reflect: add MapIter.Reset

[josharian]

Currently, iterating over a map requires two allocations: A reflect.MapIter and a runtime.hiter. By embedding a runtime.hiter (or rather, a type with the same shape and size) in a reflect.MapIter, it is possible to eliminate the second allocation. If we could re-use a reflect.MapIter to iterate over multiple maps, then we could eliminate the first allocation as well.

I propose that we add:

// Reset changes iter to iterate over v.
// It panics if v's Kind is not Map and v is not the zero Value.
func (iter *MapIter) Reset(v Value)

Allowing v to be the zero Value enables the caller to avoid pinning whatever map iter was previously iterating over.

Note that these two would be equivalent:

iter := new(reflect.MapIter)
iter.Reset(mapVal)

iter := mapVal.MapRange()

cc @bradfitz @randall77

[dsnet]

Currently, allocating over a map requires two allocations:

Did you mean to say "iterating" over a map instead?

[josharian]

@dsnet I did. Thanks, fixed.

[gopherbot]

Change https://golang.org/cl/321891 mentions this issue: reflect: add MapIter.Reset

[gopherbot]

Change https://golang.org/cl/321890 mentions this issue: reflect: improve panic when MapIter has no associated map Value

[gopherbot]

Change https://golang.org/cl/321889 mentions this issue: reflect: allocate hiter as part of MapIter