Add trace-hooks to the FsCache._atomic_open() helper, including a
primitive trace-infrastructure. They allow interrupting cache operation
and running arbitrary code.
The trace-hooks will be used by the test-suite to trigger the races we
want to protect against. During runtime, the traces should not be used
and thus will always be `None`.
This is a very primitive way to hook into the runtime execution and test
the atomicity of the operations. However, it is simple enough for our
tests and avoids pulling in huge tracing suites.
Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>
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