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, 13 insertions, 0 deletions

diff --git a/libpod/container_stat_unsupported.go b/libpod/container_stat_unsupported.go
new file mode 100644
index 000000000..c002e4d32
--- /dev/null
+++ b/libpod/container_stat_unsupported.go
@@ -0,0 +1,13 @@
+// +build !linux
+
+package libpod
+
+import (
+	"context"
+
+	"github.com/containers/podman/v3/libpod/define"
+)
+
+func (c *Container) stat(ctx context.Context, containerMountPoint string, containerPath string) (*define.FileInfo, string, string, error) {
+	return nil, "", "", nil
+}