math/rand: default Source safe for concurrent use

[awilliams]

The math/rand documentation states:

The default Source is safe for concurrent use by multiple goroutines.

This confused me because I thought the "default Source" referred to Source returned by the NewSource function. Instead, it refers to the protected internal Source used by the top-level functions.

• Introductory Commit
• Source

[bradfitz]

Agreed. We should clarify that the Source (internally: rngSource) returned by NewSource is not safe for concurrent use.

[minux]

I think the docs about default Source is pretty clear if you read it in full. ``` Top-level functions, such as Float64 and Int, use a default shared Source that produces a deterministic sequence of values each time a program is run. Use the Seed function to initialize the default Source if different behavior is required for each run. The default Source is safe for concurrent use by multiple goroutines. ``` It explicitly mentions the default Source is the one used by the top-level functions. Following Go documentation convention, anything not explicitly documented for concurrency safety is not safe for concurrent use. The NewSource doesn't say Source is safe for concurrent use, so it's not.

[bradfitz]

True. Yeah, there's not really anything to do here.

Feel free to re-open if you disagree.