Fix sha256 race condition panic

[shaodan]

No description provided.

[shaodan]

Create new hmac instance as needed since it is not goroutine safe

[eynzhang]

@shaodan , I tried your test code first, I expect it should cause panic but it didn’t. Can you double check that?

And can you explain how you reproduce the issue #8 ?

[shaodan]

@eynzhang I've fix the panic, so the test should pass. If you want to reproduce that, you need to revert my change of internal/requestbuilder/signer.go

[eynzhang]

@shaodan , I didn’t use your code for test. I used your method TestSigner_Sign_RaceCondition to test against the master branch, and it passed.

[shaodan]

@eynzhang https://play.golang.org/p/h9_pLX0jieW

[filinvadim]

@shaodan @eynzhang Hi folks! Issue was closed but nothing was actually fixed.

[eynzhang]

@shaodan thanks for your contribution, your request is merged. I am not sure why I can’t reproduce this issue last time.