Because the async_status module will read from the same file that
the async_wrapper module is writing, it's possible that the file
may not be fully synced during a read, causing spurious failures.
Use a temp file to do an atomic operation on the file. We can't
use atomic_move() here as that doesn't work properly under async.
Also, let's not read concurrently from the same file the subprocess
is writing to. Instead, capture stdout/stderr via PIPE and write to
the file to avoid nasty races.
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