Remove gzip_diff_cache feature flag

This flag was added in 12.10, and has been default_enabled: true the
entire time without incident. Removing, but leaving the rescue's for
failed encoding attempts.

changelogs/unreleased/kerrizor-remove-gzip_diff_cache-feature-flag.yml

+---
+title: Remove :gzip_diff_cache feature flag
+merge_request: 38838
+author:
+type: other

lib/gitlab/diff/highlight_cache.rb

@@ -152,34 +152,24 @@ module Gitlab
       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.
+        # Since we could 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.
         #
-        if ::Feature.enabled?(:gzip_diff_cache, default_enabled: true) && data[0..1] == "\x1F\x8B"
-          ActiveSupport::Gzip.decompress(data)
-        else
-          data
-        end
+        data[0..1] == "\x1F\x8B" ? ActiveSupport::Gzip.decompress(data) : data
       rescue Zlib::GzipFile::Error
         data
       end

spec/lib/gitlab/diff/highlight_cache_spec.rb

@@ -173,11 +173,6 @@ RSpec.describe Gitlab::Diff::HighlightCache, :clean_gitlab_redis_cache do
                              fallback_diff_refs: diffs.fallback_diff_refs)
     end
 
-    context "feature flag :gzip_diff_cache disabled" do
-      before do
-        stub_feature_flags(gzip_diff_cache: true)
-      end
-
     it "uses ActiveSupport::Gzip when reading from the cache" do
       expect(ActiveSupport::Gzip).to receive(:decompress).at_least(:once).and_call_original
 
@@ -192,27 +187,6 @@ RSpec.describe Gitlab::Diff::HighlightCache, :clean_gitlab_redis_cache do
     end
   end
 
-    context "feature flag :gzip_diff_cache disabled" do
-      before do
-        stub_feature_flags(gzip_diff_cache: false)
-      end
-
-      it "doesn't use ActiveSupport::Gzip when reading from the cache" do
-        expect(ActiveSupport::Gzip).not_to receive(:decompress)
-
-        cache.write_if_empty
-        cache.decorate(diff_file)
-      end
-
-      it "doesn't use ActiveSupport::Gzip to compress data when writing to cache" do
-        expect(ActiveSupport::Gzip).not_to receive(:compress)
-
-        expect { cache.send(:write_to_redis_hash, diff_hash) }
-          .to change { Gitlab::Redis::Cache.with { |r| r.hgetall(cache_key) } }
-      end
-    end
-  end
-
   describe 'metrics' do
     let(:transaction) { Gitlab::Metrics::WebTransaction.new({} ) }