Implement the correct linking behaviour in `WikiLinkFilter`.

Original Comments
=================

- Linking behaves as per rules documented here:
  https://gitlab.com/gitlab-org/gitlab-ce/blob/16568-document-wiki-linking-behavior/doc/markdown/wiki.md
- All links (to other wiki pages) are rewritten to be at the level of
  the app root. We can't use links relative to the current
  page ('./foo', 'foo', '../foo'), because they won't work in the
  markdown preview, where the current page is suffixed with `/edit`
- Move existing `WikiLinkFilter` specs to `WikiPipeline` spec. It makes
  sense to run these tests on the combined output of the pipeline,
  rather than a single filter, since we can catch issues with
  conflicting filters.
- Add more tests to cover the new linking

@rymai's Review
===============

- Classes nested under `WikiLinkFilter` should declare `WikiLinkFilter`'s
  inherit, so nothing changes if the nested class is loaded first.
- Add a blank line after a guard clause
- Use keyword arguments for the `Rewriter` constructor
- Invert a condition - use `if` instead of `unless`
- Inline a `let` in `WikiPipeline` spec - it was only used in a single place
- Change out of date spec names
- Add a comment for every rewrite rule in `Rewriter`

2 files changed, 46 insertions, 26 deletions

diff --git a/lib/banzai/filter/wiki_link_filter.rb b/lib/banzai/filter/wiki_link_filter.rb
index 7dc771afd71..37a2779d453 100644
--- a/lib/banzai/filter/wiki_link_filter.rb
+++ b/lib/banzai/filter/wiki_link_filter.rb
@@ -2,7 +2,8 @@ require 'uri'
 
 module Banzai
   module Filter
-    # HTML filter that "fixes" relative links to files in a repository.
+    # HTML filter that "fixes" links to pages/files in a wiki.
+    # Rewrite rules are documented in the `WikiPipeline` spec.
     #
     # Context options:
     #   :project_wiki
@@ -25,36 +26,15 @@ module Banzai
       end
 
       def process_link_attr(html_attr)
-        return if html_attr.blank? || file_reference?(html_attr) || hierarchical_link?(html_attr)
+        return if html_attr.blank?
 
-        uri = URI(html_attr.value)
-        if uri.relative? && uri.path.present?
-          html_attr.value = rebuild_wiki_uri(uri).to_s
-        end
+        html_attr.value = apply_rewrite_rules(html_attr.value)
       rescue URI::Error
         # noop
       end
 
-      def rebuild_wiki_uri(uri)
-        uri.path = ::File.join(project_wiki_base_path, uri.path)
-        uri
-      end
-
-      def project_wiki
-        context[:project_wiki]
-      end
-
-      def file_reference?(html_attr)
-        !File.extname(html_attr.value).blank?
-      end
-
-      # Of the form `./link`, `../link`, or similar
-      def hierarchical_link?(html_attr)
-        html_attr.value[0] == '.'
-      end
-
-      def project_wiki_base_path
-        project_wiki && project_wiki.wiki_base_path
+      def apply_rewrite_rules(link_string)
+        Rewriter.new(link_string, wiki: context[:project_wiki], slug: context[:page_slug]).apply_rules
       end
     end
   end
diff --git a/lib/banzai/filter/wiki_link_filter/rewriter.rb b/lib/banzai/filter/wiki_link_filter/rewriter.rb
new file mode 100644
index 00000000000..2e2c8da311e
--- /dev/null
+++ b/lib/banzai/filter/wiki_link_filter/rewriter.rb
@@ -0,0 +1,40 @@
+module Banzai
+  module Filter
+    class WikiLinkFilter < HTML::Pipeline::Filter
+      class Rewriter
+        def initialize(link_string, wiki:, slug:)
+          @uri = Addressable::URI.parse(link_string)
+          @wiki_base_path = wiki && wiki.wiki_base_path
+          @slug = slug
+        end
+
+        def apply_rules
+          apply_file_link_rules!
+          apply_hierarchical_link_rules!
+          apply_relative_link_rules!
+          @uri.to_s
+        end
+
+        private
+
+        # Of the form 'file.md'
+        def apply_file_link_rules!
+          @uri = Addressable::URI.join(@slug, @uri) if @uri.extname.present?
+        end
+
+        # Of the form `./link`, `../link`, or similar
+        def apply_hierarchical_link_rules!
+          @uri = Addressable::URI.join(@slug, @uri) if @uri.to_s[0] == '.'
+        end
+
+        # Any link _not_ of the form `http://example.com/`
+        def apply_relative_link_rules!
+          if @uri.relative? && @uri.path.present?
+            link = ::File.join(@wiki_base_path, @uri.path)
+            @uri = Addressable::URI.parse(link)
+          end
+        end
+      end
+    end
+  end
+end