Improve delegated roles loading in load_repository()

Replace the list used for the delegations graph traversal with
a deque and use a set to store already loaded roles and avoid
loops in case of cycles in the graph.
Improve comments and readability.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

tuf/repository_tool.py

@@ -39,6 +39,8 @@
 import shutil
 import json
 
+from collections import deque
+
 import tuf
 import tuf.formats
 import tuf.roledb
@@ -3100,29 +3102,50 @@ def load_repository(repository_directory, repository_name='default',
   repository, consistent_snapshot = repo_lib._load_top_level_metadata(repository,
     filenames, repository_name)
 
-  # Load the delegated targets metadata and generate their fileinfo.  The
-  # extracted fileinfo is stored in the 'meta' field of the snapshot metadata
-  # object.
+  delegated_roles_filenames = repo_lib.get_delegations_filenames(
+      metadata_directory, consistent_snapshot, storage_backend)
+
+  # Load the delegated targets metadata and their fileinfo.
+  # The delegated targets roles form a tree/graph which is traversed in a
+  # breadth-first-search manner starting from 'targets' in order to correctly
+  # load the delegations hierarchy.
   targets_objects = {}
   targets_objects['targets'] = repository.targets
-  # A list of delegated-delegating role pairs
-  delegations = []
 
-  delegations_filenames = repo_lib.get_delegations_filenames(metadata_directory,
-        consistent_snapshot, storage_backend)
+  # Keep the next delegations to be loaded in a deque structure which
+  # has the properties of a list but is designed to have fast appends
+  # and pops from both ends
+  delegations = deque()
+  # A set used to keep the already loaded delegations and avoid an infinite
+  # loop in case of cycles in the delegations graph
+  loaded_delegations = set()
 
   # Top-level roles are already loaded, fetch targets and get its delegations.
-  # Collect a list of delegated-delegating role pairs, starting from the
-  # top-level targets: [('role1', 'targets'), ('role2', 'targets'), ... ]
+  # Store the delegations in the form of delegated-delegating role tuples,
+  # starting from the top-level targets:
+  # [('role1', 'targets'), ('role2', 'targets'), ... ]
   roleinfo = tuf.roledb.get_roleinfo('targets', repository_name)
   for role in roleinfo['delegations']['roles']:
-    delegations.append([role['name'], 'targets'])
+    delegations.append((role['name'], 'targets'))
 
-  # Load the delegated roles by starting from 'targets' and continuously
-  # appending the next level delegations to the list
-  for rolename, delegating_role in delegations:
-    metadata_path = delegations_filenames[rolename]
+  # Traverse the graph by appending the next delegation to the deque and
+  # 'pop'-ing and loading the left-most element.
+  while delegations:
+    rolename, delegating_role = delegations.popleft()
+    if (rolename, delegating_role) in loaded_delegations:
+      logger.warning('Detected cycle in the delegation graph: ' +
+          repr(delegating_role) + ' -> ' +
+          repr(rolename) +
+          ' is reached more than once.')
+      continue
 
+    # Instead of adding only rolename to the set, store the already loaded
+    # delegated-delegating role tuples. This way a delegated role is added
+    # to each of its delegating roles but when the role is reached twice
+    # from the same delegating role an infinite loop is avoided.
+    loaded_delegations.add((rolename, delegating_role))
+
+    metadata_path = delegated_roles_filenames[rolename]
     signable = None
 
     try:
@@ -3149,8 +3172,6 @@ def load_repository(repository_directory, repository_name='default',
     roleinfo['paths'] = metadata_object['targets']
     roleinfo['delegations'] = metadata_object['delegations']
 
-    tuf.roledb.add_role(rolename, roleinfo, repository_name)
-
     # Generate the Targets object of the delegated role,
     # add it to the top-level 'targets' object and to its
     # direct delegating role object.
@@ -3165,10 +3186,10 @@ def load_repository(repository_directory, repository_name='default',
     targets_objects[delegating_role].add_delegated_role(rolename,
         new_targets_object)
 
-    # Append the next level delegations to the list:
+    # Append the next level delegations to the deque:
     # the 'delegated' role becomes the 'delegating'
     for delegation in metadata_object['delegations']['roles']:
-      delegations.append([delegation['name'], rolename])
+      delegations.append((delegation['name'], rolename))
 
     # Extract the keys specified in the delegations field of the Targets
     # role.  Add 'key_object' to the list of recognized keys.  Keys may be