Add a `Blob` model that wraps `Gitlab::Git::Blob`

This allows us to take advantage of Rails' `to_partial_path` to render
the correct partial based on the Blob type, rather than cluttering the
view with conditionals.

It also allows (and will allow in the future) better encapsulation for
Blob-related logic which makes sense for our Rails app but might not
make as much sense for the core `gitlab_git` library, such as detecting
if the blob is an SVG.

app/controllers/projects/blob_controller.rb

@@ -87,7 +87,7 @@ class Projects::BlobController < Projects::ApplicationController
   private
 
   def blob
-    @blob ||= @repository.blob_at(@commit.id, @path)
+    @blob ||= Blob.decorate(@repository.blob_at(@commit.id, @path))
 
     if @blob
       @blob

app/helpers/blob_helper.rb

@@ -127,10 +127,6 @@ module BlobHelper
     end
   end
 
-  def blob_svg?(blob)
-    blob.language && blob.language.name == 'SVG'
-  end
-
   # SVGs can contain malicious JavaScript; only include whitelisted
   # elements and attributes. Note that this whitelist is by no means complete
   # and may omit some elements.

app/models/blob.rb

+# Blob is a Rails-specific wrapper around Gitlab::Git::Blob objects
+class Blob < SimpleDelegator
+  # Wrap a Gitlab::Git::Blob object, or return nil when given nil
+  #
+  # This method prevents the decorated object from evaluating to "truthy" when
+  # given a nil value. For example:
+  #
+  #     blob = Blob.new(nil)
+  #     puts "truthy" if blob # => "truthy"
+  #
+  #     blob = Blob.decorate(nil)
+  #     puts "truthy" if blob # No output
+  def self.decorate(blob)
+    return if blob.nil?
+
+    new(blob)
+  end
+
+  def svg?
+    text? && language && language.name == 'SVG'
+  end
+
+  def to_partial_path
+    if lfs_pointer?
+      'download'
+    elsif image? || svg?
+      'image'
+    elsif text?
+      'text'
+    else
+      'download'
+    end
+  end
+end

app/views/projects/blob/_blob.html.haml

@@ -32,14 +32,4 @@
         = number_to_human_size(blob_size(blob))
       .file-actions.hidden-xs
         = render "actions"
-    - if blob.lfs_pointer?
-      = render "download", blob: blob
-    - elsif blob.text?
-      - if blob_svg?(blob)
-        = render "image", blob: blob
-      - else
-        = render "text", blob: blob
-    - elsif blob.image?
-      = render "image", blob: blob
-    - else
-      = render "download", blob: blob
+    = render blob, blob: blob

app/views/projects/blob/_image.html.haml

 .file-content.image_file
-  - if blob_svg?(blob)
+  - if blob.svg?
     - # We need to scrub SVG but we cannot do so in the RawController: it would
     - # be wrong/strange if RawController modified the data.
     - blob.load_all_data!(@repository)

spec/models/blob_spec.rb

+require 'rails_helper'
+
+describe Blob do
+  describe '.decorate' do
+    it 'returns NilClass when given nil' do
+      expect(described_class.decorate(nil)).to be_nil
+    end
+  end
+
+  describe '#svg?' do
+    it 'is falsey when not text' do
+      git_blob = double(text?: false)
+
+      expect(described_class.decorate(git_blob)).not_to be_svg
+    end
+
+    it 'is falsey when no language is detected' do
+      git_blob = double(text?: true, language: nil)
+
+      expect(described_class.decorate(git_blob)).not_to be_svg
+    end
+
+    it' is falsey when language is not SVG' do
+      git_blob = double(text?: true, language: double(name: 'XML'))
+
+      expect(described_class.decorate(git_blob)).not_to be_svg
+    end
+
+    it 'is truthy when language is SVG' do
+      git_blob = double(text?: true, language: double(name: 'SVG'))
+
+      expect(described_class.decorate(git_blob)).to be_svg
+    end
+  end
+
+  describe '#to_partial_path' do
+    def stubbed_blob(overrides = {})
+      overrides.reverse_merge!(
+        image?: false,
+        language: nil,
+        lfs_pointer?: false,
+        svg?: false,
+        text?: false
+      )
+
+      described_class.decorate(double).tap do |blob|
+        allow(blob).to receive_messages(overrides)
+      end
+    end
+
+    it 'handles LFS pointers' do
+      blob = stubbed_blob(lfs_pointer?: true)
+
+      expect(blob.to_partial_path).to eq 'download'
+    end
+
+    it 'handles SVGs' do
+      blob = stubbed_blob(text?: true, svg?: true)
+
+      expect(blob.to_partial_path).to eq 'image'
+    end
+
+    it 'handles images' do
+      blob = stubbed_blob(image?: true)
+
+      expect(blob.to_partial_path).to eq 'image'
+    end
+
+    it 'handles text' do
+      blob = stubbed_blob(text?: true)
+
+      expect(blob.to_partial_path).to eq 'text'
+    end
+
+    it 'defaults to download' do
+      blob = stubbed_blob
+
+      expect(blob.to_partial_path).to eq 'download'
+    end
+  end
+end