Fix retrieving image digest

The config object is the incorrect method of retrieving an image
digest as the config and layers itself creates a manifest which
is hashed to get the image digest. Instead, we will use skopeo
to retireve it. For this, we introduce a function in
tern/load/skopeo to do the remote inspection, and parse out
the image digest type and digest.

Fixes #1101

Signed-off-by: Nisha K <[email protected]>

Author: Nisha K

tern/classes/oci_image.py

@@ -11,6 +11,7 @@
 from tern.classes.image import Image
 from tern.utils.constants import manifest_file
 from tern.classes.image_layer import ImageLayer
+from tern.load import skopeo
 
 
 class OCIImage(Image):
@@ -100,12 +101,11 @@ def load_image(self, load_until_layer=0):
             self.__history = self.get_image_history(self._config)
             layer_paths = self.get_image_layers(self._manifest)
             layer_diffs = self.get_diff_ids(self._config)
-            # if the digest isn't in the repotag, get it from the config
+            # if the digest isn't in the repotag, get it using skopeo
             if not self.checksum:
-                repo_dict = general.parse_image_string(
-                    self._config.get("config").get("Image"))
-                self.set_checksum(repo_dict.get("digest_type"),
-                                  repo_dict.get("digest"))
+                digest_type, digest = skopeo.get_image_digest(self.repotag)
+                if digest_type and digest:
+                    self.set_checksum(digest_type, digest)
             layer_checksum_type = self.get_diff_checksum_type(self._config)
             layer_count = 1
             while layer_diffs and layer_paths:

tern/load/skopeo.py

@@ -7,6 +7,7 @@
 Interactions with remote container images using skopeo
 """
 
+import json
 import logging
 import sys
 import shutil
@@ -40,3 +41,23 @@ def pull_image(image_tag_string):
         logger.error("Error when downloading image: \"%s\"", error)
         return None
     return result
+
+
+def get_image_digest(image_tag_string):
+    """Use skopeo to get the remote image's digest"""
+    # check if skopeo is set up
+    check_skopeo_setup()
+    remote = f'docker://{image_tag_string}'
+    logger.debug("Inspecting remote image \"%s\"", image_tag_string)
+    result, error = rootfs.shell_command(
+        False, ['skopeo', 'inspect', remote])
+    if error or not result:
+        logger.error("Unable to retrieve image digest")
+        return None, None
+    result_string = json.loads(result)
+    digest_string = result_string.get("Digest")
+    if not digest_string:
+        logger.error("No image digest available")
+        return None, None
+    digest_list = digest_string.split(":")
+    return digest_list[0], digest_list[1]