Is getting a value using range not thread-safe in Go?

No, map operations are not atomic/thread-safe, as the commenter to your question pointed to the golang FAQ “Why are map operations not defined to be atomic?”.

To secure your accessing it, you are encouraged to use Go's channels as a means of resource access token. The channel is used to simply pass around a token. Anyone wanting to modify it will request so from the channel - blocking or non-blocking. When done with working with the map it passes the token back to the channel.

Iterating over and working with the map should be sufficiently simple and short, so you should be ok using just one token for full access.

If that is not the case, and you use the map for more complex stuff/a resource consumer needs more time with it, you may implement a reader- vs writer-access-token. So at any given time, only one writer can access the map, but when no writer is active the token is passed to any number of readers, who will not modify the map (thus they can read simultaneously).

For an introduction to channels, see the Effective Go docs on channels.