bug#24260: [PATCH 1/6] dfa: thread-safety: remove 'dfa' global in dfa.c

[Norihiro Tanaka]

On Fri, 19 Aug 2016 18:03:19 -0500
Zev Weiss <address@hidden> wrote:

> Okay -- so your question is about the necessity of making operations other 
> than dfaexec() thread-safe?  That's reasonable, though (obviously) I went 
> ahead made the other operations thread-safe anyway.
> 
> 1) It was, in some ways, simpler for me.  Rather than tracing through exactly 
> when and how each variable was used to determine which could be safely left 
> as globals, it was somewhat easier to just migrate all mutable global state 
> into a context struct of some sort.
> 
> 2) While it's not a terribly elegant approach (and certainly doesn't have to 
> be this way), my current multithreading patch set actually does compile the 
> regex separately (and concurrently) in each thread (for reasons similar to #1 
> above -- it was just simpler to do it that way).
> 
> 3) Finally, and perhaps most compellingly in my opinion, especially since 
> dfa.c is soon going to become a library component, it seems desirable for 
> compilation etc. to be made thread safe anyway.  While grep doesn't really 
> *need* to have it be thread-safe (and I'm guessing gawk and sed currently 
> don't either), it doesn't seem all that unreasonable for an application to 
> *want* to concurrently compile independent regexes, so it would seem a bit 
> unfortunate to me for a library (gnulib) to not support that at all.
> 
> 
> Zev

Thanks.  dfa exists only to improve performance of regex and regex uses
static variable for syntax which is re_syntax_options.  So we can not
still change compile phase thread-safe.

In addition, IMHO, I think that compile should run only one time even
if grep runs multithread as you say in 2, and I do not expect that
members used in only compile not used in execute is generated each more
than one.

BTW, mbcsets, nmbcsets and mbcsets_alloc use in only compile, so I think
that they may be also put out of struct dfa in future. (That isn't always
to move them to static or global)

Norihiro