814892 - Harmony direct proxies: "get" trap throws unexpectedly

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Tom Van Cutsem]

Executing the following script in Nightly 20.0a1 (2012-11-24) throws unexpectedly:

function test() {
  var t = Object.freeze({x:1});
  var p = new Proxy(t,{
    get: function(target, name, rcvr) {
      return target[name];
    }
  });
  console.log(p.x);
}

observed behavior:
TypeError: proxy must report the same value for a non-writable, non-configurable property

Expected: p.x should just return 1

This has to do with the invariant checks in direct proxies. The proxy should only throw this error if the "get" trap of the proxy returns a result that is inconsistent with the value of the target, but in this case the result should be perfectly consistent.

The bug only occurs when the target of the proxy is frozen.

Note: it would be helpful if the name of the offending property were included in the error string of the TypeError.

Comment 1

[Asaf Romano (gone)]

The target object doesn't have to be frozen:
function test() {
    let o = Object.create(Object.prototype, { p: { value: 5 } });
    new Proxy(o, { get: function(t, n) t[n] }).p
}
test()
/*
Exception: proxy must report the same value for a non-writable, non-configurable property
test@Scratchpad/3:3
@Scratchpad/3:5
*/

Comment 2

[Till Schneidereit [:till]]

Attachment: Actually compare the right values when making sure a proxy [[Get]] trap returns the correct value

Comment 3

[Eric Faust [:efaust]]

Comment on attachment 8349060 [details] [diff] [review]
Actually compare the right values when making sure a proxy [[Get]] trap returns the correct value

Thanks for finding this. r=me.

Comment 4

[Till Schneidereit [:till]]

And after a short 13 months, we've actually applied the one-token change needed to fix this. So sorry about the delay! And thanks, :Fallen, for the ping.

remote:   https://hg.mozilla.org/integration/mozilla-inbound/rev/5bcd468384a8

Comment 5

[Wes Kocher (:KWierso) (Not reading bugmail; email directly if needed)]

https://hg.mozilla.org/mozilla-central/rev/5bcd468384a8