Add consistent snapshots to all snapshot Merkle files.

In order to support third party or client auditing of Merkle trees,
auditors need to be able to access previous versions of the
snapshot merkle metadata (at least since the last timestamp
key replacement). This commit allows this by writing snapshot
Merkle files with consistent snapshots so that previous versions
can be accessed.

Signed-off-by: marinamoore <mnm678@gmail.com>

tests/test_repository_lib.py

@@ -438,6 +438,7 @@ def test_generate_targets_metadata(self):
   def test_build_merkle_tree(self):
     temporary_directory = tempfile.mkdtemp(dir=self.temporary_directory)
     storage_backend = securesystemslib.storage.FilesystemBackend()
+    version = 1
 
     # Test building the tree one node at a time to verify the hashes
 
@@ -446,11 +447,14 @@ def test_build_merkle_tree(self):
 
     root_1, leaves = repo_lib._build_merkle_tree(test_nodes)
     repo_lib._write_merkle_paths(root_1, leaves, storage_backend,
-        temporary_directory)
+        temporary_directory, version)
 
     file_path = os.path.join(temporary_directory, 'file1-snapshot.json')
     self.assertTrue(os.path.exists(file_path))
 
+    file_path = os.path.join(temporary_directory, '1.file1-snapshot.json')
+    self.assertTrue(os.path.exists(file_path))
+
     test_nodes['file2'] = tuf.formats.make_metadata_fileinfo(5, None, None)
     root_2, leaves = repo_lib._build_merkle_tree(test_nodes)
 
@@ -468,13 +472,15 @@ def test_build_merkle_tree(self):
     root_4, leaves = repo_lib._build_merkle_tree(test_nodes)
 
     repo_lib._write_merkle_paths(root_4, leaves, storage_backend,
-        temporary_directory)
+        temporary_directory, version + 1)
 
     self.assertEqual(root_4.left.digest, root_3.digest)
 
     # Ensure that the paths are written to the directory
     file_path = os.path.join(temporary_directory, 'file1-snapshot.json')
+    self.assertTrue(os.path.exists(file_path))
 
+    file_path = os.path.join(temporary_directory, '2.file1-snapshot.json')
     self.assertTrue(os.path.exists(file_path))
 
     # repo_lib.print_merkle_tree(root_4)

tuf/repository_lib.py

@@ -146,7 +146,6 @@ def _generate_and_write_metadata(rolename, metadata_filename,
       tuf.roledb.update_roleinfo('timestamp', timestamp_roleinfo,
           repository_name=repository_name)
 
-      _write_merkle_paths(root, leaves, storage_backend, metadata_directory)
 
 
     _log_warning_if_expires_soon(SNAPSHOT_FILENAME, roleinfo['expires'],
@@ -201,6 +200,9 @@ def _generate_and_write_metadata(rolename, metadata_filename,
   else:
     logger.debug('Not incrementing ' + repr(rolename) + '\'s version number.')
 
+  if rolename == 'snapshot' and snapshot_merkle:
+    _write_merkle_paths(root, leaves, storage_backend, metadata_directory, metadata['version'])
+
   if rolename in tuf.roledb.TOP_LEVEL_ROLES and not allow_partially_signed:
     # Verify that the top-level 'rolename' is fully signed.  Only a delegated
     # role should not be written to disk without full verification of its
@@ -1679,7 +1681,7 @@ def _build_merkle_tree(fileinfodict):
   # this path to the client for verification
   return root, leaves
 
-def _write_merkle_paths(root, leaves, storage_backend, merkle_directory):
+def _write_merkle_paths(root, leaves, storage_backend, merkle_directory, version):
   # The root and leaves must be part of the same fully constructed
   # Merkle tree. Create a path from
   # Each leaf to the root node. This path will be downloaded by
@@ -1729,6 +1731,13 @@ def _write_merkle_paths(root, leaves, storage_backend, merkle_directory):
     file_object = tempfile.TemporaryFile()
     file_object.write(file_content)
     filename = os.path.join(merkle_directory, l.name + '-snapshot.json')
+
+    # Also write with consistent snapshots for auditing and client verification
+    consistent_filename = os.path.join(merkle_directory, str(version) + '.'
+        + l.name + '-snapshot.json')
+    securesystemslib.util.persist_temp_file(file_object, consistent_filename,
+        should_close=False)
+
     storage_backend.put(file_object, filename)
     file_object.close()