One-line fix for bug 576 (DBUG_ASSERT failure when using CHANGE MASTER TO RELAY_LOG_POS=4).

Plus a changeset which I had committed but forgot to push (and this changeset is lost on
another computer, so I recreate it here). This changeset is "user-friendly SHOW BINLOG EVENTS
and CHANGE MASTER TO when log positions < 4 are used.


sql/slave.cc:
  fix for bug 576 (DBUG_ASSERT failure when using CHANGE MASTER TO RELAY_LOG_POS=4).
sql/sql_repl.cc:
  User-friendly SHOW BINLOG EVENTS:
  SHOW BINLOG EVENTS FROM 0: currently one gets
      MASTER> show binlog events from 0;
      ERROR 1220: Error when executing command SHOW BINLOG EVENTS: Invalid log position
  so we silently convert <4 to 4.
sql/sql_yacc.yy:
  User-friendly CHANGE MASTER TO:
  presently when one does CHANGE MASTER TO MASTER_LOG_POS=0 he gets
      030425 10:12:41  Slave I/O thread: connected to master 'root@localhost:3306',  r
      eplication started in log 'gbichot-bin.013' at position 151
      030425 10:12:41  Error reading packet from server: Client requested master to st
      art replication from impossible position (server_errno=1236)
      030425 10:12:41  Got fatal error 1236: 'Client requested master to start replica
      tion from impossible position' from master when reading data from binary log
      030425 10:12:41  Slave I/O thread exiting, read up to log 'gbichot-bin.013', pos
      ition 151
      
  while she/he probably just wanted to start at the beginning of the
  binlog, which is 4. So we silently convert <4 to 4 in sql_yacc.yy
  (i.e. in the slave code; fortunately all versions have the same
  BIN_LOG_HEADER_SIZE=4 and we should not change this). See comments
  for an explaination of why we have to do this in sql_yacc.yy,
  not in sql_repl.cc.
  Same thing for CHANGE MASTER TO RELAY_LOG_POS.
sql/unireg.h:
  warning comment

1 files changed, 14 insertions, 0 deletions

diff --git a/sql/sql_yacc.yy b/sql/sql_yacc.yy
index c79750c8014..b0c81d6f6b0 100644
--- a/sql/sql_yacc.yy
+++ b/sql/sql_yacc.yy
@@ -706,6 +706,18 @@ master_def:
        MASTER_LOG_POS_SYM EQ ulonglong_num
        {
 	 Lex->mi.pos = $3;
+         /* 
+            If the user specified a value < BIN_LOG_HEADER_SIZE, adjust it
+            instead of causing subsequent errors. 
+            We need to do it in this file, because only there we know that 
+            MASTER_LOG_POS has been explicitely specified. On the contrary
+            in change_master() (sql_repl.cc) we cannot distinguish between 0
+            (MASTER_LOG_POS explicitely specified as 0) and 0 (unspecified),
+            whereas we want to distinguish (specified 0 means "read the binlog
+            from 0" (4 in fact), unspecified means "don't change the position
+            (keep the preceding value)").
+         */
+         Lex->mi.pos = max(BIN_LOG_HEADER_SIZE, Lex->mi.pos);
        }
        |
        MASTER_CONNECT_RETRY_SYM EQ ULONG_NUM
@@ -721,6 +733,8 @@ master_def:
        RELAY_LOG_POS_SYM EQ ULONG_NUM
        {
 	 Lex->mi.relay_log_pos = $3;
+         /* Adjust if < BIN_LOG_HEADER_SIZE (same comment as Lex->mi.pos) */
+         Lex->mi.relay_log_pos = max(BIN_LOG_HEADER_SIZE, Lex->mi.relay_log_pos);
        };