Reference filters :sparkles:

Commit ranges, commits, external issues, issues, labels, merge requests,
snippets, users.

10 files changed, 928 insertions, 283 deletions

diff --git a/lib/gitlab/markdown.rb b/lib/gitlab/markdown.rb
index 47c456d8dc7..93c279edc32 100644
--- a/lib/gitlab/markdown.rb
+++ b/lib/gitlab/markdown.rb
@@ -10,11 +10,11 @@ module Gitlab
   # Supported reference formats are:
   #   * @foo for team members
   #   * #123 for issues
-  #   * #JIRA-123 for Jira issues
+  #   * JIRA-123 for Jira issues
   #   * !123 for merge requests
   #   * $123 for snippets
-  #   * 123456 for commits
-  #   * 123456...7890123 for commit ranges (comparisons)
+  #   * 1c002d for specific commit
+  #   * 1c002d...35cfb2 for commit ranges (comparisons)
   #
   # It also parses Emoji codes to insert images. See
   # http://www.emoji-cheat-sheet.com/ for a list of the supported icons.
@@ -30,10 +30,6 @@ module Gitlab
   #   >> gfm(":trollface:")
   #   => "<img alt=\":trollface:\" class=\"emoji\" src=\"/images/trollface.png" title=\":trollface:\" />
   module Markdown
-    include IssuesHelper
-
-    attr_reader :options, :html_options
-
     # Public: Parse the provided text with GitLab-Flavored Markdown
     #
     # text         - the source text
@@ -65,42 +61,13 @@ module Gitlab
         reference_only_path:  true
       )
 
-      @options      = options
-      @html_options = html_options
-
-      # TODO: add popups with additional information
-
-      # Used markdown pipelines in GitLab:
-      # GitlabEmojiFilter - performs emoji replacement.
-      # SanitizationFilter - remove unsafe HTML tags and attributes
-      #
-      # see https://gitlab.com/gitlab-org/html-pipeline-gitlab for more filters
-      filters = [
-        HTML::Pipeline::Gitlab::GitlabEmojiFilter,
-        HTML::Pipeline::SanitizationFilter
-      ]
-
-      whitelist = HTML::Pipeline::SanitizationFilter::WHITELIST
-      whitelist[:attributes][:all].push('class', 'id')
-      whitelist[:elements].push('span')
-
-      # Remove the rel attribute that the sanitize gem adds, and remove the
-      # href attribute if it contains inline javascript
-      fix_anchors = lambda do |env|
-        name, node = env[:node_name], env[:node]
-        if name == 'a'
-          node.remove_attribute('rel')
-          if node['href'] && node['href'].match('javascript:')
-            node.remove_attribute('href')
-          end
-        end
-      end
-      whitelist[:transformers].push(fix_anchors)
-
       markdown_context = {
-              asset_root: Gitlab.config.gitlab.url,
-              asset_host: Gitlab::Application.config.asset_host,
-              whitelist: whitelist
+        asset_root:      Gitlab.config.gitlab.url,
+        asset_host:      Gitlab::Application.config.asset_host,
+        whitelist:       sanitization_whitelist,
+        reference_class: html_options[:class],
+        only_path:       options[:reference_only_path],
+        project:         project
       }
 
       markdown_pipeline = HTML::Pipeline::Gitlab.new(filters).pipeline
@@ -114,21 +81,6 @@ module Gitlab
 
       text = result[:output].to_html(save_with: save_options)
 
-      # Extract pre blocks so they are not altered
-      # from http://github.github.com/github-flavored-markdown/
-      text.gsub!(%r{<pre>.*?</pre>|<code>.*?</code>}m) { |match| extract_piece(match) }
-      # Extract links with probably parsable hrefs
-      text.gsub!(%r{<a.*?>.*?</a>}m) { |match| extract_piece(match) }
-      # Extract images with probably parsable src
-      text.gsub!(%r{<img.*?>}m) { |match| extract_piece(match) }
-
-      text = parse(text, project)
-
-      # Insert pre block extractions
-      text.gsub!(/\{gfm-extraction-(\h{32})\}/) do
-        insert_piece($1)
-      end
-
       if options[:parse_tasks]
         text = parse_tasks(text)
       end
@@ -138,242 +90,48 @@ module Gitlab
 
     private
 
-    def extract_piece(text)
-      @extractions ||= {}
-
-      md5 = Digest::MD5.hexdigest(text)
-      @extractions[md5] = text
-      "{gfm-extraction-#{md5}}"
-    end
-
-    def insert_piece(id)
-      @extractions[id]
-    end
-
-    # Private: Parses text for references
-    #
-    # text - Text to parse
+    # Custom filters for html-pipeline:
     #
-    # Returns parsed text
-    def parse(text, project = @project)
-      parse_references(text, project) if project
-
-      text
-    end
-
-    NAME_STR = Gitlab::Regex::NAMESPACE_REGEX_STR
-    PROJ_STR = "(?<project>#{NAME_STR}/#{NAME_STR})"
-
-    REFERENCE_PATTERN = %r{
-      (?<prefix>\W)?                         # Prefix
-      (                                      # Reference
-         @(?<user>#{NAME_STR})               # User name
-        |~(?<label>\d+)                      # Label ID
-        |(?<issue>([A-Z\-]+-)\d+)            # JIRA Issue ID
-        |#{PROJ_STR}?\#(?<issue>([a-zA-Z\-]+-)?\d+) # Issue ID
-        |#{PROJ_STR}?!(?<merge_request>\d+)  # MR ID
-        |\$(?<snippet>\d+)                   # Snippet ID
-        |(#{PROJ_STR}@)?(?<commit_range>[\h]{6,40}\.{2,3}[\h]{6,40}) # Commit range
-        |(#{PROJ_STR}@)?(?<commit>[\h]{6,40}) # Commit ID
-        |(?<skip>gfm-extraction-[\h]{6,40})  # Skip gfm extractions. Otherwise will be parsed as commit
-      )
-      (?<suffix>\W)?                         # Suffix
-    }x.freeze
-
-    TYPES = [:user, :issue, :label, :merge_request, :snippet, :commit, :commit_range].freeze
-
-    def parse_references(text, project = @project)
-      # parse reference links
-      text.gsub!(REFERENCE_PATTERN) do |match|
-        type       = TYPES.select{|t| !$~[t].nil?}.first
-
-        actual_project = project
-        project_prefix = nil
-        project_path = $LAST_MATCH_INFO[:project]
-        if project_path
-          actual_project = ::Project.find_with_namespace(project_path)
-          actual_project = nil unless can?(current_user, :read_project, actual_project)
-          project_prefix = project_path
-        end
-
-        parse_result($LAST_MATCH_INFO, type,
-                     actual_project, project_prefix) || match
-      end
-    end
-
-    # Called from #parse_references.  Attempts to build a gitlab reference
-    # link.  Returns nil if +type+ is nil, if the match string is an HTML
-    # entity, if the reference is invalid, or if the matched text includes an
-    # invalid project path.
-    def parse_result(match_info, type, project, project_prefix)
-      prefix = match_info[:prefix]
-      suffix = match_info[:suffix]
-
-      return nil if html_entity?(prefix, suffix) || type.nil?
-      return nil if project.nil? && !project_prefix.nil?
-
-      identifier = match_info[type]
-      ref_link = reference_link(type, identifier, project, project_prefix)
-
-      if ref_link
-        "#{prefix}#{ref_link}#{suffix}"
-      else
-        nil
-      end
-    end
-
-    # Return true if the +prefix+ and +suffix+ indicate that the matched string
-    # is an HTML entity like &amp;
-    def html_entity?(prefix, suffix)
-      prefix && suffix && prefix[0] == '&' && suffix[-1] == ';'
+    # See https://gitlab.com/gitlab-org/html-pipeline-gitlab for more filters
+    def filters
+      [
+        Gitlab::Markdown::UserReferenceFilter,
+        Gitlab::Markdown::IssueReferenceFilter,
+        Gitlab::Markdown::ExternalIssueReferenceFilter,
+        Gitlab::Markdown::MergeRequestReferenceFilter,
+        Gitlab::Markdown::SnippetReferenceFilter,
+        Gitlab::Markdown::CommitRangeReferenceFilter,
+        Gitlab::Markdown::CommitReferenceFilter,
+        Gitlab::Markdown::LabelReferenceFilter,
+        HTML::Pipeline::Gitlab::GitlabEmojiFilter,
+        HTML::Pipeline::SanitizationFilter
+      ]
     end
 
-    # Private: Dispatches to a dedicated processing method based on reference
-    #
-    # reference  - Object reference ("@1234", "!567", etc.)
-    # identifier - Object identifier (Issue ID, SHA hash, etc.)
+    # Customize the SanitizationFilter whitelist
     #
-    # Returns string rendered by the processing method
-    def reference_link(type, identifier, project = @project, prefix_text = nil)
-      send("reference_#{type}", identifier, project, prefix_text)
-    end
-
-    def reference_user(identifier, project = @project, _ = nil)
-      link_options = html_options.merge(
-          class: "gfm gfm-project_member #{html_options[:class]}"
-        )
+    # - Allow `class` and `id` attributes on all elements
+    # - Allow `span` elements
+    # - Remove `rel` attributes from `a` elements
+    # - Remove `a` nodes with `javascript:` in the `href` attribute
+    def sanitization_whitelist
+      whitelist = HTML::Pipeline::SanitizationFilter::WHITELIST
+      whitelist[:attributes][:all].push('class', 'id')
+      whitelist[:elements].push('span')
 
-      if identifier == "all"
-        link_to(
-          "@all",
-          namespace_project_url(project.namespace, project, only_path: options[:reference_only_path]),
-          link_options
-        )
-      elsif namespace = Namespace.find_by(path: identifier)
-        url =
-          if namespace.is_a?(Group)
-            return nil unless can?(current_user, :read_group, namespace)
-            group_url(identifier, only_path: options[:reference_only_path])
-          else
-            user_url(identifier, only_path: options[:reference_only_path])
+      fix_anchors = lambda do |env|
+        name, node = env[:node_name], env[:node]
+        if name == 'a'
+          node.remove_attribute('rel')
+          if node['href'] && node['href'].match('javascript:')
+            node.remove_attribute('href')
           end
-
-        link_to("@#{identifier}", url, link_options)
-      end
-    end
-
-    def reference_label(identifier, project = @project, _ = nil)
-      if label = project.labels.find_by(id: identifier)
-        link_options = html_options.merge(
-          class: "gfm gfm-label #{html_options[:class]}"
-        )
-        link_to(
-          render_colored_label(label),
-          namespace_project_issues_path(project.namespace, project, label_name: label.name),
-          link_options
-        )
-      end
-    end
-
-    def reference_issue(identifier, project = @project, prefix_text = nil)
-      if project.default_issues_tracker?
-        if project.issue_exists? identifier
-          url = url_for_issue(identifier, project, only_path: options[:reference_only_path])
-          title = title_for_issue(identifier, project)
-          link_options = html_options.merge(
-            title: "Issue: #{title}",
-            class: "gfm gfm-issue #{html_options[:class]}"
-          )
-
-          link_to("#{prefix_text}##{identifier}", url, link_options)
         end
-      else
-        if project.external_issue_tracker.present?
-          reference_external_issue(identifier, project,
-                                   prefix_text)
-        end
-      end
-    end
-
-    def reference_merge_request(identifier, project = @project, prefix_text = nil)
-      if merge_request = project.merge_requests.find_by(iid: identifier)
-        link_options = html_options.merge(
-          title: "Merge Request: #{merge_request.title}",
-          class: "gfm gfm-merge_request #{html_options[:class]}"
-        )
-        url = namespace_project_merge_request_url(project.namespace, project,
-                                                  merge_request,
-                                                  only_path: options[:reference_only_path])
-        link_to("#{prefix_text}!#{identifier}", url, link_options)
-      end
-    end
-
-    def reference_snippet(identifier, project = @project, _ = nil)
-      if snippet = project.snippets.find_by(id: identifier)
-        link_options = html_options.merge(
-          title: "Snippet: #{snippet.title}",
-          class: "gfm gfm-snippet #{html_options[:class]}"
-        )
-        link_to(
-          "$#{identifier}",
-          namespace_project_snippet_url(project.namespace, project, snippet,
-                                        only_path: options[:reference_only_path]),
-          link_options
-        )
       end
-    end
 
-    def reference_commit(identifier, project = @project, prefix_text = nil)
-      if project.valid_repo? && commit = project.repository.commit(identifier)
-        link_options = html_options.merge(
-          title: commit.link_title,
-          class: "gfm gfm-commit #{html_options[:class]}"
-        )
-        prefix_text = "#{prefix_text}@" if prefix_text
-        link_to(
-          "#{prefix_text}#{identifier}",
-          namespace_project_commit_url( project.namespace, project, commit,
-                                        only_path: options[:reference_only_path]),
-          link_options
-        )
-      end
-    end
-
-    def reference_commit_range(identifier, project = @project, prefix_text = nil)
-      from_id, to_id = identifier.split(/\.{2,3}/, 2)
-
-      inclusive = identifier !~ /\.{3}/
-      from_id << "^" if inclusive
-
-      if project.valid_repo? &&
-          from = project.repository.commit(from_id) &&
-          to = project.repository.commit(to_id)
-
-        link_options = html_options.merge(
-          title: "Commits #{from_id} through #{to_id}",
-          class: "gfm gfm-commit_range #{html_options[:class]}"
-        )
-        prefix_text = "#{prefix_text}@" if prefix_text
-
-        link_to(
-          "#{prefix_text}#{identifier}",
-          namespace_project_compare_url(project.namespace, project,
-                                        from: from_id, to: to_id,
-                                        only_path: options[:reference_only_path]),
-          link_options
-        )
-      end
-    end
-
-    def reference_external_issue(identifier, project = @project, prefix_text = nil)
-      url = url_for_issue(identifier, project, only_path: options[:reference_only_path])
-      title = project.external_issue_tracker.title
+      whitelist[:transformers].push(fix_anchors)
 
-      link_options = html_options.merge(
-        title: "Issue in #{title}",
-        class: "gfm gfm-issue #{html_options[:class]}"
-      )
-      link_to("#{prefix_text}##{identifier}", url, link_options)
+      whitelist
     end
 
     # Turn list items that start with "[ ]" into HTML checkbox inputs.
diff --git a/lib/gitlab/markdown/commit_range_reference_filter.rb b/lib/gitlab/markdown/commit_range_reference_filter.rb
new file mode 100644
index 00000000000..0d93bf4f280
--- /dev/null
+++ b/lib/gitlab/markdown/commit_range_reference_filter.rb
@@ -0,0 +1,127 @@
+require 'html/pipeline'
+
+module Gitlab
+  module Markdown
+    # HTML filter that replaces commit range references with links. References
+    # within <pre>, <code>, <a>, and <style> elements are ignored.
+    #
+    # This filter supports cross-project references.
+    #
+    # Context options:
+    #   :project (required) - Current project, ignored when reference is
+    #                         cross-project.
+    #   :reference_class    - Custom CSS class added to reference links.
+    #   :only_path          - Generate path-only links.
+    #
+    class CommitRangeReferenceFilter < HTML::Pipeline::Filter
+      include CrossProjectReference
+
+      # Public: Find commit range references in text
+      #
+      #   CommitRangeReferenceFilter.references_in(text) do |match, commit_range, project_ref|
+      #     "<a href=...>#{commit_range}</a>"
+      #   end
+      #
+      # text - String text to search.
+      #
+      # Yields the String match, the String commit range, and an optional String
+      # of the external project reference.
+      #
+      # Returns a String replaced with the return of the block.
+      def self.references_in(text)
+        text.gsub(COMMIT_RANGE_PATTERN) do |match|
+          yield match, $~[:commit_range], $~[:project]
+        end
+      end
+
+      # Pattern used to extract commit range references from text
+      #
+      # The beginning and ending SHA1 sums can be between 6 and 40 hex
+      # characters, and the range selection can be double- or triple-dot.
+      #
+      # This pattern supports cross-project references.
+      COMMIT_RANGE_PATTERN = /(#{PROJECT_PATTERN}@)?(?<commit_range>\h{6,40}\.{2,3}\h{6,40})/
+
+      # Don't look for references in text nodes that are children of these
+      # elements.
+      IGNORE_PARENTS = %w(pre code a style).to_set
+
+      def call
+        doc.search('text()').each do |node|
+          content = node.to_html
+
+          next if project.nil?
+          next unless content.match(COMMIT_RANGE_PATTERN)
+          next if has_ancestor?(node, IGNORE_PARENTS)
+
+          html = commit_range_link_filter(content)
+
+          next if html == content
+
+          node.replace(html)
+        end
+
+        doc
+      end
+
+      # Replace commit range references in text with links to compare the commit
+      # ranges.
+      #
+      # text - String text to replace references in.
+      #
+      # Returns a String with commit range references replaced with links. All
+      # links have `gfm` and `gfm-commit_range` class names attached for
+      # styling.
+      def commit_range_link_filter(text)
+        self.class.references_in(text) do |match, commit_range, project_ref|
+          project = self.project_from_ref(project_ref)
+
+          from_id, to_id = split_commit_range(commit_range)
+
+          if valid_range?(project, from_id, to_id)
+            url = url_for_commit_range(project, from_id, to_id)
+
+            title = "Commits #{from_id} through #{to_id}"
+            klass = "gfm gfm-commit_range #{context[:reference_class]}".strip
+
+            project_ref += '@' if project_ref
+
+            %(<a href="#{url}"
+                 title="#{title}"
+                 class="#{klass}">#{project_ref}#{commit_range}</a>)
+          else
+            match
+          end
+        end
+      end
+
+      def validate
+        needs :project
+      end
+
+      def split_commit_range(range)
+        from_id, to_id = range.split(/\.{2,3}/, 2)
+        from_id << "^" if range !~ /\.{3}/
+
+        [from_id, to_id]
+      end
+
+      def valid_range?(project, from_id, to_id)
+        project.valid_repo? &&
+          project.repository.commit(from_id) &&
+          project.repository.commit(to_id)
+      end
+
+      def url_for_commit_range(project, from_id, to_id)
+        h = Rails.application.routes.url_helpers
+        h.namespace_project_compare_url(project.namespace, project,
+                                        from: from_id, to: to_id,
+                                        only_path: context[:only_path])
+      end
+
+      def project
+        context[:project]
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown/commit_reference_filter.rb b/lib/gitlab/markdown/commit_reference_filter.rb
new file mode 100644
index 00000000000..5697d7d97e7
--- /dev/null
+++ b/lib/gitlab/markdown/commit_reference_filter.rb
@@ -0,0 +1,108 @@
+require 'html/pipeline'
+
+module Gitlab
+  module Markdown
+    # HTML filter that replaces commit references with links. References within
+    # <pre>, <code>, <a>, and <style> elements are ignored.
+    #
+    # This filter supports cross-project references.
+    #
+    # Context options:
+    #   :project (required) - Current project, ignored when reference is
+    #                         cross-project.
+    #   :reference_class    - Custom CSS class added to reference links.
+    #   :only_path          - Generate path-only links.
+    #
+    class CommitReferenceFilter < HTML::Pipeline::Filter
+      include CrossProjectReference
+
+      # Public: Find commit references in text
+      #
+      #   CommitReferenceFilter.references_in(text) do |match, commit, project_ref|
+      #     "<a href=...>#{commit}</a>"
+      #   end
+      #
+      # text - String text to search.
+      #
+      # Yields the String match, the String commit identifier, and an optional
+      # String of the external project reference.
+      #
+      # Returns a String replaced with the return of the block.
+      def self.references_in(text)
+        text.gsub(COMMIT_PATTERN) do |match|
+          yield match, $~[:commit], $~[:project]
+        end
+      end
+
+      # Pattern used to extract commit references from text
+      #
+      # The SHA1 sum can be between 6 and 40 hex characters.
+      #
+      # This pattern supports cross-project references.
+      COMMIT_PATTERN = /(#{PROJECT_PATTERN}@)?(?<commit>\h{6,40})/
+
+      # Don't look for references in text nodes that are children of these
+      # elements.
+      IGNORE_PARENTS = %w(pre code a style).to_set
+
+      def call
+        doc.search('text()').each do |node|
+          content = node.to_html
+
+          next if project.nil?
+          next unless content.match(COMMIT_PATTERN)
+          next if has_ancestor?(node, IGNORE_PARENTS)
+
+          html = commit_link_filter(content)
+
+          next if html == content
+
+          node.replace(html)
+        end
+
+        doc
+      end
+
+      def validate
+        needs :project
+      end
+
+      # Replace commit references in text with links to the commit specified.
+      #
+      # text - String text to replace references in.
+      #
+      # Returns a String with commit references replaced with links. All links
+      # have `gfm` and `gfm-commit` class names attached for styling.
+      def commit_link_filter(text)
+        self.class.references_in(text) do |match, commit_ref, project_ref|
+          project = self.project_from_ref(project_ref)
+
+          if project.valid_repo? && commit = project.repository.commit(commit_ref)
+            url = url_for_commit(project, commit)
+
+            title = commit.link_title
+            klass = "gfm gfm-commit #{context[:reference_class]}".strip
+
+            project_ref += '@' if project_ref
+
+            %(<a href="#{url}"
+                 title="#{title}"
+                 class="#{klass}">#{project_ref}#{commit_ref}</a>)
+          else
+            match
+          end
+        end
+      end
+
+      def url_for_commit(project, commit)
+        h = Rails.application.routes.url_helpers
+        h.namespace_project_commit_url(project.namespace, project, commit,
+                                        only_path: context[:only_path])
+      end
+
+      def project
+        context[:project]
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown/cross_project_reference.rb b/lib/gitlab/markdown/cross_project_reference.rb
new file mode 100644
index 00000000000..114247d0a33
--- /dev/null
+++ b/lib/gitlab/markdown/cross_project_reference.rb
@@ -0,0 +1,26 @@
+module Gitlab
+  module Markdown
+    # Includes shared code for reference filters that support an optional
+    # cross-project reference.
+    module CrossProjectReference
+      NAMING_PATTERN  = Gitlab::Regex::NAMESPACE_REGEX_STR
+      PROJECT_PATTERN = "(?<project>#{NAMING_PATTERN}/#{NAMING_PATTERN})"
+
+      # Given a cross-project reference string, get the Project record
+      #
+      # If no valid reference is given, returns the `:project` value for the
+      # current context.
+      #
+      # ref - String reference.
+      #
+      # Returns a Project
+      def project_from_ref(ref)
+        if ref && other = Project.find_with_namespace(ref)
+          other
+        else
+          context[:project]
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown/external_issue_reference_filter.rb b/lib/gitlab/markdown/external_issue_reference_filter.rb
new file mode 100644
index 00000000000..87f58b1ae79
--- /dev/null
+++ b/lib/gitlab/markdown/external_issue_reference_filter.rb
@@ -0,0 +1,94 @@
+require 'html/pipeline'
+
+module Gitlab
+  module Markdown
+    # HTML filter that replaces external issue tracker references with links.
+    # References within <pre>, <code>, <a>, and <style> elements are ignored.
+    #
+    # Context options:
+    #   :project (required) - Current project.
+    #   :reference_class    - Custom CSS class added to reference links.
+    #   :only_path          - Generate path-only links.
+    #
+    class ExternalIssueReferenceFilter < HTML::Pipeline::Filter
+      # Public: Find `JIRA-123` issue references in text
+      #
+      #   ExternalIssueReferenceFilter.references_in(text) do |match, issue|
+      #     "<a href=...>##{issue}</a>"
+      #   end
+      #
+      # text - String text to search.
+      #
+      # Yields the String match and the String issue reference.
+      #
+      # Returns a String replaced with the return of the block.
+      def self.references_in(text)
+        text.gsub(ISSUE_PATTERN) do |match|
+          yield match, $~[:issue]
+        end
+      end
+
+      # Pattern used to extract `JIRA-123` issue references from text
+      ISSUE_PATTERN = /(?<issue>([A-Z\-]+-)\d+)/
+
+      # Don't look for references in text nodes that are children of these
+      # elements.
+      IGNORE_PARENTS = %w(pre code a style).to_set
+
+      def call
+        doc.search('text()').each do |node|
+          content = node.to_html
+
+          next if project.nil?
+          next if project.default_issues_tracker?
+          next unless content.match(ISSUE_PATTERN)
+          next if has_ancestor?(node, IGNORE_PARENTS)
+
+          html = issue_link_filter(content)
+
+          next if html == content
+
+          node.replace(html)
+        end
+
+        doc
+      end
+
+      def validate
+        needs :project
+      end
+
+      # Replace `JIRA-123` issue references in text with links to the referenced
+      # issue's details page.
+      #
+      # text - String text to replace references in.
+      #
+      # Returns a String with `JIRA-123` references replaced with links. All
+      # links have `gfm` and `gfm-issue` class names attached for styling.
+      def issue_link_filter(text)
+        project = context[:project]
+
+        self.class.references_in(text) do |match, issue|
+          url = url_for_issue(issue, project, only_path: context[:only_path])
+
+          title = "Issue in #{project.external_issue_tracker.title}"
+          klass = "gfm gfm-issue #{context[:reference_class]}".strip
+
+          %(<a href="#{url}"
+               title="#{title}"
+               class="#{klass}">#{issue}</a>)
+        end
+      end
+
+      # TODO (rspeicher): Duplicates IssueReferenceFilter
+      def project
+        context[:project]
+      end
+
+      # TODO (rspeicher): Duplicates IssueReferenceFilter
+      def url_for_issue(*args)
+        IssuesHelper.url_for_issue(*args)
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown/issue_reference_filter.rb b/lib/gitlab/markdown/issue_reference_filter.rb
new file mode 100644
index 00000000000..ab0420da199
--- /dev/null
+++ b/lib/gitlab/markdown/issue_reference_filter.rb
@@ -0,0 +1,110 @@
+require 'html/pipeline'
+
+module Gitlab
+  module Markdown
+    # HTML filter that replaces issue references with links. References within
+    # <pre>, <code>, <a>, and <style> elements are ignored. References to issues
+    # that do not exist are ignored.
+    #
+    # This filter supports cross-project references.
+    #
+    # Context options:
+    #   :project (required) - Current project, ignored when reference is
+    #                         cross-project.
+    #   :reference_class    - Custom CSS class added to reference links.
+    #   :only_path          - Generate path-only links.
+    #
+    class IssueReferenceFilter < HTML::Pipeline::Filter
+      include CrossProjectReference
+
+      # Public: Find `#123` issue references in text
+      #
+      #   IssueReferenceFilter.references_in(text) do |match, issue, project_ref|
+      #     "<a href=...>##{issue}</a>"
+      #   end
+      #
+      # text - String text to search.
+      #
+      # Yields the String match, the Integer issue ID, and an optional String of
+      # the external project reference.
+      #
+      # Returns a String replaced with the return of the block.
+      def self.references_in(text)
+        text.gsub(ISSUE_PATTERN) do |match|
+          yield match, $~[:issue].to_i, $~[:project]
+        end
+      end
+
+      # Pattern used to extract `#123` issue references from text
+      #
+      # This pattern supports cross-project references.
+      ISSUE_PATTERN = /#{PROJECT_PATTERN}?\#(?<issue>([a-zA-Z\-]+-)?\d+)/
+
+      # Don't look for references in text nodes that are children of these
+      # elements.
+      IGNORE_PARENTS = %w(pre code a style).to_set
+
+      def call
+        doc.search('text()').each do |node|
+          content = node.to_html
+
+          next if project.nil?
+          next unless project.default_issues_tracker?
+          next unless content.match(ISSUE_PATTERN)
+          next if has_ancestor?(node, IGNORE_PARENTS)
+
+          html = issue_link_filter(content)
+
+          next if html == content
+
+          node.replace(html)
+        end
+
+        doc
+      end
+
+      def validate
+        needs :project
+      end
+
+      # Replace `#123` issue references in text with links to the referenced
+      # issue's details page.
+      #
+      # text - String text to replace references in.
+      #
+      # Returns a String with `#123` references replaced with links. All links
+      # have `gfm` and `gfm-issue` class names attached for styling.
+      def issue_link_filter(text)
+        self.class.references_in(text) do |match, issue, project_ref|
+          project = self.project_from_ref(project_ref)
+
+          if project.issue_exists?(issue)
+            # FIXME: Ugly
+            url = url_for_issue(issue, project, only_path: context[:only_path])
+
+            title = "Issue: #{title_for_issue(issue, project)}"
+            klass = "gfm gfm-issue #{context[:reference_class]}".strip
+
+            %(<a href="#{url}"
+                 title="#{title}"
+                 class="#{klass}">#{project_ref}##{issue}</a>)
+          else
+            match
+          end
+        end
+      end
+
+      def project
+        context[:project]
+      end
+
+      def url_for_issue(*args)
+        IssuesHelper.url_for_issue(*args)
+      end
+
+      def title_for_issue(*args)
+        IssuesHelper.title_for_issue(*args)
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown/label_reference_filter.rb b/lib/gitlab/markdown/label_reference_filter.rb
new file mode 100644
index 00000000000..9753d0ac0f9
--- /dev/null
+++ b/lib/gitlab/markdown/label_reference_filter.rb
@@ -0,0 +1,100 @@
+require 'html/pipeline'
+
+module Gitlab
+  module Markdown
+    # HTML filter that replaces label references with links. References within
+    # <pre>, <code>, <a>, and <style> elements are ignored.
+    #
+    # Context options:
+    #   :project (required) - Current project.
+    #   :reference_class    - Custom CSS class added to reference links.
+    #   :only_path          - Generate path-only links.
+    #
+    class LabelReferenceFilter < HTML::Pipeline::Filter
+      # Public: Find label references in text
+      #
+      #   LabelReferenceFilter.references_in(text) do |match, label|
+      #     "<a href=...>#{label}</a>"
+      #   end
+      #
+      # text - String text to search.
+      #
+      # Yields the String match and the Integer label ID.
+      #
+      # Returns a String replaced with the return of the block.
+      def self.references_in(text)
+        text.gsub(LABEL_PATTERN) do |match|
+          yield match, $~[:label].to_i
+        end
+      end
+
+      # Pattern used to extract label references from text
+      #
+      # This pattern supports cross-project references.
+      LABEL_PATTERN = /~(?<label>\d+)/
+
+      # Don't look for references in text nodes that are children of these
+      # elements.
+      IGNORE_PARENTS = %w(pre code a style).to_set
+
+      def call
+        doc.search('text()').each do |node|
+          content = node.to_html
+
+          next if project.nil?
+          next unless content.match(LABEL_PATTERN)
+          next if has_ancestor?(node, IGNORE_PARENTS)
+
+          html = label_link_filter(content)
+
+          next if html == content
+
+          node.replace(html)
+        end
+
+        doc
+      end
+
+      def validate
+        needs :project
+      end
+
+      # Replace label references in text with links to the label specified.
+      #
+      # text - String text to replace references in.
+      #
+      # Returns a String with label references replaced with links. All links
+      # have `gfm` and `gfm-label` class names attached for styling.
+      def label_link_filter(text)
+        project = context[:project]
+
+        self.class.references_in(text) do |match, id|
+          if label = project.labels.find_by(id: id)
+            url = url_for_label(project, label)
+
+            klass = "gfm gfm-label #{context[:reference_class]}".strip
+
+            %(<a href="#{url}" class="#{klass}">#{render_colored_label(label)}</a>)
+          else
+            match
+          end
+        end
+      end
+
+      def url_for_label(project, label)
+        h = Rails.application.routes.url_helpers
+        h.namespace_project_issues_path(project.namespace, project,
+                                        label_name: label.name,
+                                        only_path: context[:only_path])
+      end
+
+      def render_colored_label(label)
+        LabelsHelper.render_colored_label(label)
+      end
+
+      def project
+        context[:project]
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown/merge_request_reference_filter.rb b/lib/gitlab/markdown/merge_request_reference_filter.rb
new file mode 100644
index 00000000000..96a0c92942d
--- /dev/null
+++ b/lib/gitlab/markdown/merge_request_reference_filter.rb
@@ -0,0 +1,107 @@
+require 'html/pipeline'
+
+module Gitlab
+  module Markdown
+    # HTML filter that replaces merge request references with links. References
+    # within <pre>, <code>, <a>, and <style> elements are ignored. References to
+    # merge requests that do not exist are ignored.
+    #
+    # This filter supports cross-project references.
+    #
+    # Context options:
+    #   :project (required) - Current project, ignored when reference is
+    #                         cross-project.
+    #   :reference_class    - Custom CSS class added to reference links.
+    #   :only_path          - Generate path-only links.
+    #
+    class MergeRequestReferenceFilter < HTML::Pipeline::Filter
+      include CrossProjectReference
+
+      # Public: Find `!123` merge request references in text
+      #
+      #   MergeRequestReferenceFilter.references_in(text) do |match, merge_request, project_ref|
+      #     "<a href=...>##{merge_request}</a>"
+      #   end
+      #
+      # text - String text to search.
+      #
+      # Yields the String match, the Integer merge request ID, and an optional
+      # String of the external project reference.
+      #
+      # Returns a String replaced with the return of the block.
+      def self.references_in(text)
+        text.gsub(MERGE_REQUEST_PATTERN) do |match|
+          yield match, $~[:merge_request].to_i, $~[:project]
+        end
+      end
+
+      # Pattern used to extract `!123` merge request references from text
+      #
+      # This pattern supports cross-project references.
+      MERGE_REQUEST_PATTERN = /#{PROJECT_PATTERN}?!(?<merge_request>\d+)/
+
+      # Don't look for references in text nodes that are children of these
+      # elements.
+      IGNORE_PARENTS = %w(pre code a style).to_set
+
+      def call
+        doc.search('text()').each do |node|
+          content = node.to_html
+
+          next if project.nil?
+          next unless content.match(MERGE_REQUEST_PATTERN)
+          next if has_ancestor?(node, IGNORE_PARENTS)
+
+          html = merge_request_link_filter(content)
+
+          next if html == content
+
+          node.replace(html)
+        end
+
+        doc
+      end
+
+      def validate
+        needs :project
+      end
+
+      # Replace `!123` merge request references in text with links to the
+      # referenced merge request's details page.
+      #
+      # text - String text to replace references in.
+      #
+      # Returns a String with `!123` references replaced with links. All links
+      # have `gfm` and `gfm-merge_request` class names attached for styling.
+      def merge_request_link_filter(text)
+        self.class.references_in(text) do |match, id, project_ref|
+          project = self.project_from_ref(project_ref)
+
+          if merge_request = project.merge_requests.find_by(iid: id)
+            title = "Merge Request: #{merge_request.title}"
+            klass = "gfm gfm-merge_request #{context[:reference_class]}".strip
+
+            url = url_for_merge_request(merge_request, project)
+
+            %(<a href="#{url}"
+                 title="#{title}"
+                 class="#{klass}">#{project_ref}!#{id}</a>)
+          else
+            match
+          end
+        end
+      end
+
+      def project
+        context[:project]
+      end
+
+      # TODO (rspeicher): Cleanup
+      def url_for_merge_request(mr, project)
+        h = Rails.application.routes.url_helpers
+        h.namespace_project_merge_request_url(project.namespace, project, mr,
+                                            only_path: context[:only_path])
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown/snippet_reference_filter.rb b/lib/gitlab/markdown/snippet_reference_filter.rb
new file mode 100644
index 00000000000..0e34ab4bc1e
--- /dev/null
+++ b/lib/gitlab/markdown/snippet_reference_filter.rb
@@ -0,0 +1,98 @@
+require 'html/pipeline'
+
+module Gitlab
+  module Markdown
+    # HTML filter that replaces snippet references with links. References within
+    # <pre>, <code>, <a>, and <style> elements are ignored. References to
+    # snippets that do not exist are ignored.
+    #
+    # Context options:
+    #   :project (required) - Current project.
+    #   :reference_class    - Custom CSS class added to reference links.
+    #   :only_path          - Generate path-only links.
+    #
+    class SnippetReferenceFilter < HTML::Pipeline::Filter
+      # Public: Find `$123` snippet references in text
+      #
+      #   SnippetReferenceFilter.references_in(text) do |match, snippet|
+      #     "<a href=...>$#{snippet}</a>"
+      #   end
+      #
+      # text - String text to search.
+      #
+      # Yields the String match and the Integer snippet ID.
+      #
+      # Returns a String replaced with the return of the block.
+      def self.references_in(text)
+        text.gsub(SNIPPET_PATTERN) do |match|
+          yield match, $~[:snippet].to_i
+        end
+      end
+
+      # Pattern used to extract `$123` snippet references from text
+      SNIPPET_PATTERN = /\$(?<snippet>\d+)/
+
+      # Don't look for references in text nodes that are children of these
+      # elements.
+      IGNORE_PARENTS = %w(pre code a style).to_set
+
+      def call
+        doc.search('text()').each do |node|
+          content = node.to_html
+
+          next if project.nil?
+          next unless content.match(SNIPPET_PATTERN)
+          next if has_ancestor?(node, IGNORE_PARENTS)
+
+          html = snippet_link_filter(content)
+
+          next if html == content
+
+          node.replace(html)
+        end
+
+        doc
+      end
+
+      def validate
+        needs :project
+      end
+
+      # Replace `$123` snippet references in text with links to the referenced
+      # snippets's details page.
+      #
+      # text - String text to replace references in.
+      #
+      # Returns a String with `$123` references replaced with links. All links
+      # have `gfm` and `gfm-snippet` class names attached for styling.
+      def snippet_link_filter(text)
+        project = context[:project]
+
+        self.class.references_in(text) do |match, id|
+          if snippet = project.snippets.find_by(id: id)
+            title = "Snippet: #{snippet.title}"
+            klass = "gfm gfm-snippet #{context[:reference_class]}".strip
+
+            url = url_for_snippet(snippet, project)
+
+            %(<a href="#{url}"
+                 title="#{title}"
+                 class="#{klass}">$#{id}</a>)
+          else
+            match
+          end
+        end
+      end
+
+      def project
+        context[:project]
+      end
+
+      def url_for_snippet(snippet, project)
+        h = Rails.application.routes.url_helpers
+        h.namespace_project_snippet_url(project.namespace, project, snippet,
+                                        only_path: context[:only_path])
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown/user_reference_filter.rb b/lib/gitlab/markdown/user_reference_filter.rb
new file mode 100644
index 00000000000..eaf4094338b
--- /dev/null
+++ b/lib/gitlab/markdown/user_reference_filter.rb
@@ -0,0 +1,117 @@
+require 'html/pipeline'
+
+module Gitlab
+  module Markdown
+    # HTML filter that replaces user or group references with links. References
+    # within <pre>, <code>, <a>, and <style> elements are ignored.
+    #
+    # A special `@all` reference is also supported.
+    #
+    # Context options:
+    #   :project (required) - Current project.
+    #   :reference_class    - Custom CSS class added to reference links.
+    #   :only_path          - Generate path-only links.
+    #
+    class UserReferenceFilter < HTML::Pipeline::Filter
+      # Public: Find `@user` user references in text
+      #
+      #   UserReferenceFilter.references_in(text) do |match, username|
+      #     "<a href=...>@#{user}</a>"
+      #   end
+      #
+      # text - String text to search.
+      #
+      # Yields the String match, and the String user name.
+      #
+      # Returns a String replaced with the return of the block.
+      def self.references_in(text)
+        text.gsub(USER_PATTERN) do |match|
+          yield match, $~[:user]
+        end
+      end
+
+      # Pattern used to extract `@user` user references from text
+      USER_PATTERN = /@(?<user>#{Gitlab::Regex::NAMESPACE_REGEX_STR})/
+
+      # Don't look for references in text nodes that are children of these
+      # elements.
+      IGNORE_PARENTS = %w(pre code a style).to_set
+
+      def call
+        doc.search('text()').each do |node|
+          content = node.to_html
+
+          next if project.nil?
+          next unless content.match(USER_PATTERN)
+          next if has_ancestor?(node, IGNORE_PARENTS)
+
+          html = user_link_filter(content)
+
+          next if html == content
+
+          node.replace(html)
+        end
+
+        doc
+      end
+
+      def validate
+        needs :project
+      end
+
+      # Replace `@user` user references in text with links to the referenced
+      # user's profile page.
+      #
+      # text - String text to replace references in.
+      #
+      # Returns a String with `@user` references replaced with links. All links
+      # have `gfm` and `gfm-project_member` class names attached for styling.
+      def user_link_filter(text)
+        project = context[:project]
+
+        self.class.references_in(text) do |match, user|
+          klass = "gfm gfm-project_member #{context[:reference_class]}".strip
+
+          if user == 'all'
+            url = link_to_all(project)
+
+            %(<a href="#{url}" class="#{klass}">@#{user}</a>)
+          elsif namespace = Namespace.find_by(path: user)
+            if namespace.is_a?(Group)
+              url = group_url(user, only_path: context[:only_path])
+            else
+              url = user_url(user, only_path: context[:only_path])
+            end
+
+            %(<a href="#{url}" class="#{klass}">@#{user}</a>)
+          else
+            match
+          end
+        end
+      end
+
+      def project
+        context[:project]
+      end
+
+      # TODO (rspeicher): Cleanup
+      def group_url(*args)
+        h = Rails.application.routes.url_helpers
+        h.group_url(*args)
+      end
+
+      # TODO (rspeicher): Cleanup
+      def user_url(*args)
+        h = Rails.application.routes.url_helpers
+        h.user_url(*args)
+      end
+
+      # TODO (rspeicher): Cleanup
+      def link_to_all(project)
+        h = Rails.application.routes.url_helpers
+        h.namespace_project_url(project.namespace, project,
+                                only_path: context[:only_path])
+      end
+    end
+  end
+end