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

Subject: [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: 2014-12-11 17:35:21

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).

Thanks,
-Matt