Re: [boost] Is Boost.Xpressive's sregex::compile() thread-safe for an object only used in that thread?

Subject: Re: [boost] Is Boost.Xpressive's sregex::compile() thread-safe for an object only used in that thread?
From: Chambers, Matthew (matt.chambers42_at_[hidden])
Date: 2014-12-12 15:46:12

On 12/12/2014 2:09 AM, Eric Niebler wrote:
> On 12/11/2014 2:35 PM, Chambers, Matthew wrote:
>> I have a case where I am compiling a dynamic regex based on client code
>> input, and it works except for when I run my multi-threaded unit test
>> 100-1000 times.
>>
>> When I run it with 1 thread, it never fails. When I run it with 2 or 4
>> threads it fails at this rare frequency. Except on my build server,
>> where it seems to fail most of the time, which is how I learned about
>> the issue to begin with. It's not crashing though, it's just very rarely
>> returning the wrong results, and not always in the same place in the
>> unit test.
>>
>> So I'm trying to understand Xpressive's thread safety. I understand it
>> as far as not using a regex object in multiple threads until it's
>> finished compiling and not sharing a match_results across threads, but
>> what about compiling different regex objects for each thread: if two
>> threads are compiling two different regexes (or even the same const
>> regex string but from different threads, which is possible with my test
>> code), is that ok?
>>
>> I'm seeing this failure on MSVC 12 but I think I also got it with VC10,
>> just not as reliably on the build server (but the build server had to be
>> upgraded to Windows 2012 to install VC12, so maybe that OS change had an
>> effect as well).
> Are your regex objects referring to other regex objects? That is, are
> you defining a grammar? See this caveat in the docs:
>
> http://www.boost.org/doc/libs/1_57_0/doc/html/xpressive/user_s_guide.html#boost_xpressive.user_s_guide.tips_n_tricks.create_grammars_on_a_single_thread
>
> The docs only mentions static regexes, but it's true for dynamic regexes.
>
> If you're not building grammars where some of the regexes are shared,
> then no, there are no known thread-safety problems with constructing a
> regex on a single thread.
>
> After the regex object is constructed, it is safe to use from multiple
> threads, since it's not mutated during pattern matching.

No, I'm using them as plain boost::regex-style regexes. You say "no 
thread-safety problems with constructing a regex on a single thread" - 
but I'm constructing a regex on multiple threads (it may be the same 
regex pattern or a different one, depending on how the threads are 
executing). If it's not a construction problem (a problem I may have 
ruled out by moving construction to a singleton which mutexes the 
compilation and caches the compiled patterns), then any other ideas why 
I would be seeing these very rare aberrant results?

Thanks,
-Matt