Ensure that PYTEST_ADDOPTS are valid by themselves

Fixes #4265.

changelog/4265.bugfix.rst

@@ -0,0 +1 @@
+Validate arguments from the ``PYTEST_ADDOPTS`` environment variable and the ``addopts`` ini option separately.

src/_pytest/config/__init__.py

@@ -784,12 +784,21 @@ def _mark_plugins_for_rewrite(self, hook):
         for name in _iter_rewritable_modules(package_files):
             hook.mark_rewrite(name)
 
+    def _validate_args(self, args):
+        """Validate known args."""
+        self._parser.parse_known_and_unknown_args(
+            args, namespace=copy.copy(self.option)
+        )
+        return args
+
     def _preparse(self, args, addopts=True):
         if addopts:
-            args[:] = shlex.split(os.environ.get("PYTEST_ADDOPTS", "")) + args
+            env_addopts = os.environ.get("PYTEST_ADDOPTS", "")
+            if len(env_addopts):
+                args[:] = self._validate_args(shlex.split(env_addopts)) + args
         self._initini(args)
         if addopts:
-            args[:] = self.getini("addopts") + args
+            args[:] = self._validate_args(self.getini("addopts")) + args
         self._checkversion()
         self._consider_importhook(args)
         self.pluginmanager.consider_preparse(args)

testing/test_config.py

@@ -1082,6 +1082,33 @@ def test_addopts_before_initini(self, monkeypatch):
         config._preparse([], addopts=True)
         assert config._override_ini == ["cache_dir=%s" % cache_dir]
 
+    def test_addopts_from_env_not_concatenated(self, monkeypatch):
+        """PYTEST_ADDOPTS should not take values from normal args (#4265)."""
+        from _pytest.config import get_config
+
+        monkeypatch.setenv("PYTEST_ADDOPTS", "-o")
+        config = get_config()
+        with pytest.raises(SystemExit) as excinfo:
+            config._preparse(["cache_dir=ignored"], addopts=True)
+        assert excinfo.value.args[0] == _pytest.main.EXIT_USAGEERROR
+
+    def test_addopts_from_ini_not_concatenated(self, testdir):
+        """addopts from ini should not take values from normal args (#4265)."""
+        testdir.makeini(
+            """
+            [pytest]
+            addopts=-o
+        """
+        )
+        result = testdir.runpytest("cache_dir=ignored")
+        result.stderr.fnmatch_lines(
+            [
+                "%s: error: argument -o/--override-ini: expected one argument"
+                % (testdir.request.config._parser.optparser.prog,)
+            ]
+        )
+        assert result.ret == _pytest.main.EXIT_USAGEERROR
+
     def test_override_ini_does_not_contain_paths(self):
         """Check that -o no longer swallows all options after it (#3103)"""
         from _pytest.config import get_config