sync: add Map.Clear method

[leaxoy]

Since we have landed #56351 for clear builtin map, but we seems forget the sync variant sync.Map, let's do it!

[ianlancetaylor]

CC @bcmills

[gopherbot]

Change https://go.dev/cl/515015 mentions this issue: sync/map: clear

[leaxoy]

@ianlancetaylor Maybe this should a release blocker for 1.21？

[ianlancetaylor]

Unfortunately it's too late for 1.21. And I don't see why it would be a release blocker anyhow. While I agree that the predeclared clear function is a good argument for adding a sync.Map.Clear method, they are two different things.

[leaxoy]

All right, my idea is that since the built-in clear operation is provided, sync version should also provide the same capability, but you said that it is too late for 1.21, it is make sense, after all, we can work without clear for many years.

Anyway, it's useful to land on 1.22 and it's not too late.

[bcmills]

I agree that a Clear method seems reasonable, given the built-in function.

That said, since sync.Map is sensitive to the set of keys that have recently appeared in the map, should Map.Clear retain (empty) entries for those keys so that they will be efficient to store again? Or should it be equivalent to calling Delete sequentially, which only leaves some fraction of the existing keys in the map? Or should it reset all of the keys completely?

This interacts in interesting ways with existing Map performance issues, particularly:

• sync: reduce (*Map).Load penalty for Stores with new keys #21032
• sync: reduce contention between Map operations with new-but-disjoint keys #21035

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

That said, since sync.Map is sensitive to the set of keys that have recently appeared in the map, should Map.Clear retain (empty) entries for those keys so that they will be efficient to store again? Or should it be equivalent to calling Delete sequentially, which only leaves some fraction of the existing keys in the map? Or should it reset all of the keys completely?

I think "delete everything" is fine.

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[qiulaidongfeng]

The time to freeze the code is very close. Should CL515051 be merged into go1.22?

[DenizChildir]

dam, I just saw this in the golang std reference and thought it was an actual function.

[gopherbot]

Change https://go.dev/cl/561755 mentions this issue: doc/next: add release note for sync/Map.Clear