Implement diff viewers

8 files changed, 733 insertions, 2 deletions

diff --git a/spec/features/expand_collapse_diffs_spec.rb b/spec/features/expand_collapse_diffs_spec.rb
index c4d5077e5e1..07521f50968 100644
--- a/spec/features/expand_collapse_diffs_spec.rb
+++ b/spec/features/expand_collapse_diffs_spec.rb
@@ -262,7 +262,7 @@ feature 'Expand and collapse diffs', js: true, feature: true do
 
       # Wait for elements to appear to ensure full page reload
       expect(page).to have_content('This diff was suppressed by a .gitattributes entry')
-      expect(page).to have_content('This diff could not be displayed because it is too large.')
+      expect(page).to have_content('This source diff could not be displayed because it is too large.')
       expect(page).to have_content('too_large_image.jpg')
       find('.note-textarea')
 
diff --git a/spec/features/projects/blobs/blob_show_spec.rb b/spec/features/projects/blobs/blob_show_spec.rb
index 45fdb36e506..71ffa352f80 100644
--- a/spec/features/projects/blobs/blob_show_spec.rb
+++ b/spec/features/projects/blobs/blob_show_spec.rb
@@ -17,6 +17,7 @@ feature 'File blob', :js, feature: true do
     it 'displays the blob' do
       aggregate_failures do
         # shows highlighted Ruby code
+        expect(page).to have_css(".js-syntax-highlight")
         expect(page).to have_content("require 'fileutils'")
 
         # does not show a viewer switcher
@@ -71,6 +72,7 @@ feature 'File blob', :js, feature: true do
             expect(page).to have_selector('.blob-viewer[data-type="rich"]', visible: false)
 
             # shows highlighted Markdown code
+            expect(page).to have_css(".js-syntax-highlight")
             expect(page).to have_content("[PEP-8](http://www.python.org/dev/peps/pep-0008/)")
 
             # shows an enabled copy button
@@ -114,6 +116,7 @@ feature 'File blob', :js, feature: true do
           expect(page).to have_selector('#LC1.hll')
 
           # shows highlighted Markdown code
+          expect(page).to have_css(".js-syntax-highlight")
           expect(page).to have_content("[PEP-8](http://www.python.org/dev/peps/pep-0008/)")
 
           # shows an enabled copy button
diff --git a/spec/features/projects/diffs/diff_show_spec.rb b/spec/features/projects/diffs/diff_show_spec.rb
new file mode 100644
index 00000000000..48b7f1e0f34
--- /dev/null
+++ b/spec/features/projects/diffs/diff_show_spec.rb
@@ -0,0 +1,133 @@
+require 'spec_helper'
+
+feature 'Diff file viewer', :js, feature: true do
+  let(:project) { create(:project, :public, :repository) }
+
+  def visit_commit(sha, anchor: nil)
+    visit namespace_project_commit_path(project.namespace, project, sha, anchor: anchor)
+
+    wait_for_requests
+  end
+
+  context 'Ruby file' do
+    before do
+      visit_commit('570e7b2abdd848b95f2f578043fc23bd6f6fd24d')
+    end
+
+    it 'shows highlighted Ruby code' do
+      within('.diff-file[id="2f6fcd96b88b36ce98c38da085c795a27d92a3dd"]') do
+        expect(page).to have_css(".js-syntax-highlight")
+        expect(page).to have_content("def popen(cmd, path=nil)")
+      end
+    end
+  end
+
+  context 'Ruby file (stored in LFS)' do
+    before do
+      project.add_master(project.creator)
+
+      @commit_id = Files::CreateService.new(
+        project,
+        project.creator,
+        start_branch: 'master',
+        branch_name: 'master',
+        commit_message: "Add Ruby file in LFS",
+        file_path: 'files/lfs/ruby.rb',
+        file_content: project.repository.blob_at('master', 'files/lfs/lfs_object.iso').data
+      ).execute[:result]
+    end
+
+    context 'when LFS is enabled on the project' do
+      before do
+        allow(Gitlab.config.lfs).to receive(:enabled).and_return(true)
+        project.update_attribute(:lfs_enabled, true)
+
+        visit_commit(@commit_id)
+      end
+
+      it 'shows an error message' do
+        expect(page).to have_content('This source diff could not be displayed because it is stored in LFS. You can view the blob instead.')
+      end
+    end
+
+    context 'when LFS is disabled on the project' do
+      before do
+        visit_commit(@commit_id)
+      end
+
+      it 'displays the diff' do
+        expect(page).to have_content('size 1575078')
+      end
+    end
+  end
+
+  context 'Image file' do
+    before do
+      visit_commit('2f63565e7aac07bcdadb654e253078b727143ec4')
+    end
+
+    it 'shows a rendered image' do
+      within('.diff-file[id="e986451b8f7397b617dbb6fffcb5539328c56921"]') do
+        expect(page).to have_css('img[alt="files/images/6049019_460s.jpg"]')
+      end
+    end
+  end
+
+  context 'ISO file (stored in LFS)' do
+    context 'when LFS is enabled on the project' do
+      before do
+        allow(Gitlab.config.lfs).to receive(:enabled).and_return(true)
+        project.update_attribute(:lfs_enabled, true)
+
+        visit_commit('048721d90c449b244b7b4c53a9186b04330174ec')
+      end
+
+      it 'shows that file was added' do
+        expect(page).to have_content('File added')
+      end
+    end
+
+    context 'when LFS is disabled on the project' do
+      before do
+        visit_commit('048721d90c449b244b7b4c53a9186b04330174ec')
+      end
+
+      it 'displays the diff' do
+        expect(page).to have_content('size 1575078')
+      end
+    end
+  end
+
+  context 'ZIP file' do
+    before do
+      visit_commit('ae73cb07c9eeaf35924a10f713b364d32b2dd34f')
+    end
+
+    it 'shows that file was added' do
+      expect(page).to have_content('File added')
+    end
+  end
+
+  context 'binary file that appears to be text in the first 1024 bytes' do
+    before do
+      visit_commit('7b1cf4336b528e0f3d1d140ee50cafdbc703597c')
+    end
+
+    it 'shows the diff is collapsed' do
+      expect(page).to have_content('This diff is collapsed. Click to expand it.')
+    end
+
+    context 'expanding the diff' do
+      before do
+        # We can't use `click_link` because the "link" doesn't have an `href`.
+        find('a.click-to-expand').click
+
+        wait_for_requests
+      end
+
+      it 'shows there is no preview' do
+        expect(page).to have_content('No preview for this file type')
+      end
+    end
+  end
+end
diff --git a/spec/helpers/diff_helper_spec.rb b/spec/helpers/diff_helper_spec.rb
index a74615e07f9..0ac030d3171 100644
--- a/spec/helpers/diff_helper_spec.rb
+++ b/spec/helpers/diff_helper_spec.rb
@@ -8,7 +8,7 @@ describe DiffHelper do
   let(:commit) { project.commit(sample_commit.id) }
   let(:diffs) { commit.raw_diffs }
   let(:diff) { diffs.first }
-  let(:diff_refs) { [commit.parent, commit] }
+  let(:diff_refs) { commit.diff_refs }
   let(:diff_file) { Gitlab::Diff::File.new(diff, diff_refs: diff_refs, repository: repository) }
 
   describe 'diff_view' do
@@ -207,4 +207,41 @@ describe DiffHelper do
       expect(output).not_to have_css 'td:nth-child(3)'
     end
   end
+
+  context 'viewer related' do
+    let(:viewer) { diff_file.simple_viewer }
+
+    before do
+      assign(:project, project)
+    end
+
+    describe '#diff_render_error_reason' do
+      context 'for error :too_large' do
+        before do
+          expect(viewer).to receive(:render_error).and_return(:too_large)
+        end
+
+        it 'returns an error message' do
+          expect(helper.diff_render_error_reason(viewer)).to eq('it is too large')
+        end
+      end
+
+      context 'for error :server_side_but_stored_externally' do
+        before do
+          expect(viewer).to receive(:render_error).and_return(:server_side_but_stored_externally)
+          expect(diff_file).to receive(:external_storage).and_return(:lfs)
+        end
+
+        it 'returns an error message' do
+          expect(helper.diff_render_error_reason(viewer)).to eq('it is stored in LFS')
+        end
+      end
+    end
+
+    describe '#diff_render_error_options' do
+      it 'includes a "view the blob" link' do
+        expect(helper.diff_render_error_options(viewer)).to include(/view the blob/)
+      end
+    end
+  end
 end
diff --git a/spec/lib/gitlab/diff/file_spec.rb b/spec/lib/gitlab/diff/file_spec.rb
index a9953bb0d01..f289131cc3a 100644
--- a/spec/lib/gitlab/diff/file_spec.rb
+++ b/spec/lib/gitlab/diff/file_spec.rb
@@ -92,4 +92,305 @@ describe Gitlab::Diff::File, lib: true do
       expect(diff_file.diffable?).to be_falsey
     end
   end
+
+  describe '#content_changed?' do
+    context 'when created' do
+      let(:commit) { project.commit('33f3729a45c02fc67d00adb1b8bca394b0e761d9') }
+      let(:diff_file) { commit.diffs.diff_file_with_new_path('files/images/6049019_460s.jpg') }
+
+      it 'returns false' do
+        expect(diff_file.content_changed?).to be_falsey
+      end
+    end
+
+    context 'when deleted' do
+      let(:commit) { project.commit('d59c60028b053793cecfb4022de34602e1a9218e') }
+      let(:diff_file) { commit.diffs.diff_file_with_old_path('files/js/commit.js.coffee') }
+
+      it 'returns false' do
+        expect(diff_file.content_changed?).to be_falsey
+      end
+    end
+
+    context 'when renamed' do
+      let(:commit) { project.commit('6907208d755b60ebeacb2e9dfea74c92c3449a1f') }
+      let(:diff_file) { commit.diffs.diff_file_with_new_path('files/js/commit.coffee') }
+
+      before do
+        allow(diff_file.new_blob).to receive(:id).and_return(diff_file.old_blob.id)
+      end
+
+      it 'returns false' do
+        expect(diff_file.content_changed?).to be_falsey
+      end
+    end
+
+    context 'when content changed' do
+      context 'when binary' do
+        let(:commit) { project.commit('2f63565e7aac07bcdadb654e253078b727143ec4') }
+        let(:diff_file) { commit.diffs.diff_file_with_new_path('files/images/6049019_460s.jpg') }
+
+        it 'returns true' do
+          expect(diff_file.content_changed?).to be_truthy
+        end
+      end
+
+      context 'when not binary' do
+        let(:commit) { project.commit('570e7b2abdd848b95f2f578043fc23bd6f6fd24d') }
+        let(:diff_file) { commit.diffs.diff_file_with_new_path('files/ruby/popen.rb') }
+
+        it 'returns true' do
+          expect(diff_file.content_changed?).to be_truthy
+        end
+      end
+    end
+  end
+
+  describe '#simple_viewer' do
+    context 'when the file is not diffable' do
+      before do
+        allow(diff_file).to receive(:diffable?).and_return(false)
+      end
+
+      it 'returns a Not Diffable viewer' do
+        expect(diff_file.simple_viewer).to be_a(DiffViewer::NotDiffable)
+      end
+    end
+
+    context 'when the content changed' do
+      context 'when the file represented by the diff file is binary' do
+        before do
+          allow(diff_file).to receive(:raw_binary?).and_return(true)
+        end
+
+        it 'returns a No Preview viewer' do
+          expect(diff_file.simple_viewer).to be_a(DiffViewer::NoPreview)
+        end
+      end
+
+      context 'when the diff file old and new blob types are different' do
+        before do
+          allow(diff_file).to receive(:different_type?).and_return(true)
+        end
+
+        it 'returns a No Preview viewer' do
+          expect(diff_file.simple_viewer).to be_a(DiffViewer::NoPreview)
+        end
+      end
+
+      context 'when the file represented by the diff file is text-based' do
+        it 'returns a text viewer' do
+          expect(diff_file.simple_viewer).to be_a(DiffViewer::Text)
+        end
+      end
+    end
+
+    context 'when created' do
+      let(:commit) { project.commit('913c66a37b4a45b9769037c55c2d238bd0942d2e') }
+      let(:diff_file) { commit.diffs.diff_file_with_new_path('files/ruby/popen.rb') }
+
+      before do
+        allow(diff_file).to receive(:content_changed?).and_return(nil)
+      end
+
+      context 'when the file represented by the diff file is binary' do
+        before do
+          allow(diff_file).to receive(:raw_binary?).and_return(true)
+        end
+
+        it 'returns an Added viewer' do
+          expect(diff_file.simple_viewer).to be_a(DiffViewer::Added)
+        end
+      end
+
+      context 'when the diff file old and new blob types are different' do
+        before do
+          allow(diff_file).to receive(:different_type?).and_return(true)
+        end
+
+        it 'returns an Added viewer' do
+          expect(diff_file.simple_viewer).to be_a(DiffViewer::Added)
+        end
+      end
+
+      context 'when the file represented by the diff file is text-based' do
+        it 'returns a text viewer' do
+          expect(diff_file.simple_viewer).to be_a(DiffViewer::Text)
+        end
+      end
+    end
+
+    context 'when deleted' do
+      let(:commit) { project.commit('d59c60028b053793cecfb4022de34602e1a9218e') }
+      let(:diff_file) { commit.diffs.diff_file_with_old_path('files/js/commit.js.coffee') }
+
+      before do
+        allow(diff_file).to receive(:content_changed?).and_return(nil)
+      end
+
+      context 'when the file represented by the diff file is binary' do
+        before do
+          allow(diff_file).to receive(:raw_binary?).and_return(true)
+        end
+
+        it 'returns a Deleted viewer' do
+          expect(diff_file.simple_viewer).to be_a(DiffViewer::Deleted)
+        end
+      end
+
+      context 'when the diff file old and new blob types are different' do
+        before do
+          allow(diff_file).to receive(:different_type?).and_return(true)
+        end
+
+        it 'returns a Deleted viewer' do
+          expect(diff_file.simple_viewer).to be_a(DiffViewer::Deleted)
+        end
+      end
+
+      context 'when the file represented by the diff file is text-based' do
+        it 'returns a text viewer' do
+          expect(diff_file.simple_viewer).to be_a(DiffViewer::Text)
+        end
+      end
+    end
+
+    context 'when renamed' do
+      let(:commit) { project.commit('6907208d755b60ebeacb2e9dfea74c92c3449a1f') }
+      let(:diff_file) { commit.diffs.diff_file_with_new_path('files/js/commit.coffee') }
+
+      before do
+        allow(diff_file).to receive(:content_changed?).and_return(nil)
+      end
+
+      it 'returns a Renamed viewer' do
+        expect(diff_file.simple_viewer).to be_a(DiffViewer::Renamed)
+      end
+    end
+
+    context 'when mode changed' do
+      before do
+        allow(diff_file).to receive(:content_changed?).and_return(nil)
+        allow(diff_file).to receive(:mode_changed?).and_return(true)
+      end
+
+      it 'returns a Mode Changed viewer' do
+        expect(diff_file.simple_viewer).to be_a(DiffViewer::ModeChanged)
+      end
+    end
+  end
+
+  describe '#rich_viewer' do
+    let(:commit) { project.commit('2f63565e7aac07bcdadb654e253078b727143ec4') }
+    let(:diff_file) { commit.diffs.diff_file_with_new_path('files/images/6049019_460s.jpg') }
+
+    context 'when the diff file has a matching viewer' do
+      context 'when the diff file content did not change' do
+        before do
+          allow(diff_file).to receive(:content_changed?).and_return(false)
+        end
+
+        it 'returns nil' do
+          expect(diff_file.rich_viewer).to be_nil
+        end
+      end
+
+      context 'when the diff file is not diffable' do
+        before do
+          allow(diff_file).to receive(:diffable?).and_return(false)
+        end
+
+        it 'returns nil' do
+          expect(diff_file.rich_viewer).to be_nil
+        end
+      end
+
+      context 'when the diff file old and new blob types are different' do
+        before do
+          allow(diff_file).to receive(:different_type?).and_return(true)
+        end
+
+        it 'returns nil' do
+          expect(diff_file.rich_viewer).to be_nil
+        end
+      end
+
+      context 'when the diff file has an external storage error' do
+        before do
+          allow(diff_file).to receive(:external_storage_error?).and_return(true)
+        end
+
+        it 'returns nil' do
+          expect(diff_file.rich_viewer).to be_nil
+        end
+      end
+
+      context 'when everything is right' do
+        it 'returns the viewer' do
+          expect(diff_file.rich_viewer).to be_a(DiffViewer::Image)
+        end
+      end
+    end
+
+    context 'when the diff file does not have a matching viewer' do
+      let(:commit) { project.commit('570e7b2abdd848b95f2f578043fc23bd6f6fd24d') }
+      let(:diff_file) { commit.diffs.diff_file_with_new_path('files/ruby/popen.rb') }
+
+      it 'returns nil' do
+        expect(diff_file.rich_viewer).to be_nil
+      end
+    end
+  end
+
+  describe '#rendered_as_text?' do
+    context 'when the simple viewer is text-based' do
+      let(:commit) { project.commit('570e7b2abdd848b95f2f578043fc23bd6f6fd24d') }
+      let(:diff_file) { commit.diffs.diff_file_with_new_path('files/ruby/popen.rb') }
+
+      context 'when ignoring errors' do
+        context 'when the viewer has render errors' do
+          before do
+            diff_file.diff.too_large!
+          end
+
+          it 'returns true' do
+            expect(diff_file.rendered_as_text?).to be_truthy
+          end
+        end
+
+        context "when the viewer doesn't have render errors" do
+          it 'returns true' do
+            expect(diff_file.rendered_as_text?).to be_truthy
+          end
+        end
+      end
+
+      context 'when not ignoring errors' do
+        context 'when the viewer has render errors' do
+          before do
+            diff_file.diff.too_large!
+          end
+
+          it 'returns false' do
+            expect(diff_file.rendered_as_text?(ignore_errors: false)).to be_falsey
+          end
+        end
+
+        context "when the viewer doesn't have render errors" do
+          it 'returns true' do
+            expect(diff_file.rendered_as_text?(ignore_errors: false)).to be_truthy
+          end
+        end
+      end
+    end
+
+    context 'when the simple viewer is binary' do
+      let(:commit) { project.commit('2f63565e7aac07bcdadb654e253078b727143ec4') }
+      let(:diff_file) { commit.diffs.diff_file_with_new_path('files/images/6049019_460s.jpg') }
+
+      it 'returns false' do
+        expect(diff_file.rendered_as_text?).to be_falsey
+      end
+    end
+  end
 end
diff --git a/spec/models/diff_viewer/base_spec.rb b/spec/models/diff_viewer/base_spec.rb
new file mode 100644
index 00000000000..3755f4a56f3
--- /dev/null
+++ b/spec/models/diff_viewer/base_spec.rb
@@ -0,0 +1,150 @@
+require 'spec_helper'
+
+describe DiffViewer::Base, model: true do
+  include FakeBlobHelpers
+
+  let(:project) { create(:project, :repository) }
+  let(:commit) { project.commit('570e7b2abdd848b95f2f578043fc23bd6f6fd24d') }
+  let(:diff_file) { commit.diffs.diff_file_with_new_path('files/ruby/popen.rb') }
+
+  let(:viewer_class) do
+    Class.new(described_class) do
+      include DiffViewer::ServerSide
+
+      self.extensions = %w(jpg)
+      self.binary = true
+      self.collapse_limit = 1.megabyte
+      self.size_limit = 5.megabytes
+    end
+  end
+
+  let(:viewer) { viewer_class.new(diff_file) }
+
+  describe '.can_render?' do
+    context 'when the extension is supported' do
+      let(:commit) { project.commit('2f63565e7aac07bcdadb654e253078b727143ec4') }
+      let(:diff_file) { commit.diffs.diff_file_with_new_path('files/images/6049019_460s.jpg') }
+
+      context 'when the binaryness matches' do
+        it 'returns true' do
+          expect(viewer_class.can_render?(diff_file)).to be_truthy
+        end
+      end
+
+      context 'when the binaryness does not match' do
+        before do
+          allow(diff_file.old_blob).to receive(:binary?).and_return(false)
+          allow(diff_file.new_blob).to receive(:binary?).and_return(false)
+        end
+
+        it 'returns false' do
+          expect(viewer_class.can_render?(diff_file)).to be_falsey
+        end
+      end
+    end
+
+    context 'when the file type is supported' do
+      let(:commit) { project.commit('1a0b36b3cdad1d2ee32457c102a8c0b7056fa863') }
+      let(:diff_file) { commit.diffs.diff_file_with_new_path('LICENSE') }
+
+      before do
+        viewer_class.file_types = %i(license)
+        viewer_class.binary = false
+      end
+
+      context 'when the binaryness matches' do
+        it 'returns true' do
+          expect(viewer_class.can_render?(diff_file)).to be_truthy
+        end
+      end
+
+      context 'when the binaryness does not match' do
+        before do
+          allow(diff_file.old_blob).to receive(:binary?).and_return(true)
+          allow(diff_file.new_blob).to receive(:binary?).and_return(true)
+        end
+
+        it 'returns false' do
+          expect(viewer_class.can_render?(diff_file)).to be_falsey
+        end
+      end
+    end
+
+    context 'when the extension and file type are not supported' do
+      it 'returns false' do
+        expect(viewer_class.can_render?(diff_file)).to be_falsey
+      end
+    end
+
+    context 'when the file was renamed and only the old blob is supported' do
+      let(:commit) { project.commit('2f63565e7aac07bcdadb654e253078b727143ec4') }
+      let(:diff_file) { commit.diffs.diff_file_with_new_path('files/images/6049019_460s.jpg') }
+
+      before do
+        allow(diff_file).to receive(:renamed_file?).and_return(true)
+        allow(diff_file.new_blob).to receive(:extension).and_return('jpeg')
+      end
+
+      it 'returns false' do
+        expect(viewer_class.can_render?(diff_file)).to be_falsey
+      end
+    end
+  end
+
+  describe '#collapsed?' do
+    context 'when the combined blob size is larger than the collapse limit' do
+      before do
+        allow(diff_file.old_blob).to receive(:raw_size).and_return(512.kilobytes)
+        allow(diff_file.new_blob).to receive(:raw_size).and_return(513.kilobytes)
+      end
+
+      it 'returns true' do
+        expect(viewer.collapsed?).to be_truthy
+      end
+    end
+
+    context 'when the combined blob size is smaller than the collapse limit' do
+      it 'returns false' do
+        expect(viewer.collapsed?).to be_falsey
+      end
+    end
+  end
+
+  describe '#too_large?' do
+    context 'when the combined blob size is larger than the size limit' do
+      before do
+        allow(diff_file.old_blob).to receive(:raw_size).and_return(2.megabytes)
+        allow(diff_file.new_blob).to receive(:raw_size).and_return(4.megabytes)
+      end
+
+      it 'returns true' do
+        expect(viewer.too_large?).to be_truthy
+      end
+    end
+
+    context 'when the blob size is smaller than the size limit' do
+      it 'returns false' do
+        expect(viewer.too_large?).to be_falsey
+      end
+    end
+  end
+
+  describe '#render_error' do
+    context 'when the combined blob size is larger than the size limit' do
+      before do
+        allow(diff_file.old_blob).to receive(:raw_size).and_return(2.megabytes)
+        allow(diff_file.new_blob).to receive(:raw_size).and_return(4.megabytes)
+      end
+
+      it 'returns :too_large' do
+        expect(viewer.render_error).to eq(:too_large)
+      end
+    end
+
+    context 'when the combined blob size is smaller than the size limit' do
+      it 'returns nil' do
+        expect(viewer.render_error).to be_nil
+      end
+    end
+  end
+end
diff --git a/spec/models/diff_viewer/server_side_spec.rb b/spec/models/diff_viewer/server_side_spec.rb
new file mode 100644
index 00000000000..2d926e06936
--- /dev/null
+++ b/spec/models/diff_viewer/server_side_spec.rb
@@ -0,0 +1,36 @@
+require 'spec_helper'
+
+describe DiffViewer::ServerSide, model: true do
+  let(:project) { create(:project, :repository) }
+  let(:commit) { project.commit('570e7b2abdd848b95f2f578043fc23bd6f6fd24d') }
+  let(:diff_file) { commit.diffs.diff_file_with_new_path('files/ruby/popen.rb') }
+
+  let(:viewer_class) do
+    Class.new(DiffViewer::Base) do
+      include DiffViewer::ServerSide
+    end
+  end
+
+  subject { viewer_class.new(diff_file) }
+
+  describe '#prepare!' do
+    it 'loads all diff file data' do
+      expect(diff_file.old_blob).to receive(:load_all_data!)
+      expect(diff_file.new_blob).to receive(:load_all_data!)
+
+      subject.prepare!
+    end
+  end
+
+  describe '#render_error' do
+    context 'when the diff file is stored externally' do
+      before do
+        allow(diff_file).to receive(:stored_externally?).and_return(true)
+      end
+
+      it 'return :server_side_but_stored_externally' do
+        expect(subject.render_error).to eq(:server_side_but_stored_externally)
+      end
+    end
+  end
+end
diff --git a/spec/views/projects/diffs/_viewer.html.haml_spec.rb b/spec/views/projects/diffs/_viewer.html.haml_spec.rb
new file mode 100644
index 00000000000..32469202508
--- /dev/null
+++ b/spec/views/projects/diffs/_viewer.html.haml_spec.rb
@@ -0,0 +1,71 @@
+require 'spec_helper'
+
+describe 'projects/diffs/_viewer.html.haml', :view do
+  include FakeBlobHelpers
+
+  let(:project) { create(:project, :repository) }
+  let(:commit) { project.commit('570e7b2abdd848b95f2f578043fc23bd6f6fd24d') }
+  let(:diff_file) { commit.diffs.diff_file_with_new_path('files/ruby/popen.rb') }
+
+  let(:viewer_class) do
+    Class.new(DiffViewer::Base) do
+      include DiffViewer::Rich
+
+      self.partial_name = 'text'
+    end
+  end
+
+  let(:viewer) { viewer_class.new(diff_file) }
+
+  before do
+    assign(:project, project)
+
+    controller.params[:controller] = 'projects/commit'
+    controller.params[:action] = 'show'
+    controller.params[:namespace_id] = project.namespace.to_param
+    controller.params[:project_id] = project.to_param
+    controller.params[:id] = commit.id
+  end
+
+  def render_view
+    render partial: 'projects/diffs/viewer', locals: { viewer: viewer }
+  end
+
+  context 'when there is a render error' do
+    before do
+      allow(viewer).to receive(:render_error).and_return(:too_large)
+    end
+
+    it 'renders the error' do
+      render_view
+
+      expect(view).to render_template('projects/diffs/_render_error')
+    end
+  end
+
+  context 'when the viewer is collapsed' do
+    before do
+      allow(diff_file).to receive(:collapsed?).and_return(true)
+    end
+
+    it 'renders the collapsed view' do
+      render_view
+
+      expect(view).to render_template('projects/diffs/_collapsed')
+    end
+  end
+
+  context 'when there is no render error' do
+    it 'prepares the viewer' do
+      expect(viewer).to receive(:prepare!)
+
+      render_view
+    end
+
+    it 'renders the viewer' do
+      render_view
+
+      expect(view).to render_template('projects/diffs/viewers/_text')
+    end
+  end
+end