Expose metrics element for FE consumption

Adds GFM Pipline filters to insert a placeholder in the generated
HTML from GFM based on the presence of a metrics dashboard link.

The front end should look for the class 'js-render-metrics' to
determine if it should replace the element with metrics charts.
The data element 'data-dashboard-url' should be the endpoint
the front end should hit in order to obtain a dashboard layout
in order to appropriately render the charts.

5 files changed, 210 insertions, 0 deletions

diff --git a/lib/banzai/filter/inline_embeds_filter.rb b/lib/banzai/filter/inline_embeds_filter.rb
new file mode 100644
index 00000000000..97394fd8f82
--- /dev/null
+++ b/lib/banzai/filter/inline_embeds_filter.rb
@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+module Banzai
+  module Filter
+    # HTML filter that inserts a node for each occurence of
+    # a given link format. To transform references to DB
+    # resources in place, prefer to inherit from AbstractReferenceFilter.
+    class InlineEmbedsFilter < HTML::Pipeline::Filter
+      # Find every relevant link, create a new node based on
+      # the link, and insert this node after any html content
+      # surrounding the link.
+      def call
+        return doc unless Feature.enabled?(:gfm_embedded_metrics, context[:project])
+
+        doc.xpath(xpath_search).each do |node|
+          next unless element = element_to_embed(node)
+
+          # We want this to follow any surrounding content. For example,
+          # if a link is inline in a paragraph.
+          node.parent.children.last.add_next_sibling(element)
+        end
+
+        doc
+      end
+
+      # Implement in child class.
+      #
+      # Return a Nokogiri::XML::Element to embed in the
+      # markdown.
+      def create_element(params)
+      end
+
+      # Implement in child class unless overriding #embed_params
+      #
+      # Returns the regex pattern used to filter
+      # to only matching urls.
+      def link_pattern
+      end
+
+      # Returns the xpath query string used to select nodes
+      # from the html document on which the embed is based.
+      #
+      # Override to select nodes other than links.
+      def xpath_search
+        'descendant-or-self::a[@href]'
+      end
+
+      # Creates a new element based on the parameters
+      # obtained from the target link
+      def element_to_embed(node)
+        return unless params = embed_params(node)
+
+        create_element(params)
+      end
+
+      # Returns a hash of named parameters based on the
+      # provided regex with string keys.
+      #
+      # Override to select nodes other than links.
+      def embed_params(node)
+        url = node['href']
+
+        link_pattern.match(url) { |m| m.named_captures }
+      end
+    end
+  end
+end
diff --git a/lib/banzai/filter/inline_metrics_filter.rb b/lib/banzai/filter/inline_metrics_filter.rb
new file mode 100644
index 00000000000..0120cc37d6f
--- /dev/null
+++ b/lib/banzai/filter/inline_metrics_filter.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+module Banzai
+  module Filter
+    # HTML filter that inserts a placeholder element for each
+    # reference to a metrics dashboard.
+    class InlineMetricsFilter < Banzai::Filter::InlineEmbedsFilter
+      # Placeholder element for the frontend to use as an
+      # injection point for charts.
+      def create_element(params)
+        doc.document.create_element(
+          'div',
+          class: 'js-render-metrics',
+          'data-dashboard-url': metrics_dashboard_url(params)
+        )
+      end
+
+      # Endpoint FE should hit to collect the appropriate
+      # chart information
+      def metrics_dashboard_url(params)
+        Gitlab::Metrics::Dashboard::Url.build_dashboard_url(
+          params['namespace'],
+          params['project'],
+          params['environment'],
+          embedded: true
+        )
+      end
+
+      # Search params for selecting metrics links. A few
+      # simple checks is enough to boost performance without
+      # the cost of doing a full regex match.
+      def xpath_search
+        "descendant-or-self::a[contains(@href,'metrics') and \
+          starts-with(@href, '#{Gitlab.config.gitlab.url}')]"
+      end
+
+      # Regular expression matching metrics urls
+      def link_pattern
+        Gitlab::Metrics::Dashboard::Url.regex
+      end
+    end
+  end
+end
diff --git a/lib/banzai/filter/inline_metrics_redactor_filter.rb b/lib/banzai/filter/inline_metrics_redactor_filter.rb
new file mode 100644
index 00000000000..ff91be2cbb7
--- /dev/null
+++ b/lib/banzai/filter/inline_metrics_redactor_filter.rb
@@ -0,0 +1,98 @@
+# frozen_string_literal: true
+
+module Banzai
+  module Filter
+    # HTML filter that removes embeded elements that the current user does
+    # not have permission to view.
+    class InlineMetricsRedactorFilter < HTML::Pipeline::Filter
+      include Gitlab::Utils::StrongMemoize
+
+      METRICS_CSS_CLASS = '.js-render-metrics'
+
+      # Finds all embeds based on the css class the FE
+      # uses to identify the embedded content, removing
+      # only unnecessary nodes.
+      def call
+        return doc unless Feature.enabled?(:gfm_embedded_metrics, context[:project])
+
+        nodes.each do |node|
+          path = paths_by_node[node]
+          user_has_access = user_access_by_path[path]
+
+          node.remove unless user_has_access
+        end
+
+        doc
+      end
+
+      private
+
+      def user
+        context[:current_user]
+      end
+
+      # Returns all nodes which the FE will identify as
+      # a metrics dashboard placeholder element
+      #
+      # @return [Nokogiri::XML::NodeSet]
+      def nodes
+        @nodes ||= doc.css(METRICS_CSS_CLASS)
+      end
+
+      # Maps a node to the full path of a project.
+      # Memoized so we only need to run the regex to get
+      # the project full path from the url once per node.
+      #
+      # @return [Hash<Nokogiri::XML::Node, String>]
+      def paths_by_node
+        strong_memoize(:paths_by_node) do
+          nodes.each_with_object({}) do |node, paths|
+            paths[node] = path_for_node(node)
+          end
+        end
+      end
+
+      # Gets a project's full_path from the dashboard url
+      # in the placeholder node. The FE will use the attr
+      # `data-dashboard-url`, so we want to check against that
+      # attribute directly in case a user has manually
+      # created a metrics element (rather than supporting
+      # an alternate attr in InlineMetricsFilter).
+      #
+      # @return [String]
+      def path_for_node(node)
+        url = node.attribute('data-dashboard-url').to_s
+
+        Gitlab::Metrics::Dashboard::Url.regex.match(url) do |m|
+          "#{$~[:namespace]}/#{$~[:project]}"
+        end
+      end
+
+      # Maps a project's full path to a Project object.
+      # Contains all of the Projects referenced in the
+      # metrics placeholder elements of the current document
+      #
+      # @return [Hash<String, Project>]
+      def projects_by_path
+        strong_memoize(:projects_by_path) do
+          Project.eager_load(:route, namespace: [:route])
+            .where_full_path_in(paths_by_node.values.uniq)
+            .index_by(&:full_path)
+        end
+      end
+
+      # Returns a mapping representing whether the current user
+      # has permission to view the metrics for the project.
+      # Determined in a batch
+      #
+      # @return [Hash<Project, Boolean>]
+      def user_access_by_path
+        strong_memoize(:user_access_by_path) do
+          projects_by_path.each_with_object({}) do |(path, project), access|
+            access[path] = Ability.allowed?(user, :read_environment, project)
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/banzai/pipeline/gfm_pipeline.rb b/lib/banzai/pipeline/gfm_pipeline.rb
index d67f461be57..2c1006f708a 100644
--- a/lib/banzai/pipeline/gfm_pipeline.rb
+++ b/lib/banzai/pipeline/gfm_pipeline.rb
@@ -25,6 +25,7 @@ module Banzai
           Filter::VideoLinkFilter,
           Filter::ImageLazyLoadFilter,
           Filter::ImageLinkFilter,
+          Filter::InlineMetricsFilter,
           Filter::TableOfContentsFilter,
           Filter::AutolinkFilter,
           Filter::ExternalLinkFilter,
diff --git a/lib/banzai/pipeline/post_process_pipeline.rb b/lib/banzai/pipeline/post_process_pipeline.rb
index 7eaad6d7560..5c199453638 100644
--- a/lib/banzai/pipeline/post_process_pipeline.rb
+++ b/lib/banzai/pipeline/post_process_pipeline.rb
@@ -13,6 +13,7 @@ module Banzai
       def self.internal_link_filters
         [
           Filter::RedactorFilter,
+          Filter::InlineMetricsRedactorFilter,
           Filter::RelativeLinkFilter,
           Filter::IssuableStateFilter,
           Filter::SuggestionFilter