Improve readability

* Rename ALLOWED_CONTROLLER_AND_ACTIONS to CONTROLLER_AND_ACTIONS_TO_REDIRECT
* Rename git_receive_pack? to git_receive_pack_request?
* Added comments for greater context

ee/app/controllers/ee/projects/git_http_client_controller.rb

@@ -3,6 +3,29 @@ module EE
     module GitHttpClientController
       extend ActiveSupport::Concern
 
+      # This module is responsible for determining if an incoming secondary bound
+      # HTTP request should be redirected to the primary.
+      #
+      # Why?  A secondary is not allowed to perform any write actions, so any
+      # request of this type need to be sent through to the primary.  By
+      # redirecting within code, we allow clients to git pull/push using their
+      # secondary git remote without needing an additional primary remote.
+      #
+      # Current secondary HTTP requests to redirect: -
+      #
+      # * git push
+      #   * GET   /repo.git/info/refs?service=git-receive-pack
+      #   * POST  /repo.git/git-receive-pack
+      #
+      # * git lfs push (usually happens automatically as part of a `git push`)
+      #   * POST  /repo.git/info/lfs/objects/batch (and we examine
+      #     params[:operation] to ensure we're dealing with an upload request)
+      #
+      # For more detail, see the following links:
+      #
+      # git: https://git-scm.com/book/en/v2/Git-Internals-Transfer-Protocols
+      # git-lfs: https://github.com/git-lfs/git-lfs/blob/master/docs/api
+      #
       prepended do
         before_action do
           redirect_to(primary_full_url) if redirect?
@@ -14,7 +37,7 @@ module EE
       class RouteHelper
         attr_reader :controller_name, :action_name
 
-        ALLOWED_CONTROLLER_AND_ACTIONS = {
+        CONTROLLER_AND_ACTIONS_TO_REDIRECT = {
           'git_http' => %w{git_receive_pack},
           'lfs_locks_api' => %w{create unlock verify}
         }.freeze
@@ -30,23 +53,40 @@ module EE
         end
 
         def redirect?
-          git_receive_pack? || controller_and_action_match?
+          !!CONTROLLER_AND_ACTIONS_TO_REDIRECT[controller_name]&.include?(action_name) ||
+            git_receive_pack_request?
         end
 
         private
 
         attr_reader :service
 
-        def git_receive_pack?
-          service_or_action_name == 'git-receive-pack'
+        # Examples:
+        #
+        # /repo.git/info/refs?service=git-receive-pack returns 'git-receive-pack'
+        # /repo.git/info/refs?service=git-upload-pack returns 'git-upload-pack'
+        # /repo.git/git-receive-pack returns 'git-receive-pack'
+        # /repo.git/git-upload-pack returns 'git-upload-pack'
+        #
+        def service_or_action_name
+          info_refs_request? ? service : action_name.dasherize
         end
 
-        def controller_and_action_match?
-          !!ALLOWED_CONTROLLER_AND_ACTIONS[controller_name]&.include?(action_name)
+        # Matches:
+        #
+        # GET  /repo.git/info/refs?service=git-receive-pack
+        # POST /repo.git/git-receive-pack
+        #
+        def git_receive_pack_request?
+          service_or_action_name == 'git-receive-pack'
         end
 
-        def service_or_action_name
-          action_name == 'info_refs' ? service : action_name.dasherize
+        # Matches:
+        #
+        # GET /repo.git/info/refs
+        #
+        def info_refs_request?
+          action_name == 'info_refs'
         end
       end
 
@@ -117,19 +157,27 @@ module EE
       end
 
       def redirect?
+        # Don't redirect if we're not a secondary with a primary
         return false unless ::Gitlab::Geo.secondary_with_primary?
+
+        # Redirect as the request matches RouteHelper::CONTROLLER_AND_ACTIONS_TO_REDIRECT
         return true if route_helper.redirect?
 
+        # Look to redirect, as we're an LFS batch upload request
         if git_lfs_helper.redirect?
+          # Redirect as git-lfs version is at least 2.4.2
           return true if git_lfs_helper.version_ok?
 
           # git-lfs 2.4.2 is really only required for requests that involve
           # redirection, so we only render if it's an LFS upload operation
           #
           render(git_lfs_helper.incorrect_version_response)
+
+          # Don't redirect
           return false
         end
 
+        # Don't redirect
         false
       end
     end