Avoid merging computed dependency metadata

newsfragments/fix_extends_dependency_merge.bugfix

@@ -0,0 +1 @@
+Fix extending services with dependents failing while merging internal dependency metadata.

podman_compose.py

@@ -2333,12 +2333,13 @@ def resolve_extends(
             normalize_service(from_service, subdirectory)
         else:
             from_service = services.get(from_service_name, {}).copy()
-            try:
-                del from_service["_deps"]
-            except KeyError as e:
+            if DependField.DEPENDENCIES not in from_service:
                 raise KeyError(
                     f"{from_service_name} not found at services.{name}.extends definition"
-                ) from e
+                )
+            # These fields are computed again after all extends references are resolved.
+            for field in DependField:
+                from_service.pop(field, None)
             try:
                 del from_service["extends"]
             except KeyError:

tests/unit/test_rec_merge_depends_on.py

@@ -6,7 +6,9 @@
 
 from parameterized import parameterized
 
+from podman_compose import flat_deps
 from podman_compose import rec_merge
+from podman_compose import resolve_extends
 
 
 class TestRecMergeDependsOn(unittest.TestCase):
@@ -90,3 +92,26 @@ def test_three_way_mixed_depends_on(self) -> None:
                 "environment": {"FOO": "bar"},
             },
         )
+
+    def test_extends_services_with_dependents(self) -> None:
+        services: dict[str, dict[str, Any]] = {
+            "base": {"image": "base"},
+            "child": {"extends": {"service": "base"}, "command": ["run"]},
+            "base_consumer": {
+                "depends_on": {"base": {"condition": "service_started"}},
+            },
+            "child_consumer": {
+                "depends_on": {"child": {"condition": "service_started"}},
+            },
+        }
+
+        flat_deps(services, with_extends=True)
+        service_names = sorted((len(service["_deps"]), name) for name, service in services.items())
+        resolve_extends(services, [name for _, name in service_names], {})
+
+        self.assertEqual(services["child"]["image"], "base")
+        self.assertEqual(services["child"]["command"], ["run"])
+        self.assertEqual(
+            {dependency.name for dependency in services["child"]["_dependents"]},
+            {"child_consumer"},
+        )