net/rpc handling of maps

[Jeremy Thorpe]

Hi golang,

I have a question about how net/rpc (and perhaps encoding/gob) handles maps.

I have an RPC server with code like this:

func (r *Receiver) Foo(_ struct{}, m *map[int] int) error {
*m = make(map[int] int)
(*m)[1] = 1
return nil
}

and I call it like this:

m := make(map[int] int)
m[0] = 0
err = h.Call("Receiver.Foo", struct{}{}, &m)
fmt.Println(m)

Should I expect to see 0:0 in my map after the call, or is the "Call"
supposed to create a new map? The documentation of encoder/gob seems
to support, if anything, the second theory: "...if the sender has
allocated a map, the receiver will allocate a map even no elements are
transmitted." However, what's actually printed is [0:0, 1:1], which
implies that the map is being reused (and not cleared).

Is this expected?

Thanks,
-Jeremy

[Kyle Lemons]

Looks correct.  If you have a map field in a struct or pass a reference to a nil map, I think it's implying that the map may be allocated and have zero entries (so don't compare to nil, check its length to see if its empty).