JSON-encoded binary fields should use Base64::encode64

It is not interoperable to use Base64::urlsafe_encode64 for fields in
JSON that contain binary (as opposed to UTF8-safe) data. For instance,
the Golang JSON decoder (which is what gitlab-workhorse uses) insists
upon the standard base64 encoding.

lib/gitlab/workhorse.rb

@@ -229,12 +229,17 @@ module Gitlab
 
       protected
 
+      # This is the outermost encoding of a senddata: header. It is safe for
+      # inclusion in HTTP response headers
       def encode(hash)
         Base64.urlsafe_encode64(JSON.dump(hash))
       end
 
+      # This is for encoding individual fields inside the senddata JSON that
+      # contain binary data. In workhorse, the corresponding struct field should
+      # be type []byte
       def encode_binary(binary)
-        Base64.urlsafe_encode64(binary)
+        Base64.encode64(binary)
       end
 
       def gitaly_server_hash(repository)

spec/lib/gitlab/workhorse_spec.rb

@@ -39,7 +39,7 @@ describe Gitlab::Workhorse do
           token: Gitlab::GitalyClient.token(project.repository_storage)
         },
         'ArchivePath' => metadata['ArchivePath'],
-        'GetArchiveRequest' => Base64.urlsafe_encode64(
+        'GetArchiveRequest' => Base64.encode64(
           Gitaly::GetArchiveRequest.new(
             repository: repository.gitaly_repository,
             commit_id: metadata['CommitId'],