libpod: add (*Container).ResolvePath()

Add an API to libpod to resolve a path on the container.  We can
refactor the code that was originally written for copy.  Other
functions are requiring a proper path resolution, so libpod seems
like a reasonable home for sharing that code.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

libpod/container_path_resolution.go

@@ -0,0 +1,127 @@
+package libpod
+
+import (
+	"path/filepath"
+	"strings"
+
+	securejoin "github.com/cyphar/filepath-securejoin"
+	"github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
+)
+
+// resolveContainerPaths resolves the container's mount point and the container
+// path as specified by the user.  Both may resolve to paths outside of the
+// container's mount point when the container path hits a volume or bind mount.
+func (container *Container) resolvePath(mountPoint string, containerPath string) (string, string, error) {
+	// Let's first make sure we have a path relative to the mount point.
+	pathRelativeToContainerMountPoint := containerPath
+	if !filepath.IsAbs(containerPath) {
+		// If the containerPath is not absolute, it's relative to the
+		// container's working dir.  To be extra careful, let's first
+		// join the working dir with "/", and the add the containerPath
+		// to it.
+		pathRelativeToContainerMountPoint = filepath.Join(filepath.Join("/", container.WorkingDir()), containerPath)
+	}
+	resolvedPathOnTheContainerMountPoint := filepath.Join(mountPoint, pathRelativeToContainerMountPoint)
+	pathRelativeToContainerMountPoint = strings.TrimPrefix(pathRelativeToContainerMountPoint, mountPoint)
+	pathRelativeToContainerMountPoint = filepath.Join("/", pathRelativeToContainerMountPoint)
+
+	// Now we have an "absolute container Path" but not yet resolved on the
+	// host (e.g., "/foo/bar/file.txt").  As mentioned above, we need to
+	// check if "/foo/bar/file.txt" is on a volume or bind mount.  To do
+	// that, we need to walk *down* the paths to the root.  Assuming
+	// volume-1 is mounted to "/foo" and volume-2 is mounted to "/foo/bar",
+	// we must select "/foo/bar".  Once selected, we need to rebase the
+	// remainder (i.e, "/file.txt") on the volume's mount point on the
+	// host.  Same applies to bind mounts.
+
+	searchPath := pathRelativeToContainerMountPoint
+	for {
+		volume, err := findVolume(container, searchPath)
+		if err != nil {
+			return "", "", err
+		}
+		if volume != nil {
+			logrus.Debugf("Container path %q resolved to volume %q on path %q", containerPath, volume.Name(), searchPath)
+
+			// TODO: We really need to force the volume to mount
+			// before doing this, but that API is not exposed
+			// externally right now and doing so is beyond the scope
+			// of this commit.
+			mountPoint, err := volume.MountPoint()
+			if err != nil {
+				return "", "", err
+			}
+			if mountPoint == "" {
+				return "", "", errors.Errorf("volume %s is not mounted, cannot copy into it", volume.Name())
+			}
+
+			// We found a matching volume for searchPath.  We now
+			// need to first find the relative path of our input
+			// path to the searchPath, and then join it with the
+			// volume's mount point.
+			pathRelativeToVolume := strings.TrimPrefix(pathRelativeToContainerMountPoint, searchPath)
+			absolutePathOnTheVolumeMount, err := securejoin.SecureJoin(mountPoint, pathRelativeToVolume)
+			if err != nil {
+				return "", "", err
+			}
+			return mountPoint, absolutePathOnTheVolumeMount, nil
+		}
+
+		if mount := findBindMount(container, searchPath); mount != nil {
+			logrus.Debugf("Container path %q resolved to bind mount %q:%q on path %q", containerPath, mount.Source, mount.Destination, searchPath)
+			// We found a matching bind mount for searchPath.  We
+			// now need to first find the relative path of our
+			// input path to the searchPath, and then join it with
+			// the source of the bind mount.
+			pathRelativeToBindMount := strings.TrimPrefix(pathRelativeToContainerMountPoint, searchPath)
+			absolutePathOnTheBindMount, err := securejoin.SecureJoin(mount.Source, pathRelativeToBindMount)
+			if err != nil {
+				return "", "", err
+			}
+			return mount.Source, absolutePathOnTheBindMount, nil
+
+		}
+
+		if searchPath == "/" {
+			// Cannot go beyond "/", so we're done.
+			break
+		}
+
+		// Walk *down* the path (e.g., "/foo/bar/x" -> "/foo/bar").
+		searchPath = filepath.Dir(searchPath)
+	}
+
+	// No volume, no bind mount but just a normal path on the container.
+	return mountPoint, resolvedPathOnTheContainerMountPoint, nil
+}
+
+// findVolume checks if the specified container path matches a volume inside
+// the container.  It returns a matching volume or nil.
+func findVolume(c *Container, containerPath string) (*Volume, error) {
+	runtime := c.Runtime()
+	cleanedContainerPath := filepath.Clean(containerPath)
+	for _, vol := range c.Config().NamedVolumes {
+		if cleanedContainerPath == filepath.Clean(vol.Dest) {
+			return runtime.GetVolume(vol.Name)
+		}
+	}
+	return nil, nil
+}
+
+// findBindMount checks if the specified container path matches a bind mount
+// inside the container.  It returns a matching mount or nil.
+func findBindMount(c *Container, containerPath string) *specs.Mount {
+	cleanedPath := filepath.Clean(containerPath)
+	for _, m := range c.Config().Spec.Mounts {
+		if m.Type != "bind" {
+			continue
+		}
+		if cleanedPath == filepath.Clean(m.Destination) {
+			mount := m
+			return &mount
+		}
+	}
+	return nil
+}