[BUG] Store JSON in contributors commit cache

- The code that gets contributor stats tried to store an
`map[string]*ContributorData` type in the cache, this works for the
memory cache but not for other caches such as Redis.
- The cache implementation for Redis would convert this map via
`fmt.Sprintf` to an string, which would simply print the pointer and not
the value of the pointer. Storing pointers is a no-go as this will get
GC-ed eventually within a few minutes. Therefore store everything with
json, that does properly store the value of the pointers.
- Adds unit test that verifies JSON is being used.
- Resolves https://codeberg.org/forgejo/forgejo/issues/3158

(cherry picked from commit 4c8b67c4b2)

services/repository/contributors_graph.go

@@ -20,6 +20,7 @@ import (
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/gitrepo"
 	"code.gitea.io/gitea/modules/graceful"
+	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/log"
 	api "code.gitea.io/gitea/modules/structs"
 
@@ -108,8 +109,9 @@ func GetContributorStats(ctx context.Context, cache cache.Cache, repo *repo_mode
 	switch v := cache.Get(cacheKey).(type) {
 	case error:
 		return nil, v
-	case map[string]*ContributorData:
+	case string:
-		return v, nil
+		var cachedStats map[string]*ContributorData
+		return cachedStats, json.Unmarshal([]byte(v), &cachedStats)
 	default:
 		return nil, fmt.Errorf("unexpected type in cache detected")
 	}
@@ -309,7 +311,16 @@ func generateContributorStats(genDone chan struct{}, cache cache.Cache, cacheKey
 		total.TotalCommits++
 	}
 
-	_ = cache.Put(cacheKey, contributorsCommitStats, contributorStatsCacheTimeout)
+	data, err := json.Marshal(contributorsCommitStats)
+	if err != nil {
+		err := fmt.Errorf("couldn't marshal the data: %w", err)
+		_ = cache.Put(cacheKey, err, contributorStatsCacheTimeout)
+		return
+	}
+
+	// Store the data as an string, to make it uniform what data type is returned
+	// from caches.
+	_ = cache.Put(cacheKey, string(data), contributorStatsCacheTimeout)
 	generateLock.Delete(cacheKey)
 	if genDone != nil {
 		genDone <- struct{}{}

services/repository/contributors_graph_test.go

@@ -11,6 +11,7 @@ import (
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
 	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/json"
 
 	"gitea.com/go-chi/cache"
 	"github.com/stretchr/testify/assert"
@@ -32,8 +33,14 @@ func TestRepository_ContributorsGraph(t *testing.T) {
 	assert.ErrorAs(t, err, &git.ErrNotExist{})
 
 	generateContributorStats(nil, mockCache, "key2", repo, "master")
-	data, isData := mockCache.Get("key2").(map[string]*ContributorData)
+	dataString, isData := mockCache.Get("key2").(string)
 	assert.True(t, isData)
+	// Verify that JSON is actually stored in the cache.
+	assert.EqualValues(t, `{"ethantkoenig@gmail.com":{"name":"Ethan Koenig","login":"","avatar_link":"https://secure.gravatar.com/avatar/b42fb195faa8c61b8d88abfefe30e9e3?d=identicon","home_link":"","total_commits":1,"weeks":{"1511654400000":{"week":1511654400000,"additions":3,"deletions":0,"commits":1}}},"jimmy.praet@telenet.be":{"name":"Jimmy Praet","login":"","avatar_link":"https://secure.gravatar.com/avatar/93c49b7c89eb156971d11161c9b52795?d=identicon","home_link":"","total_commits":1,"weeks":{"1624752000000":{"week":1624752000000,"additions":2,"deletions":0,"commits":1}}},"jon@allspice.io":{"name":"Jon","login":"","avatar_link":"https://secure.gravatar.com/avatar/00388ce725e6886f3e07c3733007289b?d=identicon","home_link":"","total_commits":1,"weeks":{"1607817600000":{"week":1607817600000,"additions":10,"deletions":0,"commits":1}}},"total":{"name":"Total","login":"","avatar_link":"","home_link":"","total_commits":3,"weeks":{"1511654400000":{"week":1511654400000,"additions":3,"deletions":0,"commits":1},"1607817600000":{"week":1607817600000,"additions":10,"deletions":0,"commits":1},"1624752000000":{"week":1624752000000,"additions":2,"deletions":0,"commits":1}}}}`, dataString)
+
+	var data map[string]*ContributorData
+	assert.NoError(t, json.Unmarshal([]byte(dataString), &data))
+
 	var keys []string
 	for k := range data {
 		keys = append(keys, k)