sync: clarify if it's valid to call Map methods inside the Map.Range callback

[mvdan]

Map is documented as safe for concurrent use, so I understand it is safe to call a method like LoadAndDelete while Range is running in another goroutine, for example.

However, what isn't clear is whether I can call a method like LoadAndDelete in Range's callback. Technically it's not a concurrent call, as it's happening on the same goroutine - it's just a nested call, as it happens within Range.

Is that supported? From my reading of the code it seems like it shouldn't cause a data race. I wonder if it could lead to undesirable effects however, such as if I delete a key that I haven't ranged over yet, or if I keep adding more keys and potentially make the range run for a long time.

I don't know what the answer is or what it should be, but I think the docs could be a bit clearer.

cc @rsc @ianlancetaylor @dvyukov @aclements as per https://dev.golang.org/owners

[ianlancetaylor]

CC @bcmills

[bcmills]

Yes, nested calls are supported as of CL 37342, which was back when sync.Map was still golang.org/x/syncmap.Map.

It would be fine to update the documentation to be more explicit about the fact that calls during Range (including reentrant calls to Range itself) are allowed.

[bcmills]

(The guarantees of Range are intended to parallel the guarantees for ordinary maps, but with the additional property that concurrent calls from other goroutines are allowed; see also #35239 and #46097.)

[gopherbot]

Change https://golang.org/cl/337389 mentions this issue: sync: clarify the validity to call Map methods inside Range