MDEV-12711 mariabackup --backup is refused for multi-file system tablespace

Before MDEV-12113 (MariaDB Server 10.1.25), on shutdown InnoDB would write
the current LSN to the first page of each file of the system tablespace.
This is incompatible with MariaDB's InnoDB table encryption, because
encryption repurposed the field for an encryption key ID and checksum.

buf_page_is_corrupted(): For the InnoDB system tablespace, skip
FIL_PAGE_FILE_FLUSH_LSN when checking if a page is all zero,
because the first page of each file in the system tablespace can
contain nonzero bytes in the field.

mysql-test/suite/mariabackup/absolute_ibdata_paths.opt

---innodb --innodb-data-home-dir= --innodb-data-file-path=$MYSQLTEST_VARDIR/tmp/absolute_path_ibdata1:3M;ibdata_second:1M:autoextend
\ No newline at end of file
+--innodb --innodb-data-home-dir= --innodb-data-file-path=$MYSQLTEST_VARDIR/tmp/absolute_path_ibdata1:6M;ibdata_second:1M:autoextend

mysql-test/suite/mariabackup/absolute_ibdata_paths.test

@@ -2,12 +2,32 @@
 # Innodb system tablespace is specified with absolute path in  the .opt file
 CREATE TABLE t(i INT) ENGINE INNODB;
 INSERT INTO t VALUES(1);
+
+let INNODB_PAGE_SIZE=`select @@innodb_page_size`;
+let MYSQLD_DATADIR= `SELECT @@datadir`;
+--source include/shutdown_mysqld.inc
+perl;
+my $file= "$ENV{MYSQLD_DATADIR}/ibdata_second";
+open(FILE, "+<", $file) or die "Unable to open $file\n";
+binmode FILE;
+my $ps= $ENV{INNODB_PAGE_SIZE};
+my $page;
+my $pos = $ps * 0;
+sysseek(FILE, $pos, 0) || die "Unable to seek $file\n";
+die "Unable to read $file" unless sysread(FILE, $page, $ps) == $ps;
+substr($page,26,8) = pack("NN", 1, 1);
+sysseek(FILE, $pos, 0) || die "Unable to rewind $file\n";
+syswrite(FILE, $page, $ps)==$ps || die "Unable to write $file\n";
+close(FILE) || die "Unable to close $file\n";
+EOF
+
+--source include/start_mysqld.inc
+
 echo # xtrabackup backup;
 
 let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
 let $_innodb_data_file_path=`select @@innodb_data_file_path`;
 let $_innodb_data_home_dir=`select @@innodb_data_home_dir`;
-let $_datadir= `SELECT @@datadir`;
 
 --disable_result_log
 exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir;
@@ -16,11 +36,11 @@ exec $XTRABACKUP  --prepare --target-dir=$targetdir;
 
 --source include/shutdown_mysqld.inc
 echo # remove datadir;
-rmdir $_datadir;
+rmdir $MYSQLD_DATADIR;
 #remove out-of-datadir ibdata1
 remove_file $MYSQLTEST_VARDIR/tmp/absolute_path_ibdata1;
 echo # xtrabackup copy back;
-exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --copy-back --datadir=$_datadir --target-dir=$targetdir "--innodb_data_file_path=$_innodb_data_file_path" --innodb_data_home_dir=$_innodb_data_home_dir;
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --copy-back --datadir=$MYSQLD_DATADIR --target-dir=$targetdir "--innodb_data_file_path=$_innodb_data_file_path" --innodb_data_home_dir=$_innodb_data_home_dir;
 echo # restart server;
 --source include/start_mysqld.inc
 --enable_result_log

storage/innobase/buf/buf0buf.cc

@@ -946,18 +946,31 @@ buf_page_is_corrupted(
 #error "FIL_PAGE_LSN must be 64 bit aligned"
 #endif
 
-	/* declare empty pages non-corrupted */
-	if (checksum_field1 == 0 && checksum_field2 == 0
-	    && *reinterpret_cast<const ib_uint64_t*>(read_buf +
-						     FIL_PAGE_LSN) == 0) {
-		/* make sure that the page is really empty */
-		for (ulint i = 0; i < UNIV_PAGE_SIZE; i++) {
-			if (read_buf[i] != 0) {
-				return(true);
+	/* A page filled with NUL bytes is considered not corrupted.
+	The FIL_PAGE_FILE_FLUSH_LSN field may be written nonzero for
+	the first page of each file of the system tablespace.
+	Ignore it for the system tablespace. */
+	if (!checksum_field1 && !checksum_field2) {
+		ulint i = 0;
+		do {
+			if (read_buf[i]) {
+				return true;
 			}
-		}
+		} while (++i < FIL_PAGE_FILE_FLUSH_LSN_OR_KEY_VERSION);
 
-		return(false);
+#ifndef UNIV_INNOCHECKSUM
+		if (!space || !space->id) {
+			/* Skip FIL_PAGE_FILE_FLUSH_LSN_OR_KEY_VERSION
+			in the system tablespace. */
+			i += 8;
+		}
+#endif
+		do {
+			if (read_buf[i]) {
+				return true;
+			}
+		} while (++i < srv_page_size);
+		return false;
 	}
 
 	switch (curr_algo) {

storage/xtradb/buf/buf0buf.cc

@@ -945,18 +945,29 @@ buf_page_is_corrupted(
 #error "FIL_PAGE_LSN must be 64 bit aligned"
 #endif
 
-	/* declare empty pages non-corrupted */
-	if (checksum_field1 == 0 && checksum_field2 == 0
-	    && *reinterpret_cast<const ib_uint64_t*>(read_buf +
-						     FIL_PAGE_LSN) == 0) {
-		/* make sure that the page is really empty */
-		for (ulint i = 0; i < UNIV_PAGE_SIZE; i++) {
-			if (read_buf[i] != 0) {
-				return(true);
+	/* A page filled with NUL bytes is considered not corrupted.
+	The FIL_PAGE_FILE_FLUSH_LSN field may be written nonzero for
+	the first page of each file of the system tablespace.
+	Ignore it for the system tablespace. */
+	if (!checksum_field1 && !checksum_field2) {
+		ulint i = 0;
+		do {
+			if (read_buf[i]) {
+				return true;
 			}
-		}
+		} while (++i < FIL_PAGE_FILE_FLUSH_LSN_OR_KEY_VERSION);
 
-		return(false);
+		if (!space || !space->id) {
+			/* Skip FIL_PAGE_FILE_FLUSH_LSN_OR_KEY_VERSION
+			in the system tablespace. */
+			i += 8;
+		}
+		do {
+			if (read_buf[i]) {
+				return true;
+			}
+		} while (++i < srv_page_size);
+		return false;
 	}
 
 	switch (curr_algo) {