fixtures: fix closure computation with indirect override chains

Continuation of 72ae3db.

The previous fix was a minimal change to make the existing code at least
consider fixtures overrides, and handles the common case (direct
override chains), but still wasn't correct, as it didn't handle
override chains involving an intermediary (not the overridden) fixture.

To make this work, the algorithm needs to change. Now instead of a
simple breadth-first search, we do a depth-first search, which more
closely simulates the runtime behavior, and allows us the to check the
"stack" of fixtures so we can use the correct fixture index (depth in
the override chain).

This is more expensive but should be OK.

Refs #13773.

Author: bluetech

src/_pytest/fixtures.py

@@ -1634,29 +1634,44 @@ def getfixtureclosure(
         fixturenames_closure = list(initialnames)
 
         arg2fixturedefs: dict[str, Sequence[FixtureDef[Any]]] = {}
-        lastlen = -1
-        while lastlen != len(fixturenames_closure):
-            lastlen = len(fixturenames_closure)
-            for argname in fixturenames_closure:
-                if argname in ignore_args:
-                    continue
-                if argname in arg2fixturedefs:
-                    continue
+
+        # Track the index for each fixture name in the simulated stack.
+        # Needed for handling override chains correctly, similar to _get_active_fixturedef.
+        # Using negative indices: -1 is the most specific (last), -2 is second to last, etc.
+        current_indices: dict[str, int] = {}
+
+        def process_argname(argname: str) -> None:
+            # Optimization: already processed this argname.
+            if current_indices.get(argname) == -1:
+                return
+
+            if argname in ignore_args:
+                return
+
+            if argname not in fixturenames_closure:
+                fixturenames_closure.append(argname)
+
+            fixturedefs = arg2fixturedefs.get(argname)
+            if not fixturedefs:
                 fixturedefs = self.getfixturedefs(argname, parentnode)
-                if fixturedefs:
-                    arg2fixturedefs[argname] = fixturedefs
-
-                    # Add dependencies from this fixture.
-                    # If it overrides a fixture with the same name and requests
-                    # it, also add dependencies from the overridden fixtures in
-                    # the chain. See also similar dealing in _get_active_fixturedef().
-                    for fixturedef in reversed(fixturedefs):  # pragma: no cover
-                        for arg in fixturedef.argnames:
-                            if arg not in fixturenames_closure:
-                                fixturenames_closure.append(arg)
-                        if argname not in fixturedef.argnames:
-                            # Overrides, but doesn't request super.
-                            break
+                if not fixturedefs:
+                    # Fixture not defined or not visible (will error during runtest).
+                    return
+                arg2fixturedefs[argname] = fixturedefs
+
+            index = current_indices.get(argname, -1)
+            if -index > len(fixturedefs):
+                # Exhausted the override chain (will error during runtest).
+                return
+            fixturedef = fixturedefs[index]
+
+            current_indices[argname] = index - 1
+            for dep in fixturedef.argnames:
+                process_argname(dep)
+            current_indices[argname] = index
+
+        for name in initialnames:
+            process_argname(name)
 
         def sort_by_scope(arg_name: str) -> Scope:
             try:

testing/python/fixtures.py

@@ -5110,7 +5110,6 @@ def test_something(self, request, app):
     result.assert_outcomes(passed=1)
 
 
-@pytest.mark.xfail(reason="not currently handled correctly")
 def test_fixture_closure_with_overrides_and_intermediary(pytester: Pytester) -> None:
     """Test that an item's static fixture closure properly includes transitive
     dependencies through overridden fixtures (#13773).
@@ -5255,15 +5254,14 @@ def session(db): pass
         def app(user, session): pass
 
         def test_diamond_deps(request, app):
-            assert request.node.fixturenames == ["request", "app", "user", "session", "db"]
-            assert request.fixturenames == ["request", "app", "user", "session", "db"]
+            assert request.node.fixturenames == ["request", "app", "user", "db", "session"]
+            assert request.fixturenames == ["request", "app", "user", "db", "session"]
         """
     )
     result = pytester.runpytest("-v")
     result.assert_outcomes(passed=1)
 
 
-@pytest.mark.xfail(reason="not currently handled correctly")
 def test_fixture_closure_with_complex_override_and_shared_deps(
     pytester: Pytester,
 ) -> None: