Truncate tree to max 1,000 items and display notice to users

Rendering ten thousands of tree items consumes a lot of server
time and can cause timeouts in extreme cases. Realistically,
displaying more than 1,000 files is probably not useful so
truncate and show the user a notice instead. 'Find files' can be
used to locate specific files beyond the 1,000 limit.

app/assets/stylesheets/pages/tree.scss

@@ -125,7 +125,7 @@
         color: $white-normal;
       }
-      &:hover {
+      &:hover:not(.tree-truncated-warning) {
         td {
           background-color: $row-hover;
           border-top: 1px solid $row-hover-border;
@@ -198,6 +198,11 @@
     }
   }
+  .tree-truncated-warning {
+    color: $orange-600;
+    background-color: $orange-100;
+  }
+
   .tree-time-ago {
     min-width: 135px;
     color: $gl-text-color-secondary;

app/helpers/tree_helper.rb

 module TreeHelper
+  FILE_LIMIT = 1_000
+
   # Sorts a repository's tree so that folders are before files and renders
   # their corresponding partials
   #
-  # contents - A Grit::Tree object for the current tree
+  # tree - A `Tree` object for the current tree
   def render_tree(tree)
     # Sort submodules and folders together by name ahead of files
     folders, files, submodules = tree.trees, tree.blobs, tree.submodules
-    tree = ""
+    tree = ''
     items = (folders + submodules).sort_by(&:name) + files
-    tree << render(partial: "projects/tree/tree_row", collection: items) if items.present?
+
+    if items.size > FILE_LIMIT
+      tree << render(partial: 'projects/tree/truncated_notice_tree_row',
+                     locals: { limit: FILE_LIMIT, total: items.size })
+      items = items.take(FILE_LIMIT)
+    end
+
+    tree << render(partial: 'projects/tree/tree_row', collection: items) if items.present?
     tree.html_safe
   end

app/views/projects/tree/_truncated_notice_tree_row.html.haml

+%tr.tree-truncated-warning
+  %td{ colspan: '3' }
+    = icon('exclamation-triangle fw')
+    %span
+      Too many items to show. To preserve performance only
+      %strong #{number_with_delimiter(limit)} of #{number_with_delimiter(total)}
+      items are displayed.

changelogs/unreleased/tree_item_limit.yml

+---
+title: Truncate tree to max 1,000 items and display notice to users
+merge_request:
+author:
+type: performance

spec/helpers/tree_helper_spec.rb

 require 'spec_helper'
 describe TreeHelper do
+  let(:project) { create(:project, :repository) }
+  let(:repository) { project.repository }
+  let(:sha) { 'ce369011c189f62c815f5971d096b26759bab0d1' }
+
+  describe '.render_tree' do
+    before do
+      @id = sha
+      @project = project
+    end
+
+    it 'displays all entries without a warning' do
+      tree = repository.tree(sha, 'files')
+
+      html = render_tree(tree)
+
+      expect(html).not_to have_selector('.tree-truncated-warning')
+    end
+
+    it 'truncates entries and adds a warning' do
+      stub_const('TreeHelper::FILE_LIMIT', 1)
+      tree = repository.tree(sha, 'files')
+
+      html = render_tree(tree)
+
+      expect(html).to have_selector('.tree-truncated-warning', count: 1)
+      expect(html).to have_selector('.tree-item-file-name', count: 1)
+    end
+  end
+
   describe 'flatten_tree' do
-    let(:project) { create(:project, :repository) }
-    let(:repository) { project.repository }
-    let(:sha) { 'ce369011c189f62c815f5971d096b26759bab0d1' }
     let(:tree) { repository.tree(sha, 'files') }
     let(:root_path) { 'files' }
     let(:tree_item) { tree.entries.find { |entry| entry.path == path } }