Fix #6531: Failed to load last environment object when extension added

CHANGES

@@ -48,6 +48,7 @@ Bugs fixed
 * #6507: autosummary: crashes without no autosummary_generate setting
 * #6511: LaTeX: autonumbered list can not be customized in LaTeX
   since Sphinx 1.8.0 (refs: #6533)
+* #6531: Failed to load last environment object when extension added
 
 Testing
 --------

sphinx/environment/__init__.py

@@ -237,6 +237,8 @@ class BuildEnvironment:
         self.config_status = CONFIG_OK
         if self.config is None:
             self.config_status = CONFIG_NEW
+        elif self.config.extensions != config.extensions:
+            self.config_status = CONFIG_EXTENSIONS_CHANGED
         else:
             # check if a config value was changed that affects how
             # doctrees are read
@@ -245,11 +247,6 @@ class BuildEnvironment:
                     self.config_status = CONFIG_CHANGED
                     break
 
-            # this value is not covered by the above loop because it is handled
-            # specially by the config class
-            if self.config.extensions != config.extensions:
-                self.config_status = CONFIG_EXTENSIONS_CHANGED
-
         self.config = config
 
     def _update_settings(self, config):

tests/test_environment.py

@@ -11,9 +11,32 @@ import pytest
 
 from sphinx.builders.html import StandaloneHTMLBuilder
 from sphinx.builders.latex import LaTeXBuilder
+from sphinx.environment import CONFIG_OK, CONFIG_CHANGED, CONFIG_EXTENSIONS_CHANGED, CONFIG_NEW
 from sphinx.testing.comparer import PathComparer
 
 
+@pytest.mark.sphinx('dummy', testroot='basic')
+def test_config_status(make_app, app_params):
+    args, kwargs = app_params
+
+    # clean build
+    app1 = make_app(*args, freshenv=True, **kwargs)
+    assert app1.env.config_status == CONFIG_NEW
+    app1.build()
+
+    # incremental build (no config changed)
+    app2 = make_app(*args, **kwargs)
+    assert app2.env.config_status == CONFIG_OK
+
+    # incremental build (config entry changed)
+    app3 = make_app(*args, confoverrides={'master_doc': 'content'}, **kwargs)
+    assert app3.env.config_status == CONFIG_CHANGED
+
+    # incremental build (extension changed)
+    app4 = make_app(*args, confoverrides={'extensions': ['sphinx.ext.autodoc']}, **kwargs)
+    assert app4.env.config_status == CONFIG_EXTENSIONS_CHANGED
+
+
 @pytest.mark.sphinx('dummy')
 def test_images(app):
     app.build()