perf: optimize converting releases to feed items (#7221)

- `releasesToFeedItems` is called to convert release structs to feed items, which is then used to render RSS or Atom feeds.
- Optimize the loading of attributes for the releases, introduce `ReleaseList` type which uses caching to load repository and publishers. It also no longer loads release attachments and downloads counts as that is not used in feed items.
- Optimize the composing of meta by introducing caching, this operation is especially slow when the owner is an organization.
- Add unit test (ensures new `LoadAttributes` works correctly).
- Add integration test (ensures that feed output is still as expected).

Loading https://codeberg.org/forgejo/forgejo/releases.rss reduced from ~15s to ~1s. (It is currently is deployed on codeberg.org)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7221
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

routers/web/feed/convert.go

@@ -298,14 +298,14 @@ func GetFeedType(name string, req *http.Request) (bool, string, string) {
 	return false, name, ""
 }
 
-// feedActionsToFeedItems convert gitea's Repo's Releases to feeds Item
-func releasesToFeedItems(ctx *context.Context, releases []*repo_model.Release) (items []*feeds.Item, err error) {
-	for _, rel := range releases {
-		err := rel.LoadAttributes(ctx)
-		if err != nil {
-			return nil, err
-		}
+// feedActionsToFeedItems convert repository releases into feed items.
+func releasesToFeedItems(ctx *context.Context, releases repo_model.ReleaseList) (items []*feeds.Item, err error) {
+	if err := releases.LoadAttributes(ctx); err != nil {
+		return nil, err
+	}
 
+	composeCache := make(map[int64]map[string]string)
+	for _, rel := range releases {
 		var title string
 		var content template.HTML
 
@@ -315,13 +315,19 @@ func releasesToFeedItems(ctx *context.Context, releases []*repo_model.Release) (
 			title = rel.Title
 		}
 
+		metas, ok := composeCache[rel.RepoID]
+		if !ok {
+			metas = rel.Repo.ComposeMetas(ctx)
+			composeCache[rel.RepoID] = metas
+		}
+
 		link := &feeds.Link{Href: rel.HTMLURL()}
 		content, err = markdown.RenderString(&markup.RenderContext{
 			Ctx: ctx,
 			Links: markup.Links{
 				Base: rel.Repo.Link(),
 			},
-			Metas: rel.Repo.ComposeMetas(ctx),
+			Metas: metas,
 		}, rel.Note)
 		if err != nil {
 			return nil, err