600702 - Harmony proxies: for-in loop on proxy does not suppress properties deleted during enumeration

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect, P2)

Description

[Tom Van Cutsem]

In tracemonkey, for-in loops normally suppress properties deleted during enumeration:

var obj = {a:0,b:1,c:2};
for (var p in obj) { delete obj['c']; print(p) };

prints: a,b

For proxies, this is not the case:

var target = {a:0,b:1,c:2};
var proxy = Proxy.create({
  has: function(name) {
    return name in target;
  },
  delete: function(name) {
    return delete target[name];
  },
  enumerate: function() {
    return ['a','b','c'];
  }
});

for (var prop in proxy) {
  delete proxy['c']; // delete a yet-to-be-enumerated property
  print(prop);
}

This prints a,b,c instead of the expected a,b.
It turns out enumeration on proxies does not invoke the proxy's has() trap to determine whether the property is still present on the proxy when it is enumerated.

While the semantics of proxy enumeration is still largely left unspecified in the spec, I think there was agreement that existing properties deleted during an enumeration should no longer be enumerated.

Comment 1

[Andreas Gal :gal]

Attachment: patch

Comment 2

[Brendan Eich [:brendan]]

Comment on attachment 479596 [details] [diff] [review]
patch

>diff --git a/js/src/jsproxy.cpp b/js/src/jsproxy.cpp
>--- a/js/src/jsproxy.cpp
>+++ b/js/src/jsproxy.cpp
>@@ -890,17 +890,18 @@ proxy_SetAttributes(JSContext *cx, JSObj
>     return JSProxy::defineProperty(cx, obj, id, &desc);
> }
> 
> static JSBool
> proxy_DeleteProperty(JSContext *cx, JSObject *obj, jsid id, Value *rval, JSBool strict)
> {
>     // TODO: throwing away strict
>     bool deleted;
>-    if (!JSProxy::delete_(cx, obj, id, &deleted))
>+    if (!JSProxy::delete_(cx, obj, id, &deleted) ||
>+        !js_SuppressDeletedProperty(cx, obj, id))
>         return false;

Braces for multiline any-part of if and if/else, but:

>+    if (!JSProxy::delete_(cx, obj, id, &deleted) || !js_SuppressDeletedProperty(cx, obj, id))
..1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890

this fits well within the 100-column style guide limit, so you can pull the condition up to fit in one line and avoid bracing.

r=me with that, another steal to relieve jorendorff.

/be

Comment 3

[Josh Matthews [:jdm]]

Was this patch forgotten? Does it still need pushing?

Comment 4

[Andreas Gal :gal]

Looks like it. Want to grab it jdm or should I land it?

Comment 5

[Josh Matthews [:jdm]]

Please feel free. I'm supposed to be packing to go travelling for six weeks.

Comment 6

[Andreas Gal :gal]

http://hg.mozilla.org/tracemonkey/rev/556ec2a89192

Comment 7

[Chris Leary [:cdleary] (not checking bugmail)]

http://hg.mozilla.org/mozilla-central/rev/556ec2a89192