Re: \if, \elseif, \else, \endif (was Re: [HACKERS] PSQL commands:\quit_if, \quit_unless)
| От | Corey Huinker |
|---|---|
| Тема | Re: \if, \elseif, \else, \endif (was Re: [HACKERS] PSQL commands:\quit_if, \quit_unless) |
| Дата | |
| Msg-id | CADkLM=cr2z935TF3eB9jByWiijv-q2B7--QggfUUB12gCFFByQ@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: \if, \elseif, \else, \endif (was Re: [HACKERS] PSQL commands:\quit_if, \quit_unless) (Fabien COELHO <coelho@cri.ensmp.fr>) |
| Ответы |
Re: \if, \elseif, \else, \endif (was Re: [HACKERS] PSQL commands:\quit_if, \quit_unless)
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| Список | pgsql-hackers |
On Thu, Jan 26, 2017 at 3:55 PM, Fabien COELHO <coelho@cri.ensmp.fr> wrote:
Hello Daniel,A comment about control flow and variables: in branches that are not taken, variables are expanded nonetheless, in a way that can be surprising. Case in point:
\set var 'ab''cd'
-- select :var;
\if false
select :var ;
\else
select 1;
\endif
To avoid that kind of trouble, would it make sense not to expand
variables in untaken branches?
Hmmm. This case is somehow contrived (for a string, :'var' could be used, in which case escaping would avoid the hazard), but I think that what you suggest is a better behavior, if easy to implement.
--
Fabien.
Good question, Daniel. Variable expansion seems to be done via psql_get_variable which is invoked via callback, which means that I might have to move branch_block_active into PsqlSettings. That's slightly different because the existing boolean is scoped with MainLoop(), but there's no way to get to a new MainLoop unless you're in an executing branch, and no way to legally exit a MainLoop with an unbalanced if-endif state. In short, I think it's better behavior. It does prevent someone from setting a variable to '\endif' and expecting that to work, like this:
select casewhen random() < 0.5 then '\endif'else E'\else\n\echo fooled you\n\endif'end as contrived_metaprogramming\gset\if false:contrived_metaprogramming
I'm sure someone will argue that preventing that is a good thing. Unless someone sees a good reason not to move that PsqlSettings, I'll make that change.
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