Requested changes for pypa#8078

- Condense changelog with reference to discussion pypa#8042
- Change warning to hard error with reference to pypa#8042 on multiple index URLs
- Change critical to warning on header parsing errors

news/8042.feature

@@ -1,15 +1,3 @@
-Add CLI options for enhancing requests with HTTP headers
+Add CLI option -H / --header for enhancing requests with HTTP headers.
 
--H, --header <key:val>      HTTP header to include in all requests. This option
-                            can be used multiple times. Conflicts with
-                            --extra-index-url.
-
-Example:
-
-```
-pip install \
-  --index-url http://pypi.index/simple/ \
-  --trusted-host pypi.index \
-  -H 'X-Spam: ~*~ SPAM ~*~' \
-  requests
-```
+See https://github.com/pypa/pip/issues/8042 for discussion.

src/pip/_internal/cli/req_command.py

@@ -88,11 +88,11 @@ def _get_headers(cls, options):
         # Refuse to set headers when multiple index URLs are set
         index_urls = cls._get_index_urls(options)
         if index_urls and len(index_urls) > 1:
-            logger.warning(
+            raise CommandError(
                 'Refusing to set -H / --header option(s) because multiple '
-                'index URLs are configured.',
+                'index URLs are configured. '
+                'See https://github.com/pypa/pip/issues/8042 for more info.',
             )
-            return None
 
         # Parse header inputs into dict
         headers = {}
@@ -102,7 +102,7 @@ def _get_headers(cls, options):
                 key, val = match.groups()
                 headers[key] = val
             else:
-                logger.critical('Could not parse header {!r}'.format(header))
+                logger.warning('Could not parse header {!r}'.format(header))
         return headers or None
 
     def get_default_session(self, options):