FIX: Return consistent character encodings when downloading S3 uploads

Net::HTTP always returns ASCII-8BIT encoding. File.read auto-detects the encoding. This leads to an encoding inconsistency between a fresh download, and a cached download. This commit ensures all downloaded files are treated equally, by always returning the cached version from the filesystem, even during initial download.

One symptom of this problem is during theme exports: https://meta.discourse.org/t/116907

Related ruby ticket: https://bugs.ruby-lang.org/issues/2567

lib/file_store/base_store.rb

@@ -85,6 +85,7 @@ module FileStore
             follow_redirect: true
           )
           cache_file(file, filename)
+          file = get_from_cache(filename)
         end
 
         file

spec/components/file_store/base_store_spec.rb

@@ -45,4 +45,33 @@ RSpec.describe FileStore::BaseStore do
       expect(FileStore::BaseStore.new.get_path_for_optimized_image(optimized)).to eq(optimized_path)
     end
   end
+
+  describe '#download' do
+    before do
+      `rm -rf #{FileStore::BaseStore::CACHE_DIR}`
+
+      SiteSetting.enable_s3_uploads = true
+      SiteSetting.s3_upload_bucket = "s3-upload-bucket"
+      SiteSetting.s3_access_key_id = "some key"
+      SiteSetting.s3_secret_access_key = "some secret key"
+
+      stub_request(:get, upload_s3.url).to_return(status: 200, body: "Hello world")
+    end
+
+    let(:upload_s3) { Fabricate(:upload_s3) }
+
+    it "should return consistent encodings for fresh and cached downloads" do
+      # Net::HTTP always returns binary ASCII-8BIT encoding. File.read auto-detects the encoding
+      # Make sure we File.read after downloading a file for consistency
+
+      store = FileStore::BaseStore.new
+
+      first_encoding = store.download(upload_s3).read.encoding
+
+      second_encoding = store.download(upload_s3).read.encoding
+
+      expect(first_encoding).to eq(Encoding::UTF_8)
+      expect(second_encoding).to eq(Encoding::UTF_8)
+    end
+  end
 end