I think the obvious behaviour would be to give the caller a copy of the Addr. After looking at the code I think the only way to do this is to give Addr the ability to copy itself. However, we can't change the Addr interface, so I think introducing an interface like

type AddrCopier interface{
  func Copy() Addr
}

might work around the problem. Make all the net.(.*)Addr types implement this function and let LocalAddr() (and probably RemoteAddr() as well) check if the given Addr also implements AddrCopier. If so, return a copy. If not, return the original value, because that's all we got.

I know this isn't a real fix, but I think it's the only solution.

I don't think this could be done: it's still a breaking change.

For example, user might use the result from LocalAddr() to index a map
that depended on the fact each invocation of the method on the same
connections returns pointer to the same Addr struct.

And I don't think this issue is a problem at all. Go doesn't have immutable
types, so if you modify types returned from any method, you'd better
know what you're doing.

@keks,

Please open a new issue if you have something for the immutability of concrete types that implement net.Addr interface. This issue should focus on the behavior of Addr, LocalAddr and RemteAddr.

Go doesn't have immutable types

True, except string types.

so if you modify types returned from any method, you'd better know what you're doing.

Like other APIs such as various DNS record lookups, listing nework interfaces and addresses, re-using the returned values from the net package is fine. I think this issue has been overlooked for a long time.

In Go 2, we probably should make all existing net.Addr implementations implement
the net.Addr methods on non-pointer receiver, so that Addr, LocalAddr, RemoteAddr
could return a copy of the Addr.

This is in line with other packages that returns internal state that shouldn't be modified.

Alright, I think this is reasonable. I don't see an elegant way of doing this without non-pointer receivers.