MDEV-16372 ER_BASE64_DECODE_ERROR upon replaying binary log via mysqlbinlog --verbose

(This commit is exclusively for 10.2 branch. Do not merge it to 10.3)

In case of a pattern of non-STMT_END-marked Rows-log-event (A) followed by
a STMT_END marked one (B) mysqlbinlog mixes up the base64 encoded rows events
with their pseudo sql representation produced by the verbose option:
      BINLOG '
        base64 encoded data for A
        ### verbose section for A
        base64 encoded data for B
        ### verbose section for B
      '/*!*/;
In effect the produced BINLOG '...' query is not valid and is rejected with the error.
Examples of this way malformed BINLOG could have been found in binlog_row_annotate.result
that gets corrected with the patch.

The issue is fixed with introduction an auxiliary IO_CACHE to hold on the verbose
comments until the terminal STMT_END event is found. The new cache is emptied
out after two pre-existing ones are done at that time.
The correctly produced output now for the above case is as the following:
      BINLOG '
        base64 encoded data for A
        base64 encoded data for B
      '/*!*/;
        ### verbose section for A
        ### verbose section for B

Thanks to Alexey Midenkov for the problem recognition and attempt to tackle,
and to Venkatesh Duggirala who produced a patch for the upstream whose
idea is exploited here, as well as to MDEV-23077 reporter LukeXwang who
also contributed a piece of a patch aiming at this issue.

1 files changed, 4 insertions, 2 deletions

diff --git a/sql/log_event.h b/sql/log_event.h
index 1fae201057f..3fc44a9669f 100644
--- a/sql/log_event.h
+++ b/sql/log_event.h
@@ -848,18 +848,19 @@ typedef struct st_print_event_info
   ~st_print_event_info() {
     close_cached_file(&head_cache);
     close_cached_file(&body_cache);
+    close_cached_file(&tail_cache);
 #ifdef WHEN_FLASHBACK_REVIEW_READY
     close_cached_file(&review_sql_cache);
 #endif
   }
   bool init_ok() /* tells if construction was successful */
-    { return my_b_inited(&head_cache) && my_b_inited(&body_cache)
+    { return my_b_inited(&head_cache) && my_b_inited(&body_cache) &&
+             my_b_inited(&tail_cache)
 #ifdef WHEN_FLASHBACK_REVIEW_READY
       && my_b_inited(&review_sql_cache)
 #endif
     ; }
 
-
   /* Settings on how to print the events */
   bool short_form;
   enum_base64_output_mode base64_output_mode;
@@ -885,6 +886,7 @@ typedef struct st_print_event_info
    */
   IO_CACHE head_cache;
   IO_CACHE body_cache;
+  IO_CACHE tail_cache;
 #ifdef WHEN_FLASHBACK_REVIEW_READY
   /* Storing the SQL for reviewing */
   IO_CACHE review_sql_cache;