time: allow to inject per-module or per-thread now()

[ash2k]

This is a follow up for #435.

Global var is still there in the time module:

starlark-go/lib/time/time.go

Line 74 in 2232540

var NowFunc = time.Now

I'd like to be able to customize what now() returns on a per-thread basis. Alternatively, I'd like to be able to instantiate a new time module with injected now() (or both).

Thank you!

[adonovan]

I would accept a PR to make the now function consult a starlark.Thread-local variable (of type func() time.Now) before falling back to the standard time.Now implementation.

[ash2k]

Thank you. I'll open a PR soon.

Do you want me to keep the existing package var for backward compatibility or remove it?

[adonovan]

We can't remove it, but we can deprecate it and recommend that people use the new mechanism instead, which avoids global state.