1 files changed, 114 insertions, 0 deletions
diff --git a/lib/gitlab/markdown/relative_link_filter.rb b/lib/gitlab/markdown/relative_link_filter.rb
new file mode 100644
index 00000000000..deb302c88e1
--- /dev/null
+++ b/lib/gitlab/markdown/relative_link_filter.rb
@@ -0,0 +1,114 @@
+require 'html/pipeline/filter'
+require 'uri'
+
+module Gitlab
+  module Markdown
+    # HTML filter that "fixes" relative links to files in a repository.
+    #
+    # Context options:
+    #   :commit
+    #   :project
+    #   :project_wiki
+    #   :requested_path
+    #   :ref
+    class RelativeLinkFilter < HTML::Pipeline::Filter
+
+      def call
+        if linkable_files?
+          doc.search('a').each do |el|
+            process_link_attr el.attribute('href')
+          end
+
+          doc.search('img').each do |el|
+            process_link_attr el.attribute('src')
+          end
+        end
+
+        doc
+      end
+
+      protected
+
+      def linkable_files?
+        context[:project_wiki].nil? && repository.try(:exists?) && !repository.empty?
+      end
+
+      def process_link_attr(html_attr)
+        return if html_attr.blank?
+
+        uri = URI(html_attr.value)
+        if uri.relative? && uri.path.present?
+          html_attr.value = rebuild_relative_uri(uri).to_s
+        end
+      end
+
+      def rebuild_relative_uri(uri)
+        file_path = relative_file_path(uri.path)
+
+        uri.path = [
+          relative_url_root,
+          context[:project].path_with_namespace,
+          path_type(file_path),
+          ref || 'master',  # assume that if no ref exists we can point to master
+          file_path
+        ].compact.join('/').squeeze('/').chomp('/')
+
+        uri
+      end
+
+      def relative_file_path(path)
+        nested_path = build_nested_path(path, context[:requested_path])
+        file_exists?(nested_path) ? nested_path : path
+      end
+
+      # Covering a special case, when the link is referencing file in the same
+      # directory.
+      # If we are at doc/api/README.md and the README.md contains relative
+      # links like [Users](users.md), this takes the request
+      # path(doc/api/README.md) and replaces the README.md with users.md so the
+      # path looks like doc/api/users.md.
+      # If we are at doc/api and the README.md shown in below the tree view
+      # this takes the request path(doc/api) and adds users.md so the path
+      # looks like doc/api/users.md
+      def build_nested_path(path, request_path)
+        return request_path if path.empty?
+        return path unless request_path
+
+        parts = request_path.split('/')
+        parts.pop if path_type(request_path) != 'tree'
+        parts.push(path).join('/')
+      end
+
+      def file_exists?(path)
+        return false if path.nil?
+        repository.blob_at(current_sha, path).present? ||
+          repository.tree(current_sha, path).entries.any?
+      end
+
+      # Check if the path is pointing to a directory(tree) or a file(blob)
+      # eg. doc/api is directory and doc/README.md is file.
+      def path_type(path)
+        return 'tree' if repository.tree(current_sha, path).entries.any?
+        return 'raw' if repository.blob_at(current_sha, path).try(:image?)
+        'blob'
+      end
+
+      def current_sha
+        context[:commit].try(:id) ||
+          ref ? repository.commit(ref).try(:sha) : repository.head_commit.sha
+      end
+
+      def relative_url_root
+        Gitlab.config.gitlab.relative_url_root.presence || '/'
+      end
+
+      def ref
+        context[:ref]
+      end
+
+      def repository
+        context[:project].try(:repository)
+      end
+    end
+  end
+end