Samuel Karp f3ba7c8a35 atomicfile: new package for atomic file writes ...

Certain files may need to be written atomically so that partial writes
are not visible to other processes. On Unix-like platforms such as
Linux, FreeBSD, and Darwin, this is accomplished by writing a temporary
file, syncing, and renaming over the destination file name. On Windows,
the same operations are performed, but Windows does not guarantee that a
rename operation is atomic.

Partial/inconsistent reads can occur due to:
1. A process attempting to read the file while containerd is writing it
   (both in the case of a new file with a short/incomplete write or in
   the case of an existing, updated file where new bytes may be written
   at the beginning but old bytes may still be present after).
2. Concurrent goroutines in containerd leading to multiple active
   writers of the same file.

The above mechanism explicitly protects against (1) as all writes are to
a file with a temporary name.

There is no explicit protection against multiple, concurrent goroutines
attempting to write the same file. However, atomically writing the file
should mean only one writer will "win" and a consistent file will be
visible.

Signed-off-by: Samuel Karp <samuelkarp@google.com>

2023-06-02 16:56:33 -07:00

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