Use Gitaly protobuf version as DiffStats cache key

This avoids the pitfalls we saw in a mixed-deployment setup. If an
updated server adds a field such as `old_path` to the DiffStats protobuf
message definition, it will serialize this JSON to Redis. However, a
server running an older version will fetch this data and encounter an
error while trying to initialize `DiffStats`.

Most of the time this protobuf definition shouldn't change, but to be
sure we use the loaded Gem version.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/233083

changelogs/unreleased/sh-better-diff-stats-versioning.yml

+---
+title: Use Gitaly protobuf version as DiffStats cache key
+merge_request: 38414
+author:
+type: fixed

lib/gitlab/diff/stats_cache.rb

@@ -6,7 +6,8 @@ module Gitlab
       include Gitlab::Utils::StrongMemoize
 
       EXPIRATION = 1.week
-      VERSION = 1
+      # The DiffStats#as_json representation is tied to the Gitaly protobuf version
+      VERSION = Gem.loaded_specs['gitaly'].version.to_s
 
       def initialize(cachable_key:)
         @cachable_key = cachable_key
@@ -29,6 +30,7 @@ module Gitlab
         return unless stats
 
         cache.write(key, stats.as_json, expires_in: EXPIRATION)
+        clear_memoization(:cached_values)
       end
 
       def clear

spec/lib/gitlab/diff/stats_cache_spec.rb

@@ -81,4 +81,28 @@ RSpec.describe Gitlab::Diff::StatsCache, :use_clean_rails_memory_store_caching d
       stats_cache.clear
     end
   end
+
+  it 'VERSION is set' do
+    expect(described_class::VERSION).to be_present
+  end
+
+  context 'with multiple cache versions' do
+    before do
+      stats_cache.write_if_empty(stats)
+    end
+
+    it 'does not read from a stale cache' do
+      expect(stats_cache.read.to_json).to eq(stats.to_json)
+
+      stub_const('Gitlab::Diff::StatsCache::VERSION', '1.0.new-new-thing')
+
+      stats_cache = described_class.new(cachable_key: cachable_key)
+
+      expect(stats_cache.read).to be_nil
+
+      stats_cache.write_if_empty(stats)
+
+      expect(stats_cache.read.to_json).to eq(stats.to_json)
+    end
+  end
 end