According to https://crbug.com/331459, some implementations of STL iterators can take locks for debugging; I don't know the details of which implementations are affected under which conditions. This is a problem because they're used in the IPC file descriptor remapping, which needs to be async signal safe because it runs after fork. We're using an old version of the code that was fixed in that bug, so we may need to apply similar changes. (Would we even notice if we had tests deadlocking in process creation like that? Could this be happening on automation and buried in some generic timeout bug, or scattered among a lot of bugs for whatever test case happens to get stuck?)

Fixed (by rewriting) in bug 1456911.