Redirect to documentation pages URL when configuration option is set

Contributes to issue:
https://gitlab.com/gitlab-org/gitlab/-/issues/32328

Previous MR:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42702

Build URL and redirect to documentation website when Documentation
pages URL option is populated. Fallback to default /help rendering
when it is not available.

app/controllers/help_controller.rb

@@ -27,17 +27,10 @@ class HelpController < ApplicationController
 
     respond_to do |format|
       format.any(:markdown, :md, :html) do
-        # Note: We are purposefully NOT using `Rails.root.join` because of https://gitlab.com/gitlab-org/gitlab/-/issues/216028.
-        path = File.join(Rails.root, 'doc', "#{@path}.md")
-
-        if File.exist?(path)
-          # Remove YAML frontmatter so that it doesn't look weird
-          @markdown = File.read(path).gsub(YAML_FRONT_MATTER_REGEXP, '')
-
-          render 'show.html.haml'
+        if redirect_to_documentation_website?
+          redirect_to documentation_url
         else
-          # Force template to Haml
-          render 'errors/not_found.html.haml', layout: 'errors', status: :not_found
+          render_documentation
         end
       end
 
@@ -76,4 +69,48 @@ class HelpController < ApplicationController
 
     params
   end
+
+  def redirect_to_documentation_website?
+    return false unless Feature.enabled?(:help_page_documentation_redirect)
+    return false unless Gitlab::UrlSanitizer.valid_web?(documentation_url)
+
+    true
+  end
+
+  def documentation_url
+    return unless documentation_base_url
+
+    @documentation_url ||= [
+      documentation_base_url.chomp('/'),
+      version_segment,
+      'ee',
+      "#{@path}.html"
+    ].compact.join('/')
+  end
+
+  def documentation_base_url
+    @documentation_base_url ||= Gitlab::CurrentSettings.current_application_settings.help_page_documentation_base_url.presence
+  end
+
+  def version_segment
+    return if Gitlab.pre_release?
+
+    version = Gitlab.version_info
+    [version.major, version.minor].join('.')
+  end
+
+  def render_documentation
+    # Note: We are purposefully NOT using `Rails.root.join` because of https://gitlab.com/gitlab-org/gitlab/-/issues/216028.
+    path = File.join(Rails.root, 'doc', "#{@path}.md")
+
+    if File.exist?(path)
+      # Remove YAML frontmatter so that it doesn't look weird
+      @markdown = File.read(path).gsub(YAML_FRONT_MATTER_REGEXP, '')
+
+      render 'show.html.haml'
+    else
+      # Force template to Haml
+      render 'errors/not_found.html.haml', layout: 'errors', status: :not_found
+    end
+  end
 end

changelogs/unreleased/32328_use_documentation_url_to_configure_path_to_help_pages.yml

+---
+title: Redirect to documentation pages URL when configuration option is set
+merge_request: 43157
+author:
+type: added

spec/controllers/help_controller_spec.rb

@@ -3,6 +3,8 @@
 require 'spec_helper'
 
 RSpec.describe HelpController do
+  include StubVersion
+
   let(:user) { create(:user) }
 
   before do
@@ -108,8 +110,56 @@ RSpec.describe HelpController do
         end
 
         it 'renders HTML' do
-          expect(response).to render_template('show.html.haml')
-          expect(response.media_type).to eq 'text/html'
+          aggregate_failures do
+            expect(response).to render_template('show.html.haml')
+            expect(response.media_type).to eq 'text/html'
+          end
+        end
+      end
+
+      context 'when a custom help_page_documentation_url is set' do
+        before do
+          stub_application_setting(help_page_documentation_base_url: documentation_base_url)
+          stub_version(gitlab_version, 'deadbeaf')
+        end
+
+        subject { get :show, params: { path: path }, format: 'html' }
+
+        let(:gitlab_version) { '13.4.0-ee' }
+        let(:documentation_base_url) { 'https://docs.gitlab.com' }
+        let(:path) { 'ssh/README' }
+
+        it 'redirects user to custom documentation url with a specified version' do
+          is_expected.to redirect_to("#{documentation_base_url}/13.4/ee/#{path}.html")
+        end
+
+        context 'when documentation url ends with a slash' do
+          let(:documentation_base_url) { 'https://docs.gitlab.com/' }
+
+          it 'redirects user to custom documentation url without slash duplicates' do
+            is_expected.to redirect_to("https://docs.gitlab.com/13.4/ee/#{path}.html")
+          end
+        end
+
+        context 'when it is a pre-release' do
+          let(:gitlab_version) { '13.4.0-pre' }
+
+          it 'redirects user to custom documentation url without a version' do
+            is_expected.to redirect_to("#{documentation_base_url}/ee/#{path}.html")
+          end
+        end
+
+        context 'when feature flag is disabled' do
+          before do
+            stub_feature_flags(help_page_documentation_redirect: false)
+          end
+
+          it 'renders HTML' do
+            aggregate_failures do
+              is_expected.to render_template('show.html.haml')
+              expect(response.media_type).to eq 'text/html'
+            end
+          end
         end
       end
 
@@ -129,9 +179,12 @@ RSpec.describe HelpController do
                 path: 'user/img/markdown_logo'
               },
               format: :png
-          expect(response).to be_successful
-          expect(response.media_type).to eq 'image/png'
-          expect(response.headers['Content-Disposition']).to match(/^inline;/)
+
+          aggregate_failures do
+            expect(response).to be_successful
+            expect(response.media_type).to eq 'image/png'
+            expect(response.headers['Content-Disposition']).to match(/^inline;/)
+          end
         end
       end