Ensure that we only request blobs in one batch

Blob.lazy adds a blob to a batch to load at a later point, using the
BatchLoader library. Whenever any lazy blob's attributes are accessed,
all lazy blobs requested to that point will be loaded.

BatchLoader, the library we use for this, should only request items in a
batch once. That is, if we have these batches:

1. a, b, c
2. d, e, f

Then a, b, and c should only be requested in the first batch. But if you
modify the list of items in the batch, then the second batch will
request a, b, c, d, e, f, which is almost certainly not what we want!

https://github.com/exAspArk/batch-loader/issues/44 is the upstream issue
for this, but we can also solve this in our application by not modifying
the arguments we're using inside a BatchLoader batch.

changelogs/unreleased/fix-lazy-blobs-requesting-all-previous-blobs.yml

@@ -0,0 +1,6 @@
+---
+title: Fix Blob.lazy always loading all previously-requested blobs when a new request
+  is made
+merge_request:
+author:
+type: performance

lib/gitlab/gitaly_client/blob_service.rb

@@ -55,13 +55,13 @@ module Gitlab
       def get_blobs(revision_paths, limit = -1)
         return [] if revision_paths.empty?
 
-        revision_paths.map! do |rev, path|
+        request_revision_paths = revision_paths.map do |rev, path|
           Gitaly::GetBlobsRequest::RevisionPath.new(revision: rev, path: encode_binary(path))
         end
 
         request = Gitaly::GetBlobsRequest.new(
           repository: @gitaly_repo,
-          revision_paths: revision_paths,
+          revision_paths: request_revision_paths,
           limit: limit
         )
 

spec/models/blob_spec.rb

@@ -43,6 +43,21 @@ describe Blob do
       changelog.id
       contributing.id
     end
+
+    it 'does not include blobs from previous requests in later requests' do
+      changelog = described_class.lazy(project, commit_id, 'CHANGELOG')
+      contributing = described_class.lazy(same_project, commit_id, 'CONTRIBUTING.md')
+
+      # Access property so the values are loaded
+      changelog.id
+      contributing.id
+
+      readme = described_class.lazy(project, commit_id, 'README.md')
+
+      expect(project.repository).to receive(:blobs_at).with([[commit_id, 'README.md']]).once.and_call_original
+
+      readme.id
+    end
   end
 
   describe '#data' do