MDEV-18046: Assortment of crashes, assertion failures and ASAN errors in mysql_show_binlog_events

Problem:
========
SHOW BINLOG EVENTS FROM <pos> causes a variety of failures, some of which are
listed below. It is not a race condition issue, but there is some
non-determinism in it.

Analysis:
========
"show binlog events from <pos>" code considers the user given position as a
valid event start position. The code starts reading data from this event start
position onwards and tries to map it to a set of known events. Each event has
a specific event structure and asserts have been added to ensure that read
event data satisfies the event specific requirements. When a random position
is supplied to "show binlog events command" the event structure specific
checks will fail and they result in assert.

Fix:
====
The fix is split into different parts. Each part addresses either an ASAN
issue or an assert/crash.

**Part1: Checksum based position validation when checksum is enabled**


Using checksum validate the very first event read at the user specified
position. If there is a checksum mismatch report an appropriate error for the
invalid event.

1 files changed, 17 insertions, 6 deletions

diff --git a/sql/sql_repl.cc b/sql/sql_repl.cc
index 48d2781cd46..851bff5fd24 100644
--- a/sql/sql_repl.cc
+++ b/sql/sql_repl.cc
@@ -3840,6 +3840,11 @@ bool mysql_show_binlog_events(THD* thd)
   List<Item> field_list;
   const char *errmsg = 0;
   bool ret = TRUE;
+  /*
+     Using checksum validate the correctness of event pos specified in show
+     binlog events command.
+  */
+  bool verify_checksum_once= false;
   IO_CACHE log;
   File file = -1;
   MYSQL_BIN_LOG *binary_log= NULL;
@@ -3895,6 +3900,10 @@ bool mysql_show_binlog_events(THD* thd)
       mi= 0;
     }
 
+    /* Validate user given position using checksum */
+    if (lex_mi->pos == pos && !opt_master_verify_checksum)
+      verify_checksum_once= true;
+
     unit->set_limit(thd->lex->current_select);
     limit_start= unit->offset_limit_cnt;
     limit_end= unit->select_limit_cnt;
@@ -3977,15 +3986,16 @@ bool mysql_show_binlog_events(THD* thd)
     for (event_count = 0;
          (ev = Log_event::read_log_event(&log, (mysql_mutex_t*) 0,
                                          description_event,
-                                         opt_master_verify_checksum)); )
+                                         (opt_master_verify_checksum ||
+                                          verify_checksum_once))); )
     {
       if (event_count >= limit_start &&
-	  ev->net_send(protocol, linfo.log_file_name, pos))
+          ev->net_send(protocol, linfo.log_file_name, pos))
       {
-	errmsg = "Net error";
-	delete ev;
+        errmsg = "Net error";
+        delete ev;
         mysql_mutex_unlock(log_lock);
-	goto err;
+        goto err;
       }
 
       if (ev->get_type_code() == FORMAT_DESCRIPTION_EVENT)
@@ -4011,10 +4021,11 @@ bool mysql_show_binlog_events(THD* thd)
         delete ev;
       }
 
+      verify_checksum_once= false;
       pos = my_b_tell(&log);
 
       if (++event_count >= limit_end)
-	break;
+        break;
     }
 
     if (event_count < limit_end && log.error)