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

@@ -269,7 +269,7 @@ int init_relay_log_pos(RELAY_LOG_INFO* rli,const char* log,
       goto err;
     rli->cur_log = &rli->cache_buf;
   }
-  if (pos > BIN_LOG_HEADER_SIZE)
+  if (pos >= BIN_LOG_HEADER_SIZE)
     my_b_seek(rli->cur_log,(off_t)pos);
 
 err:

sql/sql_repl.cc

@@ -961,7 +961,7 @@ int show_binlog_events(THD* thd)
   {
     LEX_MASTER_INFO *lex_mi = &thd->lex.mi;
     ha_rows event_count, limit_start, limit_end;
-    my_off_t pos = lex_mi->pos;
+    my_off_t pos = max(BIN_LOG_HEADER_SIZE, lex_mi->pos); // user-friendly
     char search_file_name[FN_REFLEN], *name;
     const char *log_file_name = lex_mi->log_file_name;
     pthread_mutex_t *log_lock = mysql_bin_log.get_log_lock();
@@ -989,12 +989,6 @@ int show_binlog_events(THD* thd)
     if ((file=open_binlog(&log, linfo.log_file_name, &errmsg)) < 0)
       goto err;
 
-    if (pos < 4)
-    {
-      errmsg = "Invalid log position";
-      goto err;
-    }
-
     pthread_mutex_lock(log_lock);
     my_b_seek(&log, pos);
 

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);
        };
 
 

sql/unireg.h

@@ -130,9 +130,13 @@ bfill((A)->null_flags,(A)->null_bytes,255);\
  */
 #define MIN_TURBOBM_PATTERN_LEN 3
 
-/* Defines for binary logging */
+/* 
+   Defines for binary logging.
+   Do not decrease the value of BIN_LOG_HEADER_SIZE.
+   Do not even increase it before checking code.
+*/
 
-#define BIN_LOG_HEADER_SIZE	4
+#define BIN_LOG_HEADER_SIZE    4 
 
 	/* Include prototypes for unireg */