cacheprovider: fix some files in packages getting lost from --lf

--lf has an optimization where it skips collecting Modules (python
files) which don't contain failing tests. The optimization works by
getting the paths of all cached failed tests and skipping the collection
of Modules whose path is not included in that list.

In pytest, Package nodes are Module nodes with the fspath being the file
`<package dir>/__init__.py`. Since it's a Module the logic above
triggered for it, and because it's an `__init__.py` file which is
unlikely to have any failing tests in it, it is skipped, which causes
its entire directory to be skipped, including any Modules inside it with
failing tests.

Fix by special-casing Packages to never filter. This means entire
Packages are never filtered, the Modules themselves are always checked.
It is reasonable to consider an optimization which does filter entire
packages bases on parent paths etc. but this wouldn't actually save any
real work so is really not worth it.

changelog/7758.bugfix.rst

@@ -0,0 +1 @@
+Fixed an issue where some files in packages are getting lost from ``--lf`` even though they contain tests that failed. Regressed in pytest 5.4.0.

src/_pytest/cacheprovider.py

@@ -28,6 +28,7 @@
 from _pytest.fixtures import FixtureRequest
 from _pytest.main import Session
 from _pytest.python import Module
+from _pytest.python import Package
 from _pytest.reports import TestReport
 
 
@@ -232,7 +233,10 @@ def __init__(self, lfplugin: "LFPlugin") -> None:
     def pytest_make_collect_report(
         self, collector: nodes.Collector
     ) -> Optional[CollectReport]:
-        if isinstance(collector, Module):
+        # Packages are Modules, but _last_failed_paths only contains
+        # test-bearing paths and doesn't try to include the paths of their
+        # packages, so don't filter them.
+        if isinstance(collector, Module) and not isinstance(collector, Package):
             if Path(str(collector.fspath)) not in self.lfplugin._last_failed_paths:
                 self.lfplugin._skipped_files += 1
 

testing/test_cacheprovider.py

@@ -7,6 +7,7 @@
 
 import pytest
 from _pytest.config import ExitCode
+from _pytest.pytester import Pytester
 from _pytest.pytester import Testdir
 
 pytest_plugins = ("pytester",)
@@ -982,6 +983,36 @@ def test_pass(): pass
         )
         assert result.ret == 0
 
+    def test_packages(self, pytester: Pytester) -> None:
+        """Regression test for #7758.
+
+        The particular issue here was that Package nodes were included in the
+        filtering, being themselves Modules for the __init__.py, even if they
+        had failed Modules in them.
+
+        The tests includes a test in an __init__.py file just to make sure the
+        fix doesn't somehow regress that, it is not critical for the issue.
+        """
+        pytester.makepyfile(
+            **{
+                "__init__.py": "",
+                "a/__init__.py": "def test_a_init(): assert False",
+                "a/test_one.py": "def test_1(): assert False",
+                "b/__init__.py": "",
+                "b/test_two.py": "def test_2(): assert False",
+            },
+        )
+        pytester.makeini(
+            """
+            [pytest]
+            python_files = *.py
+            """
+        )
+        result = pytester.runpytest()
+        result.assert_outcomes(failed=3)
+        result = pytester.runpytest("--lf")
+        result.assert_outcomes(failed=3)
+
 
 class TestNewFirst:
     def test_newfirst_usecase(self, testdir):