Remove deprecated caching class

With the launch of the new approach to caching highlighting diffs in
redis, we can drop this class as it is no longer needed. We used it in
the new highlighting class to accelerate the expiration of deprecated
cache entries, but as of 12.7 the new caching approach is enabled by
default, allowing us to completely remove this.

lib/gitlab/diff/deprecated_highlight_cache.rb

-# frozen_string_literal: true
-#
-module Gitlab
-  module Diff
-    class DeprecatedHighlightCache
-      delegate :diffable, to: :@diff_collection
-      delegate :diff_options, to: :@diff_collection
-
-      def initialize(diff_collection, backend: Rails.cache)
-        @backend = backend
-        @diff_collection = diff_collection
-      end
-
-      # - Reads from cache
-      # - Assigns DiffFile#highlighted_diff_lines for cached files
-      def decorate(diff_file)
-        if content = read_file(diff_file)
-          diff_file.highlighted_diff_lines = content.map do |line|
-            Gitlab::Diff::Line.init_from_hash(line)
-          end
-        end
-      end
-
-      # It populates a Hash in order to submit a single write to the memory
-      # cache. This avoids excessive IO generated by N+1's (1 writing for
-      # each highlighted line or file).
-      def write_if_empty
-        return if cached_content.present?
-
-        @diff_collection.diff_files.each do |diff_file|
-          next unless cacheable?(diff_file)
-
-          diff_file_id = diff_file.file_identifier
-
-          cached_content[diff_file_id] = diff_file.highlighted_diff_lines.map(&:to_hash)
-        end
-
-        cache.write(key, cached_content, expires_in: 1.week)
-      end
-
-      def clear
-        cache.delete(key)
-      end
-
-      def key
-        [diffable, 'highlighted-diff-files', Gitlab::Diff::Line::SERIALIZE_KEYS, diff_options]
-      end
-
-      private
-
-      def read_file(diff_file)
-        cached_content[diff_file.file_identifier]
-      end
-
-      def cache
-        @backend
-      end
-
-      def cached_content
-        @cached_content ||= cache.read(key) || {}
-      end
-
-      def cacheable?(diff_file)
-        diffable.present? && diff_file.text? && diff_file.diffable?
-      end
-    end
-  end
-end

lib/gitlab/diff/highlight_cache.rb

@@ -57,17 +57,6 @@ module Gitlab
 
       private
 
-      # We create a Gitlab::Diff::DeprecatedHighlightCache here in order to
-      #   expire deprecated cache entries while we make the transition. This can
-      #   be removed when :hset_redis_diff_caching is fully launched.
-      # See https://gitlab.com/gitlab-org/gitlab/issues/38008
-      #
-      def deprecated_cache
-        strong_memoize(:deprecated_cache) do
-          Gitlab::Diff::DeprecatedHighlightCache.new(@diff_collection)
-        end
-      end
-
       def cacheable_files
         strong_memoize(:cacheable_files) do
           diff_files.select { |file| cacheable?(file) && read_file(file).nil? }
@@ -104,10 +93,6 @@ module Gitlab
         #
         clear_memoization(:cached_content)
         clear_memoization(:cacheable_files)
-
-        # Clean up any deprecated hash entries
-        #
-        deprecated_cache.clear
       end
 
       def file_paths

spec/lib/gitlab/diff/deprecated_highlight_cache_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-
-describe Gitlab::Diff::DeprecatedHighlightCache do
-  let(:merge_request) { create(:merge_request_with_diffs) }
-
-  subject(:cache) { described_class.new(merge_request.diffs, backend: backend) }
-
-  describe '#decorate' do
-    let(:backend) { double('backend').as_null_object }
-
-    # Manually creates a Diff::File object to avoid triggering the cache on
-    # the FileCollection::MergeRequestDiff
-    let(:diff_file) do
-      diffs = merge_request.diffs
-      raw_diff = diffs.diffable.raw_diffs(diffs.diff_options.merge(paths: ['CHANGELOG'])).first
-      Gitlab::Diff::File.new(raw_diff,
-                             repository: diffs.project.repository,
-                             diff_refs: diffs.diff_refs,
-                             fallback_diff_refs: diffs.fallback_diff_refs)
-    end
-
-    it 'does not calculate highlighting when reading from cache' do
-      cache.write_if_empty
-      cache.decorate(diff_file)
-
-      expect_any_instance_of(Gitlab::Diff::Highlight).not_to receive(:highlight)
-
-      diff_file.highlighted_diff_lines
-    end
-
-    it 'assigns highlighted diff lines to the DiffFile' do
-      cache.write_if_empty
-      cache.decorate(diff_file)
-
-      expect(diff_file.highlighted_diff_lines.size).to be > 5
-    end
-
-    it 'submits a single reading from the cache' do
-      cache.decorate(diff_file)
-      cache.decorate(diff_file)
-
-      expect(backend).to have_received(:read).with(cache.key).once
-    end
-  end
-
-  describe '#write_if_empty' do
-    let(:backend) { double('backend', read: {}).as_null_object }
-
-    it 'submits a single writing to the cache' do
-      cache.write_if_empty
-      cache.write_if_empty
-
-      expect(backend).to have_received(:write).with(cache.key,
-                                                    hash_including('CHANGELOG-false-false-false'),
-                                                    expires_in: 1.week).once
-    end
-  end
-
-  describe '#clear' do
-    let(:backend) { double('backend').as_null_object }
-
-    it 'clears cache' do
-      cache.clear
-
-      expect(backend).to have_received(:delete).with(cache.key)
-    end
-  end
-end

spec/lib/gitlab/diff/highlight_cache_spec.rb

@@ -135,16 +135,6 @@ describe Gitlab::Diff::HighlightCache, :clean_gitlab_redis_cache do
       expect { cache.send(:write_to_redis_hash, diff_hash) }
         .to change { Gitlab::Redis::Cache.with { |r| r.hgetall(cache_key) } }
     end
-
-    # Note that this spec and the code it confirms can be removed when
-    #   :hset_redis_diff_caching is fully launched.
-    #
-    it 'attempts to clear deprecated cache entries' do
-      expect_any_instance_of(Gitlab::Diff::DeprecatedHighlightCache)
-        .to receive(:clear).and_call_original
-
-      cache.send(:write_to_redis_hash, diff_hash)
-    end
   end
 
   describe '#clear' do