collect: python: fix `AssertionError` with broken symlinks

Fixes https://github.com/pytest-dev/pytest/issues/4782.

changelog/4782.bugfix.rst

@@ -0,0 +1 @@
+Fix ``AssertionError`` with collection of broken symlinks with packages.

src/_pytest/main.py

@@ -597,7 +597,12 @@ class Session(nodes.FSCollector):
                 yield y
 
     def _collectfile(self, path, handle_dupes=True):
-        assert path.isfile()
+        assert path.isfile(), "%r is not a file (isdir=%r, exists=%r, islink=%r)" % (
+            path,
+            path.isdir(),
+            path.exists(),
+            path.islink(),
+        )
         ihook = self.gethookproxy(path)
         if not self.isinitpath(path):
             if ihook.pytest_ignore_collect(path=path, config=self.config):

src/_pytest/python.py

@@ -599,7 +599,12 @@ class Package(Module):
         return proxy
 
     def _collectfile(self, path, handle_dupes=True):
-        assert path.isfile()
+        assert path.isfile(), "%r is not a file (isdir=%r, exists=%r, islink=%r)" % (
+            path,
+            path.isdir(),
+            path.exists(),
+            path.islink(),
+        )
         ihook = self.gethookproxy(path)
         if not self.isinitpath(path):
             if ihook.pytest_ignore_collect(path=path, config=self.config):
@@ -632,7 +637,8 @@ class Package(Module):
         pkg_prefixes = set()
         for path in this_path.visit(rec=self._recurse, bf=True, sort=True):
             # We will visit our own __init__.py file, in which case we skip it.
-            if path.isfile():
+            is_file = path.isfile()
+            if is_file:
                 if path.basename == "__init__.py" and path.dirpath() == this_path:
                     continue
 
@@ -643,12 +649,14 @@ class Package(Module):
             ):
                 continue
 
-            if path.isdir():
+            if is_file:
-                if path.join("__init__.py").check(file=1):
-                    pkg_prefixes.add(path)
-            else:
                 for x in self._collectfile(path):
                     yield x
+            elif not path.isdir():
+                # Broken symlink or invalid/missing file.
+                continue
+            elif path.join("__init__.py").check(file=1):
+                pkg_prefixes.add(path)
 
 
 def _get_xunit_setup_teardown(holder, attr_name, param_obj=None):

testing/test_collection.py

@@ -1186,3 +1186,30 @@ def test_collect_pkg_init_and_file_in_args(testdir):
             "*2 passed in*",
         ]
     )
+
+
+@pytest.mark.skipif(
+    not hasattr(py.path.local, "mksymlinkto"),
+    reason="symlink not available on this platform",
+)
+@pytest.mark.parametrize("use_pkg", (True, False))
+def test_collect_sub_with_symlinks(use_pkg, testdir):
+    sub = testdir.mkdir("sub")
+    if use_pkg:
+        sub.ensure("__init__.py")
+    sub.ensure("test_file.py").write("def test_file(): pass")
+
+    # Create a broken symlink.
+    sub.join("test_broken.py").mksymlinkto("test_doesnotexist.py")
+
+    # Symlink that gets collected.
+    sub.join("test_symlink.py").mksymlinkto("test_file.py")
+
+    result = testdir.runpytest("-v", str(sub))
+    result.stdout.fnmatch_lines(
+        [
+            "sub/test_file.py::test_file PASSED*",
+            "sub/test_symlink.py::test_file PASSED*",
+            "*2 passed in*",
+        ]
+    )