Should Reader move last?

[jamesmunns]

This comment from initial implementation states that Reader should be in charge of moving last back to max when necessary. See this comment

However in my current implementation, I move the last pointer when the Writer commits a grant past the last position. I'm not sure when this changed. This still seems sound, as the following scenarios are all good:

1. read < write < last
   • OK: Reader will be blocked by writer
2. write < read < last
   • OK: Reader will be blocked by last, then will jump back to 0, where it will be back in case 1

I should probably make sure that the ordering of these transactions are still good though, and either update the code, or update the comments

CC @utaal, I need to see what his spsc buffer does in this case