to detect the end of SP definition correctly we need to know where
the parser stopped parsing the SP. lip->get_cpp_ptr() shows the
current parsing position, lip->get_cpp_tok_start() shows the start of
the last parsed token. The actual value depends on whether
the parser has performed a look-ahead. For example, in

  CREATE PROCEDURE ... BEGIN ... END ;

the parser reads 'END' and knows that this ends the procedure definition,
it does not need to read the next token for this. But in

  CREATE PROCEDURE ... SELECT 1 ;

the parser cannot know that the procedure ends at '1'. It has to read
the semicolon first (it could be '1 + 2' for example).

In the first case, the "current parsing position" is after END, before
the semicolon, in the second case it's *after* the semicolon. Note that
SP definition in both cases ends before the semicolon.

To be able to detect the end of SP deterministically, we need the parser
to do the look-ahead always or never.

The bug fix introduces a new parser token FORCE_LOOKAHEAD. Lexer never
returns it, so this token can never match. But the parser cannot know
it so it will have to perform a look-ahead to determine that the next
token is not FORCE_LOOKAHEAD. This way we deterministically end
SP parsing with a look-ahead.