Cache highlighted diff lines for merge requests

Introducing the concept of SafeDiffs which relates 
diffs with UI highlighting.

21 files changed, 174 insertions, 30 deletions

diff --git a/app/controllers/concerns/diff_for_path.rb b/app/controllers/concerns/diff_for_path.rb
index 026d8b2e1e0..aeec3009f15 100644
--- a/app/controllers/concerns/diff_for_path.rb
+++ b/app/controllers/concerns/diff_for_path.rb
@@ -1,8 +1,8 @@
 module DiffForPath
   extend ActiveSupport::Concern
 
-  def render_diff_for_path(diffs, diff_refs, project)
-    diff_file = safe_diff_files(diffs, diff_refs: diff_refs, repository: project.repository).find do |diff|
+  def render_diff_for_path(diffs)
+    diff_file = diffs.diff_files.find do |diff|
       diff.old_path == params[:old_path] && diff.new_path == params[:new_path]
     end
 
@@ -14,7 +14,7 @@ module DiffForPath
     locals = {
       diff_file: diff_file,
       diff_commit: diff_commit,
-      diff_refs: diff_refs,
+      diff_refs: diffs.diff_refs,
       blob: blob,
       project: project
     }
diff --git a/app/controllers/projects/commit_controller.rb b/app/controllers/projects/commit_controller.rb
index 7ae034f9398..6060b6e55bc 100644
--- a/app/controllers/projects/commit_controller.rb
+++ b/app/controllers/projects/commit_controller.rb
@@ -28,7 +28,7 @@ class Projects::CommitController < Projects::ApplicationController
   end
 
   def diff_for_path
-    render_diff_for_path(@diffs, @commit.diff_refs, @project)
+    render_diff_for_path(SafeDiffs::Commit.new(@commit, diff_options: diff_options))
   end
 
   def builds
@@ -110,7 +110,7 @@ class Projects::CommitController < Projects::ApplicationController
     opts = diff_options
     opts[:ignore_whitespace_change] = true if params[:format] == 'diff'
 
-    @diffs = commit.diffs(opts)
+    @diffs = SafeDiffs::Commit.new(commit, diff_options: opts)
     @notes_count = commit.notes.count
   end
 
diff --git a/app/controllers/projects/compare_controller.rb b/app/controllers/projects/compare_controller.rb
index 8c004724f02..2eda950a1bd 100644
--- a/app/controllers/projects/compare_controller.rb
+++ b/app/controllers/projects/compare_controller.rb
@@ -21,7 +21,7 @@ class Projects::CompareController < Projects::ApplicationController
   def diff_for_path
     return render_404 unless @compare
 
-    render_diff_for_path(@diffs, @diff_refs, @project)
+    render_diff_for_path(SafeDiffs::Compare.new(@compare, project: @project, diff_options: diff_options))
   end
 
   def create
@@ -46,12 +46,12 @@ class Projects::CompareController < Projects::ApplicationController
       @commit = @project.commit(@head_ref)
       @base_commit = @project.merge_base_commit(@start_ref, @head_ref)
 
-      @diffs = @compare.diffs(diff_options)
-      @diff_refs = Gitlab::Diff::DiffRefs.new(
+      diff_refs = Gitlab::Diff::DiffRefs.new(
         base_sha: @base_commit.try(:sha),
         start_sha: @start_commit.try(:sha),
         head_sha: @commit.try(:sha)
       )
+      @diffs = SafeDiffs::Compare.new(@compare, project: @project, diff_options: diff_options, diff_refs: diff_refs)
 
       @diff_notes_disabled = true
       @grouped_diff_discussions = {}
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 116e7904a4e..78a6a3c5715 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -103,9 +103,8 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     end
 
     define_commit_vars
-    diffs = @merge_request.diffs(diff_options)
 
-    render_diff_for_path(diffs, @merge_request.diff_refs, @merge_request.project)
+    render_diff_for_path(SafeDiffs::MergeRequest.new(merge_request, diff_options: diff_options))
   end
 
   def commits
@@ -153,7 +152,12 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     @commits = @merge_request.compare_commits.reverse
     @commit = @merge_request.diff_head_commit
     @base_commit = @merge_request.diff_base_commit
-    @diffs = @merge_request.compare.diffs(diff_options) if @merge_request.compare
+    if @merge_request.compare
+      @diffs = SafeDiffs::Compare.new(@merge_request.compare,
+        project:      @merge_request.project,
+        diff_refs:    @merge_request.diff_refs,
+        diff_options: diff_options)
+    end
     @diff_notes_disabled = true
 
     @pipeline = @merge_request.pipeline
diff --git a/app/helpers/commits_helper.rb b/app/helpers/commits_helper.rb
index f497626e21a..7a02d0b10d9 100644
--- a/app/helpers/commits_helper.rb
+++ b/app/helpers/commits_helper.rb
@@ -206,10 +206,10 @@ module CommitsHelper
     end
   end
 
-  def view_file_btn(commit_sha, diff, project)
+  def view_file_btn(commit_sha, diff_new_path, project)
     link_to(
       namespace_project_blob_path(project.namespace, project,
-                                  tree_join(commit_sha, diff.new_path)),
+                                  tree_join(commit_sha, diff_new_path)),
       class: 'btn view-file js-view-file btn-file-option'
     ) do
       raw('View file @') + content_tag(:span, commit_sha[0..6],
diff --git a/app/helpers/diff_helper.rb b/app/helpers/diff_helper.rb
index f35e2f6ddcd..6497282af57 100644
--- a/app/helpers/diff_helper.rb
+++ b/app/helpers/diff_helper.rb
@@ -23,18 +23,17 @@ module DiffHelper
   end
 
   def diff_options
-    options = { ignore_whitespace_change: hide_whitespace?, no_collapse: expand_all_diffs? }
+    options = SafeDiffs.default_options.merge(
+      ignore_whitespace_change: hide_whitespace?,
+      no_collapse: expand_all_diffs?
+    )
 
     if action_name == 'diff_for_path'
       options[:no_collapse] = true
       options[:paths] = params.values_at(:old_path, :new_path)
     end
 
-    Commit.max_diff_options.merge(options)
-  end
-
-  def safe_diff_files(diffs, diff_refs: nil, repository: nil)
-    diffs.decorate! { |diff| Gitlab::Diff::File.new(diff, diff_refs: diff_refs, repository: repository) }
+    options
   end
 
   def unfold_bottom_class(bottom)
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index a99c4ba52a4..774851cc90f 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -313,6 +313,8 @@ class MergeRequest < ActiveRecord::Base
 
     merge_request_diff.reload_content
 
+    MergeRequests::MergeRequestDiffCacheService.new.execute(self)
+
     new_diff_refs = self.diff_refs
 
     update_diff_notes_positions(
diff --git a/app/models/safe_diffs.rb b/app/models/safe_diffs.rb
new file mode 100644
index 00000000000..8ca9ec4cc39
--- /dev/null
+++ b/app/models/safe_diffs.rb
@@ -0,0 +1,5 @@
+module SafeDiffs
+  def self.default_options
+    ::Commit.max_diff_options.merge(ignore_whitespace_change: false, no_collapse: false)
+  end
+end
diff --git a/app/models/safe_diffs/base.rb b/app/models/safe_diffs/base.rb
new file mode 100644
index 00000000000..dfc4708e293
--- /dev/null
+++ b/app/models/safe_diffs/base.rb
@@ -0,0 +1,55 @@
+module SafeDiffs
+  class Base
+    attr_reader :project, :diff_options, :diff_view, :diff_refs
+
+    delegate :count, :real_size, to: :diff_files
+
+    def initialize(diffs, project:, diff_options:, diff_refs: nil)
+      @diffs = diffs
+      @project = project
+      @diff_options = diff_options
+      @diff_refs = diff_refs
+    end
+
+    def diff_files
+      @diff_files ||= begin
+        diffs = @diffs.decorate! do |diff|
+          Gitlab::Diff::File.new(diff, diff_refs: @diff_refs, repository: @project.repository)
+        end
+
+        highlight!(diffs)
+        diffs
+      end
+    end
+
+    private
+
+    def highlight!(diff_files)
+      if cacheable?
+        cache_highlight!(diff_files)
+      else
+        diff_files.each { |diff_file| highlight_diff_file!(diff_file) }
+      end
+    end
+
+    def cacheable?
+      false
+    end
+
+    def cache_highlight!
+      raise NotImplementedError
+    end
+
+    def highlight_diff_file_from_cache!(diff_file, cache_diff_lines)
+      diff_file.diff_lines = cache_diff_lines.map do |line|
+        Gitlab::Diff::Line.init_from_hash(line)
+      end
+    end
+
+    def highlight_diff_file!(diff_file)
+      diff_file.diff_lines = Gitlab::Diff::Highlight.new(diff_file, repository: diff_file.repository).highlight
+      diff_file.highlighted_diff_lines = diff_file.diff_lines # To be used on parallel diff
+      diff_file
+    end
+  end
+end
diff --git a/app/models/safe_diffs/commit.rb b/app/models/safe_diffs/commit.rb
new file mode 100644
index 00000000000..338878f32e0
--- /dev/null
+++ b/app/models/safe_diffs/commit.rb
@@ -0,0 +1,10 @@
+module SafeDiffs
+  class Commit < Base
+    def initialize(commit, diff_options:)
+      super(commit.diffs(diff_options),
+        project: commit.project,
+        diff_options: diff_options,
+        diff_refs: commit.diff_refs)
+    end
+  end
+end
diff --git a/app/models/safe_diffs/compare.rb b/app/models/safe_diffs/compare.rb
new file mode 100644
index 00000000000..6b64b81137d
--- /dev/null
+++ b/app/models/safe_diffs/compare.rb
@@ -0,0 +1,10 @@
+module SafeDiffs
+  class Compare < Base
+    def initialize(compare, project:, diff_options:, diff_refs: nil)
+      super(compare.diffs(diff_options),
+        project: project,
+        diff_options: diff_options,
+        diff_refs: diff_refs)
+    end
+  end
+end
diff --git a/app/models/safe_diffs/merge_request.rb b/app/models/safe_diffs/merge_request.rb
new file mode 100644
index 00000000000..111b9a54f91
--- /dev/null
+++ b/app/models/safe_diffs/merge_request.rb
@@ -0,0 +1,52 @@
+module SafeDiffs
+  class MergeRequest < Base
+    def initialize(merge_request, diff_options:)
+      @merge_request = merge_request
+
+      super(merge_request.diffs(diff_options),
+        project: merge_request.project,
+        diff_options: diff_options,
+        diff_refs: merge_request.diff_refs)
+    end
+
+    private
+
+    #
+    # If we find the highlighted diff files lines on the cache we replace existing diff_files lines (no highlighted)
+    # for the highlighted ones, so we just skip their execution.
+    # If the highlighted diff files lines are not cached we calculate and cache them.
+    #
+    # The content of the cache is and Hash where the key correspond to the file_path and the values are Arrays of
+    # hashes than represent serialized diff lines.
+    #
+    def cache_highlight!(diff_files)
+      highlighted_cache = Rails.cache.read(cache_key) || {}
+      highlighted_cache_was_empty = highlighted_cache.empty?
+
+      diff_files.each do |diff_file|
+        file_path = diff_file.file_path
+
+        if highlighted_cache[file_path]
+          highlight_diff_file_from_cache!(diff_file, highlighted_cache[file_path])
+        else
+          highlight_diff_file!(diff_file)
+          highlighted_cache[file_path] = diff_file.diff_lines.map(&:to_hash)
+        end
+      end
+
+      if highlighted_cache_was_empty
+        Rails.cache.write(cache_key, highlighted_cache)
+      end
+
+      diff_files
+    end
+
+    def cacheable?
+      @merge_request.merge_request_diff.present?
+    end
+
+    def cache_key
+      [@merge_request.merge_request_diff, 'highlighted-safe-diff-files', diff_options]
+    end
+  end
+end
diff --git a/app/services/merge_requests/merge_request_diff_cache_service.rb b/app/services/merge_requests/merge_request_diff_cache_service.rb
new file mode 100644
index 00000000000..0a1905f7137
--- /dev/null
+++ b/app/services/merge_requests/merge_request_diff_cache_service.rb
@@ -0,0 +1,8 @@
+module MergeRequests
+  class MergeRequestDiffCacheService
+    def execute(merge_request)
+      # Executing the iteration we cache all the highlighted diff information
+      SafeDiffs::MergeRequest.new(merge_request, diff_options: SafeDiffs.default_options).diff_files.to_a
+    end
+  end
+end
diff --git a/app/views/notify/repository_push_email.html.haml b/app/views/notify/repository_push_email.html.haml
index c161ecc3463..2d1a98caeaa 100644
--- a/app/views/notify/repository_push_email.html.haml
+++ b/app/views/notify/repository_push_email.html.haml
@@ -75,7 +75,7 @@
             - blob = diff_file.blob
             - if blob && blob.respond_to?(:text?) && blob_text_viewable?(blob)
               %table.code.white
-                - diff_file.highlighted_diff_lines.each do |line|
+                - diff_file.diff_lines.each do |line|
                   = render "projects/diffs/line", line: line, diff_file: diff_file, plain: true
             - else
               No preview for this file type
diff --git a/app/views/projects/commit/show.html.haml b/app/views/projects/commit/show.html.haml
index d0da2606587..11b2020f99b 100644
--- a/app/views/projects/commit/show.html.haml
+++ b/app/views/projects/commit/show.html.haml
@@ -7,7 +7,7 @@
   = render "ci_menu"
 - else
   %div.block-connector
-= render "projects/diffs/diffs", diffs: @diffs, project: @project, diff_refs: @commit.diff_refs
+= render "projects/diffs/diffs", diff_files: @diffs.diff_files, project: @diffs.project, diff_refs: @diffs.diff_refs
 = render "projects/notes/notes_with_form"
 - if can_collaborate_with_project?
   - %w(revert cherry-pick).each do |type|
diff --git a/app/views/projects/compare/show.html.haml b/app/views/projects/compare/show.html.haml
index 28a50e7031a..eb8a1bd5289 100644
--- a/app/views/projects/compare/show.html.haml
+++ b/app/views/projects/compare/show.html.haml
@@ -8,7 +8,7 @@
 
   - if @commits.present?
     = render "projects/commits/commit_list"
-    = render "projects/diffs/diffs", diffs: @diffs, project: @project, diff_refs: @diff_refs
+    = render "projects/diffs/diffs", diff_files: @diffs.diff_files, project: @diffs.project, diff_refs: @diffs.diff_refs
   - else
     .light-well
       .center
diff --git a/app/views/projects/diffs/_diffs.html.haml b/app/views/projects/diffs/_diffs.html.haml
index 4bf3ccace20..45895a9a3de 100644
--- a/app/views/projects/diffs/_diffs.html.haml
+++ b/app/views/projects/diffs/_diffs.html.haml
@@ -2,8 +2,6 @@
 - if diff_view == 'parallel'
   - fluid_layout true
 
-- diff_files = safe_diff_files(diffs, diff_refs: diff_refs, repository: project.repository)
-
 .content-block.oneline-block.files-changed
   .inline-parallel-buttons
     - if !expand_all_diffs? && diff_files.any? { |diff_file| diff_file.collapsed? }
diff --git a/app/views/projects/diffs/_file.html.haml b/app/views/projects/diffs/_file.html.haml
index 1854c64cbd7..f914e13a1ec 100644
--- a/app/views/projects/diffs/_file.html.haml
+++ b/app/views/projects/diffs/_file.html.haml
@@ -15,6 +15,6 @@
                 from_merge_request_id: @merge_request.id,
                 skip_visible_check: true)
 
-        = view_file_btn(diff_commit.id, diff_file, project)
+        = view_file_btn(diff_commit.id, diff_file.new_path, project)
 
-  = render 'projects/diffs/content', diff_file: diff_file, diff_commit: diff_commit, diff_refs: diff_refs, blob: blob, project: project
+  = render 'projects/diffs/content', diff_file: diff_file, diff_commit: diff_commit, blob: blob, project: project
diff --git a/app/views/projects/diffs/_text_file.html.haml b/app/views/projects/diffs/_text_file.html.haml
index 5970b9abf2b..a483927671e 100644
--- a/app/views/projects/diffs/_text_file.html.haml
+++ b/app/views/projects/diffs/_text_file.html.haml
@@ -5,7 +5,7 @@
 
 %table.text-file.code.js-syntax-highlight{ data: diff_view_data, class: too_big ? 'hide' : '' }
   - last_line = 0
-  - diff_file.highlighted_diff_lines.each do |line|
+  - diff_file.diff_lines.each do |line|
     - last_line = line.new_pos
     = render "projects/diffs/line", line: line, diff_file: diff_file
 
diff --git a/app/views/projects/merge_requests/_new_submit.html.haml b/app/views/projects/merge_requests/_new_submit.html.haml
index a5e67b95727..cb2b623691c 100644
--- a/app/views/projects/merge_requests/_new_submit.html.haml
+++ b/app/views/projects/merge_requests/_new_submit.html.haml
@@ -42,7 +42,7 @@
           %h4 This comparison includes more than #{MergeRequestDiff::COMMITS_SAFE_SIZE} commits.
           %p To preserve performance the line changes are not shown.
       - else
-        = render "projects/diffs/diffs", diffs: @diffs, project: @project, diff_refs: @merge_request.diff_refs, show_whitespace_toggle: false
+        = render "projects/diffs/diffs", diff_files: @diffs.diff_files, project: @diffs.project, diff_refs: @merge_request.diff_refs, show_whitespace_toggle: false
     - if @pipeline
       #builds.builds.tab-pane
         = render "projects/merge_requests/show/builds"
diff --git a/app/views/projects/merge_requests/show/_diffs.html.haml b/app/views/projects/merge_requests/show/_diffs.html.haml
index 1b0bae86ad4..ed2765356db 100644
--- a/app/views/projects/merge_requests/show/_diffs.html.haml
+++ b/app/views/projects/merge_requests/show/_diffs.html.haml
@@ -1,6 +1,7 @@
 - if @merge_request_diff.collected?
-  = render "projects/diffs/diffs", diffs: @merge_request.diffs(diff_options),
-    project: @merge_request.project, diff_refs: @merge_request.diff_refs
+  - diffs = SafeDiffs::MergeRequest.new(@merge_request, diff_options: diff_options)
+  = render "projects/diffs/diffs", diff_files: diffs.diff_files,
+    diff_refs: diffs.diff_refs, project: diffs.project
 - elsif @merge_request_diff.empty?
   .nothing-here-block Nothing to merge from #{@merge_request.source_branch} into #{@merge_request.target_branch}
 - else