Add more logging to track encoding errors appending CI traces

We are seeing a recent increase in `Encoding::CompatibilityError`
errors. This commit adds more logging to understand the root cause.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/333424

Changelog: other

app/models/ci/build_trace_chunks/fog.rb

@@ -33,6 +33,20 @@ module Ci
 
         set_data(model, new_data)
         new_data.bytesize
+      rescue Encoding::CompatibilityError => e
+        Gitlab::ErrorTracking.track_and_raise_exception(
+          e,
+          build_id: model.build_id,
+          chunk_index: model.chunk_index,
+          chunk_start_offset: model.start_offset,
+          chunk_end_offset: model.end_offset,
+          chunk_size: model.size,
+          chunk_data_store: model.data_store,
+          offset: offset,
+          old_data_encoding: truncated_data.encoding.to_s,
+          new_data: new_data,
+          new_data_size: new_data.bytesize,
+          new_data_encoding: new_data.encoding.to_s)
       end
 
       def size(model)

spec/models/ci/build_trace_chunks/fog_spec.rb

@@ -102,6 +102,23 @@ RSpec.describe Ci::BuildTraceChunks::Fog do
     end
   end
 
+  describe '#append_data' do
+    let(:model) { create(:ci_build_trace_chunk, :fog_with_data, initial_data: (+'😺').force_encoding('ASCII-8BIT')) }
+    let(:data) { data_store.data(model) }
+
+    it 'appends ASCII data' do
+      data_store.append_data(model, 'hello world', 4)
+
+      expect(data.encoding).to eq(Encoding.find('ASCII-8BIT'))
+      expect(data.force_encoding('UTF-8')).to eq('😺hello world')
+    end
+
+    it 'throws an exception when appending UTF-8 data' do
+      expect(Gitlab::ErrorTracking).to receive(:track_and_raise_exception).and_call_original
+      expect { data_store.append_data(model, 'Résumé', 4) }.to raise_exception(Encoding::CompatibilityError)
+    end
+  end
+
   describe '#delete_data' do
     subject { data_store.delete_data(model) }