Fix crash when passing a very long cmdline argument (#11404)

Fixes #11394

src/_pytest/main.py

@@ -36,6 +36,7 @@ from _pytest.outcomes import exit
 from _pytest.pathlib import absolutepath
 from _pytest.pathlib import bestrelpath
 from _pytest.pathlib import fnmatch_ex
+from _pytest.pathlib import safe_exists
 from _pytest.pathlib import visit
 from _pytest.reports import CollectReport
 from _pytest.reports import TestReport
@@ -888,7 +889,7 @@ def resolve_collection_argument(
         strpath = search_pypath(strpath)
     fspath = invocation_path / strpath
     fspath = absolutepath(fspath)
-    if not fspath.exists():
+    if not safe_exists(fspath):
         msg = (
             "module or package not found: {arg} (missing __init__.py?)"
             if as_pypath

src/_pytest/pathlib.py

@@ -1,5 +1,6 @@
 import atexit
 import contextlib
+import errno
 import fnmatch
 import importlib.util
 import itertools
@@ -773,3 +774,13 @@ def bestrelpath(directory: Path, dest: Path) -> str:
         # Forward from base to dest.
         *reldest.parts,
     )
+
+
+def safe_exists(p: Path) -> bool:
+    """Like Path.exists(), but account for input arguments that might be too long (#11394)."""
+    try:
+        return p.exists()
+    except OSError as e:
+        if e.errno == errno.ENAMETOOLONG:
+            return False
+        raise