I committed the same changeset in my tree yesterday, but broke the tree since,

so I commit again in a fresh tree.
Fix for bug#763 (Relay_log_space too big by 4 bytes),
plus comments and DBUG_PRINT, and we don't start replication
if --bootstrap.

mysql-test/r/rpl_log.result

@@ -93,6 +93,6 @@ slave-bin.002	62	Query	1	168	use test; insert into t1 values (1)
 slave-bin.002	122	Query	1	228	use test; drop table t1
 show slave status;
 Master_Host	Master_User	Master_Port	Connect_retry	Master_Log_File	Read_Master_Log_Pos	Relay_Log_File	Relay_Log_Pos	Relay_Master_Log_File	Slave_IO_Running	Slave_SQL_Running	Replicate_do_db	Replicate_ignore_db	Last_errno	Last_error	Skip_counter	Exec_master_log_pos	Relay_log_space
-127.0.0.1	root	MASTER_PORT	1	master-bin.002	276	slave-relay-bin.002	1563	master-bin.002	Yes	Yes			0		0	276	1567
+127.0.0.1	root	MASTER_PORT	1	master-bin.002	276	slave-relay-bin.002	1563	master-bin.002	Yes	Yes			0		0	276	1563
 show binlog events in 'slave-bin.005' from 4;
 Error when executing command SHOW BINLOG EVENTS: Could not find target log

mysql-test/r/rpl_log_pos.result

@@ -9,7 +9,7 @@ File	Position	Binlog_do_db	Binlog_ignore_db
 master-bin.001	79		
 show slave status;
 Master_Host	Master_User	Master_Port	Connect_retry	Master_Log_File	Read_Master_Log_Pos	Relay_Log_File	Relay_Log_Pos	Relay_Master_Log_File	Slave_IO_Running	Slave_SQL_Running	Replicate_do_db	Replicate_ignore_db	Last_errno	Last_error	Skip_counter	Exec_master_log_pos	Relay_log_space
-127.0.0.1	root	MASTER_PORT	1	master-bin.001	79	slave-relay-bin.002	120	master-bin.001	Yes	Yes			0		0	79	124
+127.0.0.1	root	MASTER_PORT	1	master-bin.001	79	slave-relay-bin.002	120	master-bin.001	Yes	Yes			0		0	79	120
 slave stop;
 change master to master_log_pos=73;
 slave start;

sql/mysqld.cc

@@ -2293,6 +2293,8 @@ int main(int argc, char **argv)
   if (!opt_noacl)
     udf_init();
 #endif
+  if (opt_bootstrap) /* If running with bootstrap, do not start replication. */
+    opt_skip_slave_start= 1;
   /* init_slave() must be called after the thread keys are created */
   init_slave();
 

sql/slave.cc

@@ -1424,6 +1424,12 @@ static int count_relay_log_space(RELAY_LOG_INFO* rli)
     if (add_relay_log(rli,&linfo))
       DBUG_RETURN(1);
   } while (!rli->relay_log.find_next_log(&linfo, 1));
+  /* 
+     As we have counted everything, including what may have written in a
+     preceding write, we must reset bytes_written, or we may count some space 
+     twice.
+  */
+  rli->relay_log.reset_bytes_written();
   DBUG_RETURN(0);
 }
 
@@ -3213,8 +3219,25 @@ Log_event* next_event(RELAY_LOG_INFO* rli)
 	hot_log=0;				// Using old binary log
       }
     }
+#ifndef DBUG_OFF
+    {
       DBUG_ASSERT(my_b_tell(cur_log) >= BIN_LOG_HEADER_SIZE);
+      /* The next assertion sometimes (very rarely) fails, let's try to track it */
+      char llbuf1[22], llbuf2[22];
+      /* Merging man, please be careful with this; in 4.1, the assertion below is
+         replaced by
+         DBUG_ASSERT(my_b_tell(cur_log) == rli->event_relay_log_pos);
+         so you should not merge blindly (fortunately it won't build then), and
+         instead modify the merged code. Thanks. */
+      DBUG_PRINT("info", ("Before assert, my_b_tell(cur_log)=%s \
+rli->relay_log_pos=%s rli->pending=%lu",
+                          llstr(my_b_tell(cur_log),llbuf1), 
+                          llstr(rli->relay_log_pos,llbuf2),
+                          rli->pending));
       DBUG_ASSERT(my_b_tell(cur_log) == rli->relay_log_pos + rli->pending);
+    }
+#endif
+
     /*
       Relay log is always in new format - if the master is 3.23, the
       I/O thread will convert the format for us

sql/sql_acl.cc

@@ -2250,7 +2250,7 @@ int mysql_grant (THD *thd, const char *db, List <LEX_USER> &list,
   if (!initialized)
   {
     send_error(&(thd->net), ER_UNKNOWN_COM_ERROR); /* purecov: tested */
-    return 1; /* purecov: tested */
+    DBUG_RETURN(1); /* purecov: tested */
   }
 
   if (lower_case_table_names && db)

sql/sql_repl.cc

@@ -796,6 +796,25 @@ err:
   DBUG_RETURN(error);
 }
 
+/*
+
+  Kill all Binlog_dump threads which previously talked to the same slave
+  ("same" means with the same server id). Indeed, if the slave stops, if the
+  Binlog_dump thread is waiting (pthread_cond_wait) for binlog update, then it
+  will keep existing until a query is written to the binlog. If the master is
+  idle, then this could last long, and if the slave reconnects, we could have 2
+  Binlog_dump threads in SHOW PROCESSLIST, until a query is written to the
+  binlog. To avoid this, when the slave reconnects and sends COM_BINLOG_DUMP,
+  the master kills any existing thread with the slave's server id (if this id is
+  not zero; it will be true for real slaves, but false for mysqlbinlog when it
+  sends COM_BINLOG_DUMP to get a remote binlog dump).
+
+  SYNOPSIS
+    kill_zombie_dump_threads()
+    slave_server_id     the slave's server id
+
+*/
+  
 
 void kill_zombie_dump_threads(uint32 slave_server_id)
 {