pytester: use per test tmproot

src/_pytest/pytester.py

@@ -496,6 +496,8 @@ class Testdir(object):
         self._mod_collections = WeakKeyDictionary()
         name = request.function.__name__
         self.tmpdir = tmpdir_factory.mktemp(name, numbered=True)
+        self.test_tmproot = tmpdir_factory.mktemp("tmp-" + name, numbered=True)
+        os.environ["PYTEST_DEBUG_TEMPROOT"] = str(self.test_tmproot)
         self.plugins = []
         self._cwd_snapshot = CwdSnapshot()
         self._sys_path_snapshot = SysPathsSnapshot()
@@ -522,6 +524,7 @@ class Testdir(object):
         self._sys_modules_snapshot.restore()
         self._sys_path_snapshot.restore()
         self._cwd_snapshot.restore()
+        del os.environ["PYTEST_DEBUG_TEMPROOT"]
 
     def __take_sys_modules_snapshot(self):
         # some zope modules used by twisted-related tests keep internal state

src/_pytest/tmpdir.py

@@ -1,6 +1,7 @@
 """ support for providing temporary directories to test functions.  """
 from __future__ import absolute_import, division, print_function
 
+import os
 import re
 import pytest
 import py
@@ -51,7 +52,8 @@ class TempPathFactory(object):
                 basetemp = Path(self.given_basetemp)
                 ensure_reset_dir(basetemp)
             else:
-                temproot = Path(tempfile.gettempdir())
+                from_env = os.environ.get("PYTEST_DEBUG_TEMPROOT")
+                temproot = Path(from_env or tempfile.gettempdir())
                 user = get_user() or "unknown"
                 # use a sub-directory in the temproot to speed-up
                 # make_numbered_dir() call

testing/test_tmpdir.py

@@ -143,7 +143,6 @@ def break_getuser(monkeypatch):
         monkeypatch.delenv(envvar, raising=False)
 
 
-@pytest.mark.skip(reason="creates random tmpdirs as part of a system level test")
 @pytest.mark.usefixtures("break_getuser")
 @pytest.mark.skipif(sys.platform.startswith("win"), reason="no os.getuid on windows")
 def test_tmpdir_fallback_uid_not_found(testdir):
@@ -162,7 +161,6 @@ def test_tmpdir_fallback_uid_not_found(testdir):
     reprec.assertoutcome(passed=1)
 
 
-@pytest.mark.skip(reason="creates random tmpdirs as part of a system level test")
 @pytest.mark.usefixtures("break_getuser")
 @pytest.mark.skipif(sys.platform.startswith("win"), reason="no os.getuid on windows")
 def test_get_user_uid_not_found():