Fix resolve_extends crash when an extends target has dependents

newsfragments/fix-extends-dependents-set-merge.bugfix

@@ -0,0 +1 @@
+Fix `AttributeError: 'set' object has no attribute 'extend'` in `resolve_extends()` when a service used as an `extends:` target was itself depended on by another service.

podman_compose.py

@@ -2343,6 +2343,7 @@ def resolve_extends(
                 del from_service["extends"]
             except KeyError:
                 pass
+            from_service.pop(DependField.DEPENDENTS, None)
         new_service = rec_merge({}, from_service, service)
         services[name] = new_service
 

tests/unit/test_resolve_extends_dependents.py

@@ -0,0 +1,46 @@
+# SPDX-License-Identifier: GPL-2.0
+from __future__ import annotations
+
+import unittest
+from typing import Any
+
+from podman_compose import flat_deps
+from podman_compose import normalize
+from podman_compose import resolve_extends
+
+
+class TestResolveExtendsDependents(unittest.TestCase):
+    """Regression test: resolve_extends used to crash with
+    AttributeError: 'set' object has no attribute 'extend'
+    whenever a service that is itself depended on by another service
+    was also used as the target of an `extends:`.
+
+    flat_deps(services, with_extends=True) populates a "_dependents" set on
+    every service that has a dependent. resolve_extends() already stripped
+    the internal "_deps" set from the extends source before merging it, but
+    not "_dependents" - so when both the extends source and the extending
+    service carried a "_dependents" set, rec_merge_one() tried to
+    list.extend() a set and blew up.
+    """
+
+    def test_extends_target_with_dependents_does_not_crash(self) -> None:
+        compose: dict[str, Any] = {
+            "services": {
+                "base": {"image": "busybox"},
+                "app": {
+                    "extends": {"service": "base"},
+                    "depends_on": ["base"],
+                },
+                "app2": {
+                    "image": "busybox",
+                    "depends_on": ["app"],
+                },
+            }
+        }
+        services = normalize(compose)["services"]
+        flat_deps(services, with_extends=True)
+
+        # should not raise AttributeError: 'set' object has no attribute 'extend'
+        resolve_extends(services, list(services.keys()), {})
+
+        self.assertEqual(services["app"]["image"], "busybox")