Patch for Bug#27863 (excessive memory usage for many small queries in a

multiquery packet).

Background:

  - a query can contain multiple SQL statements;

  - the server frees resources allocated to process a query when the
    whole query is handled. In other words, resources allocated to process
    one SQL statement from a multi-statement query are freed when all SQL
    statements are handled.

The problem was that the parser allocated a buffer of size of the whole
query for each SQL statement in a multi-statement query. Thus, if a query
had many SQL-statements (so, the query was long), but each SQL statement
was short, ther parser tried to allocate huge amount of memory (number of
small SQL statements * length of the whole query).

The memory was allocated for a so-called "cpp buffer", which is intended to
store pre-processed SQL statement -- SQL text without version specific
comments.

The fix is to allocate memory for the "cpp buffer" once for all SQL
statements (once for a query).

mysql-test/t/parser_stack.test

@@ -399,4 +399,12 @@ delimiter ;$$
 
 drop procedure p_37228;
 
+#
+# Bug#27863 (excessive memory usage for many small queries in a multiquery
+# packet).
+#
 
+let $i=`select repeat("set @a=1;", 65535)`;
+--disable_query_log
+eval $i;
+--enable_query_log