Consolidate parsing of stratum artifacts in one place

This allows handing parse errors that might occur if the files in the
cache have become corrupted, instead of crashing with a traceback.

Change-Id: Ibaa372ba6a14e7a04de907cb3a664b92cf61fbf3

1 files changed, 22 insertions, 0 deletions

diff --git a/morphlib/util.py b/morphlib/util.py
index 91880988..8ae95b65 100644
--- a/morphlib/util.py
+++ b/morphlib/util.py
@@ -14,6 +14,7 @@
 
 import contextlib
 import itertools
+import json
 import os
 import pipes
 import re
@@ -21,9 +22,11 @@ import subprocess
 import textwrap
 import sys
 
+import cliapp
 import fs.osfs
 
 import morphlib
+from morphlib.artifactcachereference import ArtifactCacheReference
 
 '''Utility functions for morph.'''
 
@@ -692,3 +695,22 @@ def write_from_dict(filepath, d, validate=lambda x, y: True): #pragma: no cover
 
         os.fchown(f.fileno(), 0, 0)
         os.fchmod(f.fileno(), 0644)
+
+
+def get_stratum_contents(cache, stratum_artifact):
+    '''Load a stratum from a local artifact cache.
+
+    Returns a list of ArtifactCacheReference instances for the chunks
+    contained in the stratum.
+
+    '''
+
+    with open(cache.get(stratum_artifact), 'r') as stratum_file:
+        try:
+            artifact_list = json.load(stratum_file,
+                                        encoding='unicode-escape')
+        except ValueError as e:
+            raise cliapp.AppException(
+                'Corruption detected: %s while loading %s' %
+                (e, cache.artifact_filename(stratum_artifact)))
+    return [ArtifactCacheReference(a) for a in artifact_list]