Improve pip config set error when PIP_CONFIG_FILE is non-regular

Author: gcomneno

src/pip/_internal/configuration.py

@@ -378,9 +378,18 @@ def _get_parser_to_modify(self) -> tuple[str, RawConfigParser]:
         assert self.load_only
         parsers = self._parsers[self.load_only]
         if not parsers:
-            # This should not happen if everything works correctly.
+            # When PIP_CONFIG_FILE points to a non-regular file (e.g. /dev/null),
+            # we can't load/modify it as an INI file. Surface a clear error instead
+            # of an internal one.
+            env_config_file = os.environ.get("PIP_CONFIG_FILE")
+            if env_config_file and not os.path.isfile(env_config_file):
+                raise ConfigurationError(
+                    "Cannot modify pip configuration: PIP_CONFIG_FILE points to a "
+                    f"non-regular file: {env_config_file}"
+                )
+
             raise ConfigurationError(
-                "Fatal Internal error [id=2]. Please report as a bug."
+                "Cannot modify pip configuration: no configuration file is available for the selected scope."
             )
 
         # Use the highest priority parser.

tests/functional/test_configuration.py

@@ -216,6 +216,24 @@ def test_config_separated(
             result.stdout,
         )
 
+
+    def test_config_set_with_pip_config_file_devnull_shows_human_error(
+        self, script: PipTestEnvironment
+    ) -> None:
+        script.environ["PIP_CONFIG_FILE"] = "/dev/null"
+
+        result = script.pip(
+            "config",
+            "set",
+            "global.index-url",
+            "https://example.com/",
+            expect_error=True,
+        )
+        assert "Fatal Internal error [id=2]" not in result.stderr
+        assert "PIP_CONFIG_FILE" in result.stderr
+        assert "non-regular file" in result.stderr
+
+
     @pytest.mark.network
     def test_editable_mode_default_config(
         self, script: PipTestEnvironment, data: TestData