Yesterday I removed process_io_create_file; I shouldn't have.

Let's say the lack of comments did not help me ;)
Copying it back again and adding comments; now 3.23->4.0
replication of LOAD DATA INFILE works again.

sql/slave.cc

@@ -57,6 +57,7 @@ typedef enum { SLAVE_THD_IO, SLAVE_THD_SQL} SLAVE_THD_TYPE;
 
 void skip_load_data_infile(NET* net);
 static int process_io_rotate(MASTER_INFO* mi, Rotate_log_event* rev);
+static int process_io_create_file(MASTER_INFO* mi, Create_file_log_event* cev);
 static bool wait_for_relay_log_space(RELAY_LOG_INFO* rli);
 static inline bool io_slave_killed(THD* thd,MASTER_INFO* mi);
 static inline bool sql_slave_killed(THD* thd,RELAY_LOG_INFO* rli);
@@ -2728,6 +2729,102 @@ the slave SQL thread with \"SLAVE START\". We stopped at log \
 }
 
 
+static int process_io_create_file(MASTER_INFO* mi, Create_file_log_event* cev)
+{
+  int error = 1;
+  ulong num_bytes;
+  bool cev_not_written;
+  THD* thd;
+  NET* net = &mi->mysql->net;
+  DBUG_ENTER("process_io_create_file");
+
+  if (unlikely(!cev->is_valid()))
+    DBUG_RETURN(1);
+  /*
+    TODO: fix to honor table rules, not only db rules
+  */
+  if (!db_ok(cev->db, replicate_do_db, replicate_ignore_db))
+  {
+    skip_load_data_infile(net);
+    DBUG_RETURN(0);
+  }
+  DBUG_ASSERT(cev->inited_from_old);
+  thd = mi->io_thd;
+  thd->file_id = cev->file_id = mi->file_id++;
+  thd->server_id = cev->server_id;
+  cev_not_written = 1;
+  
+  if (unlikely(net_request_file(net,cev->fname)))
+  {
+    sql_print_error("Slave I/O: failed requesting download of '%s'",
+                   cev->fname);
+    goto err;
+  }
+
+  /*
+    This dummy block is so we could instantiate Append_block_log_event
+    once and then modify it slightly instead of doing it multiple times
+    in the loop
+  */
+  {
+    Append_block_log_event aev(thd,0,0,0,0);
+  
+    for (;;)
+    {
+      if (unlikely((num_bytes=my_net_read(net)) == packet_error))
+      {
+       sql_print_error("Network read error downloading '%s' from master",
+                       cev->fname);
+       goto err;
+      }
+      if (unlikely(!num_bytes)) /* eof */
+      {
+       send_ok(net); /* 3.23 master wants it */
+       Execute_load_log_event xev(thd,0,0);
+       xev.log_pos = mi->master_log_pos;
+       if (unlikely(mi->rli.relay_log.append(&xev)))
+       {
+         sql_print_error("Slave I/O: error writing Exec_load event to \
+relay log");
+         goto err;
+       }
+       mi->rli.relay_log.harvest_bytes_written(&mi->rli.log_space_total);
+       break;
+      }
+      if (unlikely(cev_not_written))
+      {
+       cev->block = (char*)net->read_pos;
+       cev->block_len = num_bytes;
+       cev->log_pos = mi->master_log_pos;
+       if (unlikely(mi->rli.relay_log.append(cev)))
+       {
+         sql_print_error("Slave I/O: error writing Create_file event to \
+relay log");
+         goto err;
+       }
+       cev_not_written=0;
+       mi->rli.relay_log.harvest_bytes_written(&mi->rli.log_space_total);
+      }
+      else
+      {
+       aev.block = (char*)net->read_pos;
+       aev.block_len = num_bytes;
+       aev.log_pos = mi->master_log_pos;
+       if (unlikely(mi->rli.relay_log.append(&aev)))
+       {
+         sql_print_error("Slave I/O: error writing Append_block event to \
+relay log");
+         goto err;
+       }
+       mi->rli.relay_log.harvest_bytes_written(&mi->rli.log_space_total) ;
+      }
+    }
+  }
+  error=0;
+err:
+  DBUG_RETURN(error);
+}
+
 /*
   Start using a new binary log on the master
 
@@ -2803,6 +2900,12 @@ static int queue_old_event(MASTER_INFO *mi, const char *buf,
     tmp_buf[event_len]=0; // Create_file constructor wants null-term buffer
     buf = (const char*)tmp_buf;
   }
+  /*
+    This will transform LOAD_EVENT into CREATE_FILE_EVENT, ask the master to
+    send the loaded file, and write it to the relay log in the form of
+    Append_block/Exec_load (the SQL thread needs the data, as that thread is not
+    connected to the master).
+  */
   Log_event *ev = Log_event::read_log_event(buf,event_len, &errmsg,
 					    1 /*old format*/ );
   if (unlikely(!ev))
@@ -2831,6 +2934,24 @@ static int queue_old_event(MASTER_INFO *mi, const char *buf,
     mi->ignore_stop_event=1;
     inc_pos= 0;
     break;
+  case CREATE_FILE_EVENT:
+    /*
+      Yes it's possible to have CREATE_FILE_EVENT here, even if we're in
+      queue_old_event() which is for 3.23 events which don't comprise
+      CREATE_FILE_EVENT. This is because read_log_event() above has just
+      transformed LOAD_EVENT into CREATE_FILE_EVENT.
+    */
+  {
+    /* We come here when and only when tmp_buf != 0 */
+    DBUG_ASSERT(tmp_buf);
+    int error = process_io_create_file(mi,(Create_file_log_event*)ev);
+    delete ev;
+    mi->master_log_pos += event_len;
+    DBUG_PRINT("info", ("master_log_pos: %d", (ulong) mi->master_log_pos));
+    pthread_mutex_unlock(&mi->data_lock);
+    my_free((char*)tmp_buf, MYF(0));
+    DBUG_RETURN(error);
+  }
   default:
     mi->ignore_stop_event=0;
     inc_pos= event_len;