Race on global variable: factorials

[nasermirzaei89]

We get a RACE error on running tests in the production with the flag -race enabled.
And the stack shows it comes from here:

I believe that the reason is multiple goroutines are calling method ExpTaylor and this method manipulates the shared variable factorials (https://github.com/shopspring/decimal/blob/master/decimal.go#L84).
As you used a global variable for caching factorials (#229), it's not goroutine safe and should have a lock.

[mwoss]

Hi @nasermirzaei89! Thanks for bringing this issue up.
I was aware of this potential race condition problem during the implementation of ExpTaylor, but I though my lock-less approach would be sufficient for - https://github.com/shopspring/decimal/blob/master/decimal.go#L1089:L1099. I guess I was wrong, I would need to take a closer look on that 👀