Resolve Yajl encoding incompatibility

When Yajl encoder was provided a string with ASCII-8BIT,
"incompatible character encodings: ASCII-8BIT and UTF-8" error
was raised.

Let's use StringIO in order to build the result out of chunks.

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63379

lib/gitlab/json.rb

@@ -242,7 +242,7 @@ module Gitlab
       def self.encode(object, limit: 25.megabytes)
         return ::Gitlab::Json.dump(object) unless Feature.enabled?(:json_limited_encoder)
 
-        buffer = []
+        buffer = StringIO.new
         buffer_size = 0
 
         ::Yajl::Encoder.encode(object) do |data_chunk|
@@ -254,7 +254,7 @@ module Gitlab
           buffer_size += chunk_size
         end
 
-        buffer.join('')
+        buffer.string
       end
     end
   end

spec/lib/gitlab/json_spec.rb

@@ -411,7 +411,7 @@ RSpec.describe Gitlab::Json do
   end
 
   describe Gitlab::Json::LimitedEncoder do
-    subject { described_class.encode(obj, limit: 8.kilobytes) }
+    subject { described_class.encode(obj, limit: 10.kilobytes) }
 
     context 'when object size is acceptable' do
       let(:obj) { { test: true } }
@@ -431,6 +431,16 @@ RSpec.describe Gitlab::Json do
       end
     end
 
+    context 'when object contains ASCII-8BIT encoding' do
+      let(:obj) { [{ a: "\x8F" }] * 1000 }
+
+      it 'does not raise encoding error' do
+        expect { subject }.not_to raise_error
+        expect(subject).to be_a(String)
+        expect(subject.size).to eq(10001)
+      end
+    end
+
     context 'when json_limited_encoder is disabled' do
       let(:obj) { [{ test: true }] * 1000 }