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Traditionally, the path resolution for containers has been resolved on
the *host*; relative to the container's mount point or relative to
specified bind mounts or volumes.
While this works nicely for non-running containers, it poses a problem
for running ones. In that case, certain kinds of mounts (e.g., tmpfs)
will not resolve correctly. A tmpfs is held in memory and hence cannot
be resolved relatively to the container's mount point. A copy operation
will succeed but the data will not show up inside the container.
To support these kinds of mounts, we need to join the *running*
container's mount namespace (and PID namespace) when copying.
Note that this change implies moving the copy and stat logic into
`libpod` since we need to keep the container locked to avoid race
conditions. The immediate benefit is that all logic is now inside
`libpod`; the code isn't scattered anymore.
Further note that Docker does not support copying to tmpfs mounts.
Tests have been extended to cover *both* path resolutions for running
and created containers. New tests have been added to exercise the
tmpfs-mount case.
For the record: Some tests could be improved by using `start -a` instead
of a start-exec sequence. Unfortunately, `start -a` is flaky in the CI
which forced me to use the more expensive start-exec option.
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>
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