podman cp: support copying on tmpfs mounts

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>

1 files changed, 6 insertions, 0 deletions

diff --git a/libpod/container.go b/libpod/container.go
index ee6e243ac..65abbfd5e 100644
--- a/libpod/container.go
+++ b/libpod/container.go
@@ -904,6 +904,12 @@ func (c *Container) NamespacePath(linuxNS LinuxNS) (string, error) { //nolint:in
 		}
 	}
 
+	return c.namespacePath(linuxNS)
+}
+
+// namespacePath returns the path of one of the container's namespaces
+// If the container is not running, an error will be returned
+func (c *Container) namespacePath(linuxNS LinuxNS) (string, error) { //nolint:interfacer
 	if c.state.State != define.ContainerStateRunning && c.state.State != define.ContainerStatePaused {
 		return "", errors.Wrapf(define.ErrCtrStopped, "cannot get namespace path unless container %s is running", c.ID())
 	}