Running the gdb test suite with the thread sanitizer enabled shows a race when bfd_check_format_matches and bfd_cache_close_all are called simultaneously on different threads. This patch fixes this race by having bfd_check_format_matches temporarily remove the BFD from the file descriptor cache -- leaving it open while format-checking proceeds. In this setup, the BFD client is responsible for closing the BFD again on the "checking" thread, should that be desired. gdb does this by calling bfd_cache_close in the relevant worker thread. An earlier version of this patch omitted the "possibly_cached" helper function. However, this ran into crashes in the binutils test suite involving the archive-checking abort in bfd_cache_lookup_worker. I do not understand the purpose of this check, so I've simply had the new function work around it. I couldn't find any comments explaining this situation, either. I suspect that there may still be races related to this case, but I don't think I have access to the platforms where gdb deals with archives. Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31264