Memoize Repository#empty? instead of double caching the value

We saw that in a customer instance, `empty?` was cached to be `true` even
though `has_visible_content?` and `exists?` were `true`. This double caching
can run into edge cases because there's no guarantee that the inner values
will properly expire the outer one, especially if there is Redis replication lag.
Consider this scenario:

1. `exists?` and `has_visible_content?` are false
2. `empty?` is expired
3. A subsequent call to `empty?` returns `true` because `exists?` is false even though `empty` is true
4. `exists?` and `has_visible_content?` are then expired
5. `exists?` and `has_visible_content?` are set to true
6. `empty?` is still stuck in the wrong value as `true`

Closes #43882

app/models/repository.rb

@@ -35,7 +35,7 @@ class Repository
   CACHED_METHODS = %i(size commit_count rendered_readme contribution_guide
                       changelog license_blob license_key gitignore koding_yml
                       gitlab_ci_yml branch_names tag_names branch_count
-                      tag_count avatar exists? empty? root_ref has_visible_content?
+                      tag_count avatar exists? root_ref has_visible_content?
                       issue_template_names merge_request_template_names).freeze
   # Methods that use cache_method but only memoize the value
@@ -360,7 +360,7 @@ class Repository
   def expire_emptiness_caches
     return unless empty?
-    expire_method_caches(%i(empty? has_visible_content?))
+    expire_method_caches(%i(has_visible_content?))
   end
   def lookup_cache
@@ -506,12 +506,14 @@ class Repository
   end
   cache_method :exists?
+  # We don't need to cache the output of this method because both exists? and
+  # has_visible_content? are already memoized and cached. There's no guarantee
+  # that the values are expired and loaded atomically.
   def empty?
     return true unless exists?
     !has_visible_content?
   end
-  cache_method :empty?
   # The size of this repository in megabytes.
   def size

changelogs/unreleased/sh-remove-double-caching-repo-empty.yml

+---
+title: Remove double caching of Repository#empty?
+merge_request:
+author:
+type: fixed

spec/models/repository_spec.rb

@@ -1447,7 +1447,6 @@ describe Repository do
     it 'expires the caches for an empty repository' do
       allow(repository).to receive(:empty?).and_return(true)
-      expect(cache).to receive(:expire).with(:empty?)
       expect(cache).to receive(:expire).with(:has_visible_content?)
       repository.expire_emptiness_caches
@@ -1456,7 +1455,6 @@ describe Repository do
     it 'does not expire the cache for a non-empty repository' do
       allow(repository).to receive(:empty?).and_return(false)
-      expect(cache).not_to receive(:expire).with(:empty?)
       expect(cache).not_to receive(:expire).with(:has_visible_content?)
       repository.expire_emptiness_caches