_page(
 			goto reset_bit;
 		}
 
+		/* Do NOT merge to the 4.1 code base! */
+		if (trx_sys_downgrading_from_4_1_1) {
+			fprintf(stderr,
+"InnoDB: Fatal error: you are downgrading from >= 4.1.1 to 4.0, but\n"
+"InnoDB: the insert buffer was not empty.\n");
+			ut_a(0);
+		}
+
 		if (corruption_noticed) {
 			rec_sprintf(err_buf, 450, ibuf_rec);
 

innobase/include/srv0srv.h

@@ -17,6 +17,8 @@ Created 10/10/1995 Heikki Tuuri
 #include "que0types.h"
 #include "trx0types.h"
 
+extern char*	srv_main_thread_op_info;
+
 /* Buffer which can be used in printing fatal error messages */
 extern char	srv_fatal_errbuf[];
 

innobase/include/trx0sys.h

@@ -24,6 +24,18 @@ Created 3/26/1996 Heikki Tuuri
 #include "fsp0fsp.h"
 #include "read0types.h"
 
+/* Do NOT merge this to the 4.1 code base! */
+extern ibool		trx_sys_downgrading_from_4_1_1;
+
+/********************************************************************
+Do NOT merge this to the 4.1 code base!
+Marks the trx sys header when we have successfully downgraded from the >= 4.1.1
+multiple tablespace format back to the 4.0 format. */
+
+void
+trx_sys_mark_downgraded_from_4_1_1(void);
+/*====================================*/
+
 /* In a MySQL replication slave, in crash recovery we store the master log
 file name and position here. We have successfully got the updates to InnoDB
 up to this position. If .._pos is -1, it means no crash recovery was needed,
@@ -354,8 +366,14 @@ this contains the same fields as TRX_SYS_MYSQL_LOG_INFO below */
 						sys header is half-written
 						to disk, we still may be able
 						to recover the information */
+#define TRX_SYS_DOUBLEWRITE_SPACE_ID_STORED (24 + FSEG_HEADER_SIZE)
+						/* If this is set to
+						.._N, then we are
+						DOWNGRADING from >= 4.1.1 to
+						4.0 */
 /*-------------------------------------------------------------*/
 #define TRX_SYS_DOUBLEWRITE_MAGIC_N	536853855
+#define TRX_SYS_DOUBLEWRITE_SPACE_ID_STORED_N 1783657386
 
 #define TRX_SYS_DOUBLEWRITE_BLOCK_SIZE	FSP_EXTENT_SIZE	
 

innobase/srv/srv0start.c

@@ -1479,6 +1479,31 @@ NetWare. */
 
 	os_fast_mutex_free(&srv_os_test_mutex);
 
+	/***********************************************************/
+	/* Do NOT merge to the 4.1 code base! */
+	if (trx_sys_downgrading_from_4_1_1) {
+		fprintf(stderr,
+"InnoDB: You are downgrading from an InnoDB version which allows multiple\n"
+"InnoDB: tablespaces. Wait that purge and insert buffer merge run to\n"
+"InnoDB: completion...\n");
+		for (;;) {
+			os_thread_sleep(10000000);
+
+			if (0 == strcmp(srv_main_thread_op_info,
+					"waiting for server activity")) {
+				break;
+			}
+		}
+		fprintf(stderr,
+"InnoDB: Full purge and insert buffer merge completed.\n");
+
+	        trx_sys_mark_downgraded_from_4_1_1();
+
+		fprintf(stderr,
+"InnoDB: Downgraded from >= 4.1.1 to 4.0\n");
+	}
+	/***********************************************************/
+
 	if (srv_print_verbose_log) {
 	  	ut_print_timestamp(stderr);
 	  	fprintf(stderr, "  InnoDB: Started\n");

innobase/trx/trx0sys.c

@@ -34,6 +34,43 @@ or there was no master log position info inside InnoDB. */
 char 		trx_sys_mysql_master_log_name[TRX_SYS_MYSQL_LOG_NAME_LEN];
 ib_longlong	trx_sys_mysql_master_log_pos	= -1;
 
+/* Do NOT merge this to the 4.1 code base! */
+ibool		trx_sys_downgrading_from_4_1_1	= FALSE;
+
+/********************************************************************
+Do NOT merge this to the 4.1 code base!
+Marks the trx sys header when we have successfully downgraded from the >= 4.1.1
+multiple tablespace format back to the 4.0 format. */
+
+void
+trx_sys_mark_downgraded_from_4_1_1(void)
+/*====================================*/
+{
+	page_t*	page;
+	byte*	doublewrite;
+	mtr_t	mtr;
+
+	/* Let us mark to the trx_sys header that the downgrade has been
+	done. */
+
+	mtr_start(&mtr);
+
+	page = buf_page_get(TRX_SYS_SPACE, TRX_SYS_PAGE_NO, RW_X_LATCH, &mtr);
+	buf_page_dbg_add_level(page, SYNC_NO_ORDER_CHECK);
+
+	doublewrite = page + TRX_SYS_DOUBLEWRITE;
+
+	mlog_write_ulint(doublewrite + TRX_SYS_DOUBLEWRITE_SPACE_ID_STORED,
+				TRX_SYS_DOUBLEWRITE_SPACE_ID_STORED_N + 1,
+				MLOG_4BYTES, &mtr);
+	mtr_commit(&mtr);
+		
+	/* Flush the modified pages to disk and make a checkpoint */
+	log_make_checkpoint_at(ut_dulint_max, TRUE);
+
+	trx_sys_downgrading_from_4_1_1 = FALSE;
+}
+
 /********************************************************************
 Determines if a page number is located inside the doublewrite buffer. */
 
@@ -138,12 +175,36 @@ trx_sys_create_doublewrite_buf(void)
 	
 	if (mach_read_from_4(doublewrite + TRX_SYS_DOUBLEWRITE_MAGIC)
 					== TRX_SYS_DOUBLEWRITE_MAGIC_N) {
+		/* Do NOT merge to the 4.1 code base! */
+        	if (mach_read_from_4(doublewrite
+				+ TRX_SYS_DOUBLEWRITE_SPACE_ID_STORED)
+            		== TRX_SYS_DOUBLEWRITE_SPACE_ID_STORED_N) {
+			
+			fprintf(stderr,
+"InnoDB: You are downgrading from the multiple tablespace format of\n"
+"InnoDB: >= MySQL-4.1.1 back to the old format of MySQL-4.0.\n"
+"InnoDB:\n"
+"InnoDB: MAKE SURE that the mysqld server is idle, and purge and the insert\n"
+"InnoDB: buffer merge have run to completion under >= 4.1.1 before trying to\n"
+"InnoDB: downgrade! You can determine this by looking at SHOW INNODB STATUS:\n"
+"InnoDB: if the Main thread is 'waiting for server activity' and SHOW\n"
+"InnoDB: PROCESSLIST shows that you have ended all other connections\n"
+"InnoDB: to mysqld, then purge and the insert buffer merge have been\n"
+"InnoDB: completed.\n"
+"InnoDB: If you have already created .ibd tables in >= 4.1.1, then those\n"
+"InnoDB: tables cannot be used under 4.0.\n"
+"InnoDB: NOTE THAT this downgrade procedure has not been properly tested!\n"
+"InnoDB: The safe way to downgrade is to dump all InnoDB tables and recreate\n"
+"InnoDB: the whole tablespace.\n");
+
+			trx_sys_downgrading_from_4_1_1 = TRUE;
+		}
 
 		/* The doublewrite buffer has already been created:
 		just read in some numbers */
 
 		trx_doublewrite_init(doublewrite);
-		
+
 		mtr_commit(&mtr);
 	} else {
 		fprintf(stderr,
@@ -295,6 +356,31 @@ trx_sys_doublewrite_restore_corrupt_pages(void)
 					== TRX_SYS_DOUBLEWRITE_MAGIC_N) {
 		/* The doublewrite buffer has been created */
 		
+		/* Do NOT merge to the 4.1 code base! */
+        	if (mach_read_from_4(doublewrite
+				+ TRX_SYS_DOUBLEWRITE_SPACE_ID_STORED)
+            		== TRX_SYS_DOUBLEWRITE_SPACE_ID_STORED_N) {
+			
+			fprintf(stderr,
+"InnoDB: You are downgrading from the multiple tablespace format of\n"
+"InnoDB: >= MySQL-4.1.1 back to the old format of MySQL-4.0.\n"
+"InnoDB:\n"
+"InnoDB: MAKE SURE that the mysqld server is idle, and purge and the insert\n"
+"InnoDB: buffer merge have run to completion under >= 4.1.1 before trying to\n"
+"InnoDB: downgrade! You can determine this by looking at SHOW INNODB STATUS:\n"
+"InnoDB: if the Main thread is 'waiting for server activity' and SHOW\n"
+"InnoDB: PROCESSLIST shows that you have ended all other connections\n"
+"InnoDB: to mysqld, then purge and the insert buffer merge have been\n"
+"InnoDB: completed.\n"
+"InnoDB: If you have already created .ibd tables in >= 4.1.1, then those\n"
+"InnoDB: tables cannot be used under 4.0.\n"
+"InnoDB: NOTE THAT this downgrade procedure has not been properly tested!\n"
+"InnoDB: The safe way to downgrade is to dump all InnoDB tables and recreate\n"
+"InnoDB: the whole tablespace.\n");
+
+			trx_sys_downgrading_from_4_1_1 = TRUE;
+		}
+
 		trx_doublewrite_init(doublewrite);
 
 		block1 = trx_doublewrite->block1;