[WIP] Track visited files and directories when collecting

AUTHORS

@@ -223,6 +223,7 @@ Virgil Dupras
 Vitaly Lashmanov
 Vlad Dragos
 Wil Cooley
+Will Thompson
 William Lee
 Wim Glenn
 Wouter van Ackooy

src/_pytest/main.py

@@ -392,6 +392,8 @@ def __init__(self, config):
         self._initialpaths = frozenset()
         # Keep track of any collected nodes in here, so we don't duplicate fixtures
         self._node_cache = {}
+        # Keep track of visited directories in here, so we don't end up in a symlink-induced loop.
+        self._visited = set()
 
         self.config.pluginmanager.register(self, name="session")
 
@@ -558,7 +560,17 @@ def _collectfile(self, path):
                 return ()
         return ihook.pytest_collect_file(path=path, parent=self)
 
+    def _check_visited(self, path):
+        st = path.stat()
+        key = (st.dev, st.ino)
+        if key in self._visited:
+            return True
+        self._visited.add(key)
+        return False
+
     def _recurse(self, path):
+        if self._check_visited(path):
+            return False
         ihook = self.gethookproxy(path.dirpath())
         if ihook.pytest_ignore_collect(path=path, config=self.config):
             return

testing/examples/test_issue624.py

@@ -0,0 +1,56 @@
+# -*- coding: utf-8 -*-
+from __future__ import absolute_import, division, print_function
+import sys
+
+import six
+
+import py
+import pytest
+
+
+@pytest.mark.skipif(
+    not hasattr(py.path.local, "mksymlinkto"),
+    reason="symlink not available on this platform",
+)
+def test_624(testdir):
+    """
+    Runs tests in the following directory tree:
+
+        testdir/
+          test_noop.py
+          symlink-0 -> .
+          symlink-1 -> .
+
+    On Linux, the maximum number of symlinks in a path is 40, after which ELOOP
+    is returned when trying to read the path. This means that if we walk the
+    directory tree naively, following symlinks, naively, this will attempt to
+    visit test_noop.py via 2 ** 41 paths:
+
+        testdir/symlink-0/test_noop.py
+        testdir/symlink-1/test_noop.py
+        testdir/symlink-0/symlink-0/test_noop.py
+        testdir/symlink-0/symlink-1/test_noop.py
+        .. and eventually ..
+        testdir/symlink-0/.. 2 ** 39 more combinations ../test_noop.py
+        testdir/symlink-1/.. 2 ** 39 more combinations ../test_noop.py
+
+    Instead, we should stop recursing when we reach a directory we've seen
+    before.  In this test, this means visiting the test once at the root, and
+    once via a symlink, then stopping.
+    """
+
+    test_noop_py = testdir.makepyfile(test_noop="def test_noop():\n    pass")
+
+    # dummy check that we can actually create symlinks: on Windows `py.path.mksymlinkto` is
+    # available, but normal users require special admin privileges to create symlinks.
+    if sys.platform == "win32":
+        try:
+            (testdir.tmpdir / ".dummy").mksymlinkto(test_noop_py)
+        except OSError as e:
+            pytest.skip(six.text_type(e.args[0]))
+
+    for i in range(2):
+        (testdir.tmpdir / "symlink-{}".format(i)).mksymlinkto(testdir.tmpdir)
+
+    result = testdir.runpytest()
+    result.assert_outcomes(passed=2)

tox.ini

@@ -18,7 +18,7 @@ envlist =
 
 [testenv]
 commands =
-    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest --lsof
+    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest --lsof {posargs}
     coverage: coverage combine
     coverage: coverage report
 passenv = USER USERNAME COVERAGE_* TRAVIS
@@ -41,7 +41,7 @@ deps =
     py27: mock
     nose
 commands =
-    pytest -n auto --runpytest=subprocess
+    pytest -n auto --runpytest=subprocess {posargs}
 
 
 [testenv:linting]
@@ -58,7 +58,7 @@ deps =
     hypothesis>=3.56
     {env:_PYTEST_TOX_EXTRA_DEP:}
 commands =
-    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest -n auto
+    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest -n auto {posargs}
 
 [testenv:py36-xdist]
 # NOTE: copied from above due to https://github.com/tox-dev/tox/issues/706.