MDEV-6364: Migrate a slave from MySQL 5.6 to MariaDB 10 break replication

MySQL 5.6 implemented WL#344, which is about a MASTER_DELAY option to CHANGE
MASTER. But as part of this worklog, the format of the realy-log.info file was
changed. The new format is not understood by earlier versions, and nor by
MariaDB 10.0, so changing server to those versions would cause the slave to
abort with an error due to reading incorrect data out of relay-log.info.

Fix this by backporting from the WL#344 patch just the code that understands
the new relay-log.info format. We still write out the old format, and none of
the MASTER_DELAY feature is backported with this commit.

1 files changed, 67 insertions, 7 deletions

diff --git a/sql/rpl_rli.cc b/sql/rpl_rli.cc
index a162d1d79f8..9de23e3cca6 100644
--- a/sql/rpl_rli.cc
+++ b/sql/rpl_rli.cc
@@ -305,20 +305,80 @@ Failed to open the existing relay log info file '%s' (errno %d)",
     }
 
     rli->info_fd = info_fd;
-    int relay_log_pos, master_log_pos;
+    int relay_log_pos, master_log_pos, lines;
+    char *first_non_digit;
+    /*
+      In MySQL 5.6, there is a MASTER_DELAY option to CHANGE MASTER. This is
+      not yet merged into MariaDB (as of 10.0.13). However, we detect the
+      presense of the new option in relay-log.info, as a placeholder for
+      possible later merge of the feature, and to maintain file format
+      compatibility with MySQL 5.6+.
+    */
+    int dummy_sql_delay;
+
+    /*
+      Starting from MySQL 5.6.x, relay-log.info has a new format.
+      Now, its first line contains the number of lines in the file.
+      By reading this number we can determine which version our master.info
+      comes from. We can't simply count the lines in the file, since
+      versions before 5.6.x could generate files with more lines than
+      needed. If first line doesn't contain a number, or if it
+      contains a number less than LINES_IN_RELAY_LOG_INFO_WITH_DELAY,
+      then the file is treated like a file from pre-5.6.x version.
+      There is no ambiguity when reading an old master.info: before
+      5.6.x, the first line contained the binlog's name, which is
+      either empty or has an extension (contains a '.'), so can't be
+      confused with an integer.
+
+      So we're just reading first line and trying to figure which
+      version is this.
+    */
+
+    /*
+      The first row is temporarily stored in mi->master_log_name, if
+      it is line count and not binlog name (new format) it will be
+      overwritten by the second row later.
+    */
     if (init_strvar_from_file(rli->group_relay_log_name,
                               sizeof(rli->group_relay_log_name),
+                              &rli->info_file, ""))
+    {
+      msg="Error reading slave log configuration";
+      goto err;
+    }
+
+    lines= strtoul(rli->group_relay_log_name, &first_non_digit, 10);
+
+    if (rli->group_relay_log_name[0] != '\0' &&
+        *first_non_digit == '\0' &&
+        lines >= LINES_IN_RELAY_LOG_INFO_WITH_DELAY)
+    {
+      DBUG_PRINT("info", ("relay_log_info file is in new format."));
+      /* Seems to be new format => read relay log name from next line */
+      if (init_strvar_from_file(rli->group_relay_log_name,
+                                sizeof(rli->group_relay_log_name),
+                                &rli->info_file, ""))
+      {
+        msg="Error reading slave log configuration";
+        goto err;
+      }
+    }
+    else
+      DBUG_PRINT("info", ("relay_log_info file is in old format."));
+
+    if (init_intvar_from_file(&relay_log_pos,
+                              &rli->info_file, BIN_LOG_HEADER_SIZE) ||
+        init_strvar_from_file(rli->group_master_log_name,
+                              sizeof(rli->group_master_log_name),
                               &rli->info_file, "") ||
-       init_intvar_from_file(&relay_log_pos,
-                             &rli->info_file, BIN_LOG_HEADER_SIZE) ||
-       init_strvar_from_file(rli->group_master_log_name,
-                             sizeof(rli->group_master_log_name),
-                             &rli->info_file, "") ||
-       init_intvar_from_file(&master_log_pos, &rli->info_file, 0))
+        init_intvar_from_file(&master_log_pos, &rli->info_file, 0) ||
+        (lines >= LINES_IN_RELAY_LOG_INFO_WITH_DELAY &&
+         init_intvar_from_file(&dummy_sql_delay, &rli->info_file, 0)))
     {
       msg="Error reading slave log configuration";
       goto err;
     }
+
     strmake_buf(rli->event_relay_log_name,rli->group_relay_log_name);
     rli->group_relay_log_pos= rli->event_relay_log_pos= relay_log_pos;
     rli->group_master_log_pos= master_log_pos;