BUG#55675 rpl.rpl_log_pos fails sporadically with error binlog truncated in the middle

There are two calls to read_log_event() on master in mysql_binlog_send().
Each call reads 19 bytes in this test case and the error of the second
read_log_event() is reported to the slave.

The second read_log_event() starts from position 94 (75 + 19) to 113
(75 + 19 + 19). Usually, there are two events in the binary log:

    . 0   - 3   - Header
    . 4   - 105 - Format Descriptor Event
    . 106 - 304 - Query Event

and both reads fail because operations are reading from invalid positions
as expected.

However, mysql_binlog_send() does not use the same IO_CACHE that is used to
write into binary log (i.e. mysql_bin_log.log_file) for the hot binary log.
It opens the binary log file directly by calling open_binlog() and creates a
separated IO_CACHE. So there is a possibly that after a master has flushed
the binary log file, the content has been cached by the filesystem, and has
not updated the disk file. If this happens, then a slave will only see part
of the file, and thus the second read_log_event() will report event truncated
error.

To fix the problem, if the first read_log_event() has failed, we ensure that
the second one will try to read from the same position.

1 files changed, 8 insertions, 1 deletions

diff --git a/sql/sql_repl.cc b/sql/sql_repl.cc
index 8c769ce6acf..2a1efab13a9 100644
--- a/sql/sql_repl.cc
+++ b/sql/sql_repl.cc
@@ -545,8 +545,10 @@ impossible position";
 
   while (!net->error && net->vio != 0 && !thd->killed)
   {
+    my_off_t prev_pos= pos;
     while (!(error = Log_event::read_log_event(&log, packet, log_lock)))
     {
+      prev_pos= my_b_tell(&log);
 #ifndef DBUG_OFF
       if (max_binlog_dump_events && !left_events--)
       {
@@ -613,8 +615,13 @@ impossible position";
       here we were reading binlog that was not closed properly (as a result
       of a crash ?). treat any corruption as EOF
     */
-    if (binlog_can_be_corrupted && error != LOG_READ_MEM)
+    if (binlog_can_be_corrupted &&
+        error != LOG_READ_MEM && error != LOG_READ_EOF)
+    {
+      my_b_seek(&log, prev_pos);
       error=LOG_READ_EOF;
+    }
+
     /*
       TODO: now that we are logging the offset, check to make sure
       the recorded offset and the actual match.