Merge branch 'validate-readme-format' into 'master'

Validate README format

Take the first previewable README file as project's README, otherwise if
none file is available, or we can't preview any of them, we assume that
project doesn't have a README file.

Fixes #3978

CHANGELOG

@@ -19,6 +19,7 @@ v 8.4.0 (unreleased)
   - Show 'All' tab by default in the builds page
   - Fix API project lookups when querying with a namespace with dots (Stan Hu)
   - Update version check images to use SVG
+  - Validate README format before displaying
 
 v 8.3.3 (unreleased)
   - Fix project transfer e-mail sending incorrect paths in e-mail notification (Stan Hu)

app/models/tree.rb

@@ -17,17 +17,15 @@ class Tree
   def readme
     return @readme if defined?(@readme)
 
-    available_readmes = blobs.select(&:readme?)
-
-    if available_readmes.count == 0
-      return @readme = nil
+    # Take the first previewable readme, or return nil if none is available or
+    # we can't preview any of them
+    readme_tree = blobs.find do |blob|
+      blob.readme? && (previewable?(blob.name) || plain?(blob.name))
     end
 
-    # Take the first previewable readme, or the first available readme, if we
-    # can't preview any of them
-    readme_tree = available_readmes.find do |readme|
-      previewable?(readme.name)
-    end || available_readmes.first
+    if readme_tree.nil?
+      return @readme = nil
+    end
 
     readme_path = path == '/' ? readme_tree.name : File.join(path, readme_tree.name)