If you interrupt JarMaker when it has created a lockfile, the lockfile will stick around and hang your next attempt to build. You can reproduce this pretty easily by hitting Ctrl+C while a jar is being built.
Wrapping the while True: loop at http://mxr.mozilla.org/mozilla-central/source/config/JarMaker.py#271 with a try-finally and closing the jf if not-None should fix this. Taking.
Created attachment 339773 [details] [diff] [review] add try-finally around the loop, adjust comment, patch -w This is a diff -w for the change, I'll attach a regular patch for reference in a second. Requesting review on this one.
Pushed, http://hg.mozilla.org/mozilla-central/rev/e9794a8b579d. Marking FIXED.