feat: detect dependencies resolution error

ahriman-core/src/ahriman/core/exceptions.py

@@ -83,6 +83,15 @@ Process stderr:
 {self.stderr}"""
 
 
+class DependencyResolutionError(RuntimeError):
+    """
+    exception raised when package dependencies cannot be resolved
+    """
+
+    def __init__(self, unprocessed: list[str]) -> None:
+        RuntimeError.__init__(self, f"Could not process dependencies. Unresolved packages: {unprocessed}")
+
+
 class DuplicateRunError(RuntimeError):
     """
     exception which will be raised if there is another application instance

ahriman-core/src/ahriman/core/tree.py

@@ -22,7 +22,7 @@ from __future__ import annotations
 from collections.abc import Iterable
 from functools import partial
 
-from ahriman.core.exceptions import PartitionError
+from ahriman.core.exceptions import DependencyResolutionError, PartitionError
 from ahriman.core.utils import minmax, partition
 from ahriman.models.package import Package
 
@@ -226,6 +226,9 @@ class Tree:
 
         Returns:
             list[list[Package]]: sorted list of packages lists based on their dependencies
+
+        Raises:
+            DependencyResolutionError: dependency cycle prevents ordering packages
         """
         unsorted: list[list[Leaf]] = []
 
@@ -235,6 +238,8 @@ class Tree:
             # additional workaround with partial in order to hide cell-var-from-loop pylint warning
             predicate = partial(Leaf.is_root, packages=unprocessed)
             new_level, unprocessed = partition(unprocessed, predicate)
+            if not new_level:
+                raise DependencyResolutionError([leaf.package.base for leaf in unprocessed])
             unsorted.append(new_level)
 
         # move leaves to the end if they are not required at the next level

ahriman-core/tests/ahriman/core/test_tree.py

@@ -1,6 +1,6 @@
 import pytest
 
-from ahriman.core.exceptions import PartitionError
+from ahriman.core.exceptions import DependencyResolutionError, PartitionError
 from ahriman.core.tree import Leaf, Tree
 from ahriman.models.package import Package
 from ahriman.models.package_description import PackageDescription
@@ -270,3 +270,14 @@ def test_tree_partitions() -> None:
     first, second = tree.partitions(count=3)
     assert first == [leaf1.package, leaf2.package, leaf3.package, leaf5.package]
     assert second == [leaf4.package]
+
+
+def test_tree_levels_cycle(leaf_ahriman: Leaf, leaf_python_schedule: Leaf) -> None:
+    """
+    must raise an exception if dependencies contain a cycle
+    """
+    leaf_ahriman.dependencies = set(leaf_python_schedule.items)
+    leaf_python_schedule.dependencies = set(leaf_ahriman.items)
+
+    with pytest.raises(DependencyResolutionError):
+        Tree([leaf_ahriman, leaf_python_schedule]).levels()