1. Build Gecko
2. Run ./mach xpcshell-test netwerk/test/unit/test_resumable_channel.js and check it succeeds
3. Replace obj-x86_64-pc-mingw32\dist\bin\modules\Promise-backend.js with a symlink to toolkit\modules\Promise-backend.js.
4. Run step 2 again

Expected: It should also succeed
Actual: It does not, the failure log says:

pid:17812 JavaScript error: resource://testing-common/PromiseTestUtils.jsm, line 70: TypeError: can't access property "Debugging", JSMPromise is undefined

Since 1) Promise.jsm loads Promise-backend.js via mozJSSubScriptLoader 2) Promise.jsm itself runs as expected (try adding throw new Error there), I think it's a problem in mozJSSubScriptLoader.

Further debugging says NS_ReadInputStreamToString inside mozJSSubScriptLoader::ReadScript reads a zero-length string.

Sigh. During further digging I found that nsLocalFileWin has a total misunderstanding of Windows symlinks. An .lnk file is a shortcut, not a symlink.

Remember that Gecko has more than 20-years history. When nsIFile was implemented, Windows had no symlink support and Mozilla decided to treat .lnk files as symlinks.

To fix this bug, nsLocalFile::GetFileSize should support (native NTFS) symlinks.

Unrelated to the JS engine. Moving to XPCOM based on comment 4.

https://hg.mozilla.org/mozilla-central/rev/c7b27737901a