Use cached readme blobs where appropriate

GitLab keeps a cache of the rendered HTML for a repository's README as
stored in the HEAD branch. However, it was not used in all
circumstances. In particular, the new blob viewer framework bypassed
this cache entirely.

This MR ensures a ::ReadmeBlob is returned instead of a ::Blob when
asking a repository for an individual blob, if the commit and path
match the readme for HEAD. This makes the cached HTML available to
consumers, including the blob viewer.

The ReadmeBlob is a simple delegator to the Blob, so should be
compatible in all cases. Adding the rendered_markdown method is the
only additional behaviour it contains.

2 files changed, 43 insertions, 27 deletions

diff --git a/app/models/repository.rb b/app/models/repository.rb
index 6ce480c32c4..37a1dd64052 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -487,7 +487,20 @@ class Repository
   end
 
   def blob_at(sha, path)
-    Blob.decorate(raw_repository.blob_at(sha, path), project)
+    blob = Blob.decorate(raw_repository.blob_at(sha, path), project)
+
+    # Don't attempt to return a special result if there is no blob at all
+    return unless blob
+
+    # Don't attempt to return a special result unless we're looking at HEAD
+    return blob unless head_commit&.sha == sha
+
+    case path
+    when head_tree&.readme_path
+      ReadmeBlob.new(blob, self)
+    else
+      blob
+    end
   rescue Gitlab::Git::Repository::NoRepository
     nil
   end
@@ -569,9 +582,7 @@ class Repository
   cache_method :merge_request_template_names, fallback: []
 
   def readme
-    if readme = tree(:head)&.readme
-      ReadmeBlob.new(readme, self)
-    end
+    head_tree&.readme
   end
 
   def rendered_readme
diff --git a/app/models/tree.rb b/app/models/tree.rb
index 3641c33254c..cd385872171 100644
--- a/app/models/tree.rb
+++ b/app/models/tree.rb
@@ -2,6 +2,7 @@
 
 class Tree
   include Gitlab::MarkupHelper
+  include Gitlab::Utils::StrongMemoize
 
   attr_accessor :repository, :sha, :path, :entries
 
@@ -16,32 +17,36 @@ class Tree
     @entries = Gitlab::Git::Tree.where(git_repo, @sha, @path, recursive)
   end
 
-  def readme
-    return @readme if defined?(@readme)
-
-    available_readmes = blobs.select do |blob|
-      Gitlab::FileDetector.type_of(blob.name) == :readme
-    end
-
-    previewable_readmes = available_readmes.select do |blob|
-      previewable?(blob.name)
-    end
-
-    plain_readmes = available_readmes.select do |blob|
-      plain?(blob.name)
+  def readme_path
+    strong_memoize(:readme_path) do
+      available_readmes = blobs.select do |blob|
+        Gitlab::FileDetector.type_of(blob.name) == :readme
+      end
+
+      previewable_readmes = available_readmes.select do |blob|
+        previewable?(blob.name)
+      end
+
+      plain_readmes = available_readmes.select do |blob|
+        plain?(blob.name)
+      end
+
+      # Prioritize previewable over plain readmes
+      entry = previewable_readmes.first || plain_readmes.first
+      next nil unless entry
+
+      if path == '/'
+        entry.name
+      else
+        File.join(path, entry.name)
+      end
     end
+  end
 
-    # Prioritize previewable over plain readmes
-    readme_tree = previewable_readmes.first || plain_readmes.first
-
-    # Return if we can't preview any of them
-    if readme_tree.nil?
-      return @readme = nil
+  def readme
+    strong_memoize(:readme) do
+      repository.blob_at(sha, readme_path) if readme_path
     end
-
-    readme_path = path == '/' ? readme_tree.name : File.join(path, readme_tree.name)
-
-    @readme = repository.blob_at(sha, readme_path)
   end
 
   def trees