- Closing and re-opening to upgrade to write will lose all existing read
locks on this process.
- If we didn't allow ranges, sleeping until no reads would work.
- With ranges, we may never be able to take some legal write locks
without invalidating all reads. e.g., if a write lock has distinct
range from all reads, it should just work, but we'd have to close the
file, reopen, and re-take reads.
- It's easier to just check whether the file is writable in the first
place and open for writing from the start.
- Lock now only opens files read-only if we *can't* write them.
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