Execute the supplied block exactly once per file, no matter how many threads have asked for it to run. If another thread is already executing it, wait for it to finish. If this thread is already executing it, return immediately without executing the block.

Note: the reason for returning immediately if this thread is already executing the block is to handle the case of a circular import—when this happens, we attempt to recursively re-parse a file that we are already in the process of parsing. To prevent an infinite regress we need to simply do nothing when the recursive import is attempted.