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, 16 insertions, 0 deletions

diff --git a/libpod/container_copy_unsupported.go b/libpod/container_copy_unsupported.go
new file mode 100644
index 000000000..b2bdd3e3d
--- /dev/null
+++ b/libpod/container_copy_unsupported.go
@@ -0,0 +1,16 @@
+// +build !linux
+
+package libpod
+
+import (
+	"context"
+	"io"
+)
+
+func (c *Container) copyFromArchive(ctx context.Context, path string, reader io.Reader) (func() error, error) {
+	return nil, nil
+}
+
+func (c *Container) copyToArchive(ctx context.Context, path string, writer io.Writer) (func() error, error) {
+	return nil, nil
+}