Optimize the calculation of length and hashes

After we had given the option to use or not hashes and length
for timestamp and snapshot roles, it's good to make sure we are
calculating them only when they are needed.

This optimization could be important for the bigger tuf adopters.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

tuf/repository_lib.py

@@ -1521,6 +1521,28 @@ def _generate_targets_fileinfo(target_files, targets_directory,
 
 
 
+def _get_hashes_and_length_if_needed(use_length, use_hashes, full_file_path,
+    storage_backend):
+  """
+  Calculate length and hashes only if they are required.
+  """
+
+  length = None
+  hashes = None
+  # We want to make sure we are calculating length and hashes only when
+  # they are needed. Otherwise, for adoptors of tuf with
+  # lots of delegations, this will cause unnecessary overhead.
+  if use_length:
+    length = securesystemslib.util.get_file_length(full_file_path, storage_backend)
+
+  if use_hashes:
+    hashes = securesystemslib.util.get_file_hashes(full_file_path,
+        tuf.settings.FILE_HASH_ALGORITHMS, storage_backend)
+
+  return length, hashes
+
+
+
 def generate_snapshot_metadata(metadata_directory, version, expiration_date,
     storage_backend, consistent_snapshot=False,
     repository_name='default', use_length=False, use_hashes=False):
@@ -1604,12 +1626,8 @@ def generate_snapshot_metadata(metadata_directory, version, expiration_date,
   # Targets, and all delegated roles of the repository.
   fileinfodict = {}
 
-  length, hashes = securesystemslib.util.get_file_details(
-      os.path.join(metadata_directory, TARGETS_FILENAME),
-      tuf.settings.FILE_HASH_ALGORITHMS, storage_backend)
-
-  length = (use_length and length) or None
-  hashes = (use_hashes and hashes) or None
+  length, hashes = _get_hashes_and_length_if_needed(use_length, use_hashes,
+      os.path.join(metadata_directory, TARGETS_FILENAME), storage_backend)
 
   targets_role = TARGETS_FILENAME[:-len(METADATA_EXTENSION)]
 
@@ -1645,18 +1663,8 @@ def generate_snapshot_metadata(metadata_directory, version, expiration_date,
       if tuf.roledb.role_exists(rolename, repository_name) and \
           rolename not in tuf.roledb.TOP_LEVEL_ROLES:
 
-        length = None
-        hashes = None
-        # We want to make sure we are calculating length and hashes only when
-        # at least one of them is needed. Otherwise, for adoptors of tuf with
-        # lots of delegations, this will cause unnecessary overhead.
-        if use_length or use_hashes:
-          length, hashes = securesystemslib.util.get_file_details(
-              os.path.join(metadata_directory, metadata_filename),
-              tuf.settings.FILE_HASH_ALGORITHMS)
-
-          length = (use_length and length) or None
-          hashes = (use_hashes and hashes) or None
+        length, hashes = _get_hashes_and_length_if_needed(use_length, use_hashes,
+            os.path.join(metadata_directory, metadata_filename), storage_backend)
 
         file_version = get_metadata_versioninfo(rolename,
             repository_name)
@@ -1752,11 +1760,8 @@ def generate_timestamp_metadata(snapshot_file_path, version, expiration_date,
 
   snapshot_fileinfo = {}
 
-  length, hashes = securesystemslib.util.get_file_details(snapshot_file_path,
-      tuf.settings.FILE_HASH_ALGORITHMS, storage_backend)
-
-  length = (use_length and length) or None
-  hashes = (use_hashes and hashes) or None
+  length, hashes = _get_hashes_and_length_if_needed(use_length, use_hashes,
+      snapshot_file_path, storage_backend)
 
   snapshot_filename = os.path.basename(snapshot_file_path)
   # Retrieve the versioninfo of the Snapshot metadata file.