Make tmpdir resiliant to user ids that do not exist

Previously, the tmpdir fixture would fail if the current process's user
id does not correspond to a valid user (e.g. running pytest in a Docker
container with 'docker run -u').

_pytest/tmpdir.py

@@ -78,7 +78,7 @@ def get_user():
     import getpass
     try:
         return getpass.getuser()
-    except ImportError:
+    except (ImportError, KeyError):
         return None
 
 # backward compatibility

testing/test_tmpdir.py

@@ -136,6 +136,40 @@ def test_tmpdir_fallback_tox_env(testdir, monkeypatch):
     reprec.assertoutcome(passed=1)
 
 
+@pytest.mark.skipif(sys.platform.startswith('win'), reason='no os.getuid on windows')
+def test_tmpdir_fallback_uid_not_found(testdir, monkeypatch):
+    """Test that tmpdir works even if the current process's user id does not
+    correspond to a valid user.
+    """
+    import os
+    monkeypatch.setattr(os, 'getuid', lambda: -1)
+    monkeypatch.delenv('USER', raising=False)
+    monkeypatch.delenv('USERNAME', raising=False)
+
+    testdir.makepyfile("""
+        import pytest
+        def test_some(tmpdir):
+            assert tmpdir.isdir()
+    """)
+    reprec = testdir.inline_run()
+    reprec.assertoutcome(passed=1)
+
+
+@pytest.mark.skipif(sys.platform.startswith('win'), reason='no os.getuid on windows')
+def test_get_user_uid_not_found(monkeypatch):
+    """Test that get_user() function works even if the current process's
+    user id does not correspond to a valid user (e.g. running pytest in a
+    Docker container with 'docker run -u'.
+    """
+    import os
+    monkeypatch.setattr(os, 'getuid', lambda: -1)
+    monkeypatch.delenv('USER', raising=False)
+    monkeypatch.delenv('USERNAME', raising=False)
+
+    from _pytest.tmpdir import get_user
+    assert get_user() is None
+
+
 @pytest.mark.skipif(not sys.platform.startswith('win'), reason='win only')
 def test_get_user(monkeypatch):
     """Test that get_user() function works even if environment variables