Fixed bug #42496 - the server could crash on a debug assert after a failure

to write into a closed socket

sql/protocol.cc:
  Protocol::flush modified: set thd->main_da.can_overwrite_status= TRUE
  before call to net_flush() in order to prevent crash on assert in case
  of socket write failure, reset it to FALSE when net_flush() returned;
  Protocol::send_fields modified: return from method with error if call to
  my_net_write(), proto.write() or write_eof_packet() failed.
sql/sql_cache.cc:
  Query_cache::send_result_to_client modified: call to
  thd->main_da.disable_status() only if write to socket
  was successful.
sql/sql_cursor.cc:
  Materialized_cursor::fetch modified: leave method if call to
  result->send_data() failed.
sql/sql_prepare.cc:
  send_prep_stmt() modified: call to thd->main_da.disable_status()
  only if thd->protocol_text.send_fields() completed successfully.

sql/protocol.cc

@@ -534,7 +534,11 @@ void Protocol::end_partial_result_set(THD *thd_arg)
 bool Protocol::flush()
 {
 #ifndef EMBEDDED_LIBRARY
-  return net_flush(&thd->net);
+  bool error;
+  thd->main_da.can_overwrite_status= TRUE;
+  error= net_flush(&thd->net);
+  thd->main_da.can_overwrite_status= FALSE;
+  return error;
 #else
   return 0;
 #endif
@@ -574,7 +578,8 @@ bool Protocol::send_fields(List<Item> *list, uint flags)
   if (flags & SEND_NUM_ROWS)
   {				// Packet with number of elements
     uchar *pos= net_store_length(buff, list->elements);
-    (void) my_net_write(&thd->net, buff, (size_t) (pos-buff));
+    if (my_net_write(&thd->net, buff, (size_t) (pos-buff)))
+      DBUG_RETURN(1);
   }
 
 #ifndef DBUG_OFF
@@ -698,7 +703,7 @@ bool Protocol::send_fields(List<Item> *list, uint flags)
     if (flags & SEND_DEFAULTS)
       item->send(&prot, &tmp);			// Send default value
     if (prot.write())
-      break;					/* purecov: inspected */
+      DBUG_RETURN(1);
 #ifndef DBUG_OFF
     field_types[count++]= field.type;
 #endif
@@ -711,7 +716,9 @@ bool Protocol::send_fields(List<Item> *list, uint flags)
       to show that there is no cursor.
       Send no warning information, as it will be sent at statement end.
     */
-    write_eof_packet(thd, &thd->net, thd->server_status, thd->total_warn_count);
+    if (write_eof_packet(thd, &thd->net, thd->server_status,
+                         thd->total_warn_count))
+      DBUG_RETURN(1);
   }
   DBUG_RETURN(prepare_for_send(list));
 

sql/sql_cache.cc

@@ -1653,6 +1653,7 @@ def_week_frmt: %lu, in_trans: %d, autocommit: %d",
 
   thd->limit_found_rows = query->found_rows();
   thd->status_var.last_query_cost= 0.0;
+  if (!thd->main_da.is_set())
     thd->main_da.disable_status();
 
   BLOCK_UNLOCK_RD(query_block);

sql/sql_cursor.cc

@@ -658,7 +658,12 @@ void Materialized_cursor::fetch(ulong num_rows)
     if ((res= table->file->rnd_next(table->record[0])))
       break;
     /* Send data only if the read was successful. */
-    result->send_data(item_list);
+    /*
+      If network write failed (i.e. due to a closed socked),
+      the error has already been set. Just return.
+    */
+    if (result->send_data(item_list))
+      return;
   }
 
   switch (res) {

sql/sql_prepare.cc

@@ -263,8 +263,11 @@ static bool send_prep_stmt(Prepared_statement *stmt, uint columns)
                                           &stmt->lex->param_list,
                                           Protocol::SEND_EOF);
   }
+
+  if (!error)
     /* Flag that a response has already been sent */
     thd->main_da.disable_status();
+
   DBUG_RETURN(error);
 }
 #else