GDALGetCacheMax64() TSAN complaint

[schwehr]

Super low priority...

In the I don't really care category, but TSAN complained about GDALGetCacheMax64(). It doesn't like the bool check: if( !bCacheMaxInitialized ). I don't think that two threads getting into this code would really be a big deal. This would easily solve with call_once, but this would be the first use of call_once in GDAL. But maybe it's not so easy since there interaction in GDALSetCacheMax64() for bCacheMaxInitialized

namespace {
std::once_flag flagSetupGDALGetCacheMax64;
}

GIntBig CPL_STDCALL GDALGetCacheMax64()
{
    std::call_once(flagSetupGDALGetCacheMax64, [](){
        // All the setup code here.
    });
    return nCacheMax;
}

https://en.cppreference.com/w/cpp/thread/call_once

[rouault]

@schwehr Could you confirm #9253 fixes this issue ? I couldn't reproduce a TSAN warning with gcc 9 -sanitize=thread