The image cache is keyed on the original URI of each image - that is, the URI before redirects. To make this work, we rely in places on nsIChannel::GetOriginalURI() actually returning that original URI - the one passed to LoadImage(). I want to add an assertion that this is always the case. However, we seem to have a lot of bugs in this area! I've already discovered and fixed bug 1163866 while trying to add this assertion in bug 1160703, but there seem to be more issues. I think it's better to land the assertion in this bug, which can depend on the fixes for any further Necko-level issues that get discovered.
To my understanding, the problem here is that for an image loaded via a .sjs that redirects to a data URI, GetOriginalURI returns the data URI instead of the .sjs. This problem only seems to appear on the test infrastructure, and not when the test is run locally. https://treeherder.mozilla.org/#/jobs?repo=try&revision=0e95c06b5045 Also, from the try run, I'm inclined to believe this is only for e10s.
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