This needs cycle collector macrology. We need to get this in one of our testing harnesses in such a way that the test will fail if we leak, ideally by hooking up mochitests to leak-gauge and failing whenever anything leaks. This bug was *fixed* once, but the cycle collector landing didn't include cycle collection in nsTreeWalker. This is *trivially* catchable with leak-gauge; we should ensure it never happens again.

I filed bug 393993 on getting support in Mochitest to fail on window/document/docshell leaks. In the meantime, hacking in the environment variables to runtests.pl shows that without this patch, we leak a bunch of stuff related to the included test (running only dom/tests/mochitest/bugs), whereas with it, we only leak about:blank (to be dealt with in the nuclear holocaust I intend to wage upon test leaks, so that we can make this test actually fail without the patch).

Comment on attachment 278583 [details] [diff] [review] Patch, with mochitest that would leak without it sr=me, but I think there are macros that implement cycle collecting addref/release/unlink/traverse using a single macro, for simple unlink/traverses like this.

NS_IMPL_CYCLE_COLLECTION_3 in nsCycleCollectionParticipant.h

That macro wasn't working for me when I originally tried it, but a closer look at the error message showed the fairly obvious mistake. This patch, to be committed momentarily, uses the _3 macro.

Checked in on trunk, with a test, but since the test wouldn't fail if this were regressed I'm leaving the flag as-is; I'll flip it when bug 393993 is fixed.