Revert "Implement _preorder_depth_first_walk recursively"

This reverts commit ebf7a3d.

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

Author: sechkova

tuf/ngclient/updater.py

@@ -171,10 +171,7 @@ def get_one_valid_targetinfo(
             RepositoryError: Metadata failed to verify in some way
             TODO: download-related errors
         """
-        targetinfo, dummy = self._preorder_depth_first_walk(
-            target_path, set(), ("targets", "root"), self.config.max_delegations
-        )
-        return targetinfo
+        return self._preorder_depth_first_walk(target_path)
 
     @staticmethod
     def updated_targets(
@@ -373,63 +370,77 @@ def _load_targets(self, role: str, parent_role: str) -> None:
             self._persist_metadata(role, data)
 
     def _preorder_depth_first_walk(
-        self,
-        target_filepath: str,
-        visited_role_names: Set[Tuple[str, str]],
-        current_role_pair: Tuple[str, str],
-        number_of_delegations: int,
-    ) -> Tuple[Union[Dict[str, Any], None], bool]:
+        self, target_filepath: str
+    ) -> Union[Dict[str, Any], None]:
         """
         Interrogates the tree of target delegations in order of appearance
         (which implicitly order trustworthiness), and returns the matching
         target found in the most trusted role.
         """
-        targetinfo = None
-        terminated = False
+
+        role_names = [("targets", "root")]
+        visited_role_names: Set[Tuple[str, str]] = set()
+        number_of_delegations = self.config.max_delegations
+
         # Preorder depth-first traversal of the graph of target delegations.
-        if number_of_delegations <= 0:
-            return targetinfo, terminated
-
-        # Pop the role name from the top of the stack.
-        role_name, parent_role = current_role_pair
-
-        # The metadata for 'role_name' must be downloaded/updated before
-        # its targets, delegations, and child roles can be inspected.
-        self._load_targets(role_name, parent_role)
-        role_metadata: Targets = self._trusted_set[role_name].signed
-        target = role_metadata.targets.get(target_filepath)
-
-        if target is not None:
-            logger.debug("Found target in current role %s", role_name)
-            targetinfo = {"filepath": target_filepath, "fileinfo": target}
-            return targetinfo, terminated
-
-        # After preorder check, add current role to set of visited roles.
-        visited_role_names.add((role_name, parent_role))
-
-        # And also decrement number of visited roles.
-        number_of_delegations -= 1
-        if role_metadata.delegations is not None:
-            for child_role in role_metadata.delegations.roles:
-                # Skip any visited current role to prevent cycles.
-                if (child_role.name, role_name) in visited_role_names:
-                    continue
-
-                if child_role.is_in_trusted_paths(target_filepath):
-
-                    targetinfo, terminated = self._preorder_depth_first_walk(
-                        target_filepath,
-                        visited_role_names,
-                        (child_role.name, role_name),
-                        number_of_delegations,
-                    )
-
-                    if child_role.terminating or terminated:
-                        terminated = True
-                        logger.debug("Not backtracking to other roles.")
-                        break
-
-        return targetinfo, terminated
+        while number_of_delegations > 0 and len(role_names) > 0:
+
+            # Pop the role name from the top of the stack.
+            role_name, parent_role = role_names.pop(-1)
+
+            # Skip any visited current role to prevent cycles.
+            if (role_name, parent_role) in visited_role_names:
+                logger.debug("Skipping visited current role %s", role_name)
+                continue
+
+            # The metadata for 'role_name' must be downloaded/updated before
+            # its targets, delegations, and child roles can be inspected.
+            self._load_targets(role_name, parent_role)
+
+            role_metadata: Targets = self._trusted_set[role_name].signed
+            target = role_metadata.targets.get(target_filepath)
+
+            if target is not None:
+                logger.debug("Found target in current role %s", role_name)
+                return {"filepath": target_filepath, "fileinfo": target}
+
+            # After preorder check, add current role to set of visited roles.
+            visited_role_names.add((role_name, parent_role))
+
+            # And also decrement number of visited roles.
+            number_of_delegations -= 1
+
+            if role_metadata.delegations is not None:
+                child_roles_to_visit = []
+                # NOTE: This may be a slow operation if there are many
+                # delegated roles.
+                for child_role in role_metadata.delegations.roles:
+                    if child_role.is_in_trusted_paths(target_filepath):
+                        logger.debug("Adding child role %s", child_role.name)
+
+                        child_roles_to_visit.append(
+                            (child_role.name, role_name)
+                        )
+                        if child_role.terminating:
+                            logger.debug("Not backtracking to other roles.")
+                            role_names = []
+                            break
+                # Push 'child_roles_to_visit' in reverse order of appearance
+                # onto 'role_names'.  Roles are popped from the end of
+                # the 'role_names' list.
+                child_roles_to_visit.reverse()
+                role_names.extend(child_roles_to_visit)
+
+        if number_of_delegations == 0 and len(role_names) > 0:
+            logger.debug(
+                "%d roles left to visit, but allowed to "
+                "visit at most %d delegations.",
+                len(role_names),
+                self.config.max_delegations,
+            )
+
+        # If this point is reached then target is not found, return None
+        return None
 
 
 def _ensure_trailing_slash(url: str):