1 files changed, 39 insertions, 2 deletions
diff --git a/lib/gitlab/diff/highlight_cache.rb b/lib/gitlab/diff/highlight_cache.rb
index 055eae2c0fd..01ec9798fe4 100644
--- a/lib/gitlab/diff/highlight_cache.rb
+++ b/lib/gitlab/diff/highlight_cache.rb
@@ -94,7 +94,11 @@ module Gitlab
         Gitlab::Redis::Cache.with do |redis|
           redis.pipelined do
             hash.each do |diff_file_id, highlighted_diff_lines_hash|
-              redis.hset(key, diff_file_id, highlighted_diff_lines_hash.to_json)
+              redis.hset(
+                key,
+                diff_file_id,
+                compose_data(highlighted_diff_lines_hash.to_json)
+              )
             end
 
             # HSETs have to have their expiration date manually updated
@@ -152,12 +156,45 @@ module Gitlab
         end
 
         results.map! do |result|
-          JSON.parse(result, symbolize_names: true) unless result.nil?
+          JSON.parse(extract_data(result), symbolize_names: true) unless result.nil?
         end
 
         file_paths.zip(results).to_h
       end
 
+      def compose_data(json_data)
+        if ::Feature.enabled?(:gzip_diff_cache, default_enabled: true)
+          # #compress returns ASCII-8BIT, so we need to force the encoding to
+          #   UTF-8 before caching it in redis, else we risk encoding mismatch
+          #   errors.
+          #
+          ActiveSupport::Gzip.compress(json_data).force_encoding("UTF-8")
+        else
+          json_data
+        end
+      rescue Zlib::GzipFile::Error
+        json_data
+      end
+
+      def extract_data(data)
+        # Since when we deploy this code, we'll be dealing with an already
+        #   populated cache full of data that isn't gzipped, we want to also
+        #   check to see if the data is gzipped before we attempt to #decompress
+        #   it, thus we check the first 2 bytes for "\x1F\x8B" to confirm it is
+        #   a gzipped string. While a non-gzipped string will raise a
+        #   Zlib::GzipFile::Error, which we're rescuing, we don't want to count
+        #   on rescue for control flow. This check can be removed in the release
+        #   after this change is released.
+        #
+        if ::Feature.enabled?(:gzip_diff_cache, default_enabled: true) && data[0..1] == "\x1F\x8B"
+          ActiveSupport::Gzip.decompress(data)
+        else
+          data
+        end
+      rescue Zlib::GzipFile::Error
+        data
+      end
+
       def cacheable?(diff_file)
         diffable.present? && diff_file.text? && diff_file.diffable?
       end