wait for in-flight ops before cloning File

[djmitche]

If there is an ongoing operation on a file, wait for that to complete before performing the clone in File::try_clone. This avoids a race between the ongoing operation and any subsequent operations performed on the clone.

Fixes: #5759

Motivation

#5759 shows a situation where a file is written to with write_all, and once that completes (Poll::Ready), cloned with try_clone. The file contents are then read back from the clone. In unlucky situations, this read produces an empty result because the read occurs before the write has completed.

Solution

Like most methods on File, the try_clone function should wait until any previous asynchronously-completing operations on the file are complete before actually cloning the file.

No Tests?

The failure fixed by this PR appears very difficult to test:

• It is a race condition, and one which almost always succeeds (that is, the write almost always completes before the read).
• It's not well-defined what an operating system will do if a file is duplicated while a write(2) syscall is in progress. Reproducing the race requires that the read happen before the invocation of write(2). In practice, this bug reproduces best on a very busy system, such as in CI, where the worker thread that calls wait(2) is not scheduled immediately.
• Tests run with a mocked spawn_blocking which does not exhibit this bug.
• If I was able to produce a test case where the closure passed to spawn_blocking was blocked from executing, then when the fix was applied the test would block indefinitely.

I'm open to suggestions as to how to test this, but perhaps the fix is simple enough to accept without them?