Race condition if multiple threads are parsing CSV and somebody calls SetHeaderNormalizer

[brianV]

Hi all.

We've noted a race condition with SetHeaderNormalizer():

func SetHeaderNormalizer(f Normalizer) {
	normalizeName = f
	// Need to clear the cache hen the header normalizer changes.
	structInfoCache = sync.Map{}
}

If we have threads currently working and performing operations on structInfoCache, our copying of a blank map into structInfoCache leads to various errors including:

fatal error: sync: unlock of unlocked mutex

structInfoCache should be defined as a *sync.Map{}. When we need to clear the cache, we then should run:

structInfoCache = &sync.Map{}

In-progress calls to the old structInfoCache would complete, and new calls will operate against the new one.