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

3 files changed, 27 insertions, 23 deletions

diff --git a/morphlib/builder.py b/morphlib/builder.py
index b0c95bb3..44e26d06 100644
--- a/morphlib/builder.py
+++ b/morphlib/builder.py
@@ -28,7 +28,6 @@ import tempfile
 import cliapp
 
 import morphlib
-from morphlib.artifactcachereference import ArtifactCacheReference
 from morphlib.util import error_message_for_containerised_commandline
 import morphlib.gitversion
 
@@ -558,29 +557,12 @@ class SystemBuilder(BuilderBase):  # pragma: no cover
         self.save_build_times()
         return self.source.artifacts.itervalues()
 
-    def load_stratum(self, stratum_artifact):
-        '''Load a stratum from the local artifact cache.
-
-        Returns a list of ArtifactCacheReference instances for the chunks
-        contained in the stratum.
-
-        '''
-        cache = self.local_artifact_cache
-        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]
-
     def unpack_one_stratum(self, stratum_artifact, target):
         '''Unpack a single stratum into a target directory'''
 
         cache = self.local_artifact_cache
-        for chunk in self.load_stratum(stratum_artifact):
+        chunks = morphlib.util.get_stratum_contents(cache, stratum_artifact)
+        for chunk in chunks:
             self.app.status(msg='Checkout chunk %(basename)s',
                             basename=chunk.basename(), chatty=True)
             cache.get(chunk, target)
@@ -606,7 +588,8 @@ class SystemBuilder(BuilderBase):  # pragma: no cover
 
                 # download the chunk artifacts if necessary
                 for stratum_artifact in self.source.dependencies:
-                    chunks = self.load_stratum(stratum_artifact)
+                    chunks = morphlib.util.get_stratum_contents(
+                        self.local_artifact_cache, stratum_artifact)
                     download_depends(chunks,
                                      self.local_artifact_cache,
                                      self.remote_artifact_cache)
diff --git a/morphlib/plugins/deploy_plugin.py b/morphlib/plugins/deploy_plugin.py
index dbca2d10..45c516b1 100644
--- a/morphlib/plugins/deploy_plugin.py
+++ b/morphlib/plugins/deploy_plugin.py
@@ -554,8 +554,7 @@ class DeployPlugin(cliapp.Plugin):
         a morphlib.remoteartifactcache.GetError is raised.
 
         """
-        with open(lac.get(artifact), 'r') as stratum:
-            chunks = [ArtifactCacheReference(c) for c in json.load(stratum)]
+        chunks = morphlib.util.get_stratum_contents(lac, artifact)
         morphlib.builder.download_depends(chunks, lac, rac)
         for chunk in chunks:
             self.app.status(msg='Checkout chunk %(name)s.',
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]