Fix bug when creating pull request (#36166)

Extract from #36105 

Fix #36116
Fix #35912
Fix #20906

routers/common/compare.go

@@ -4,6 +4,8 @@
 package common
 
 import (
+	"context"
+
 	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/git"
@@ -20,3 +22,54 @@ type CompareInfo struct {
 	HeadBranch       string
 	DirectComparison bool
 }
+
+// maxForkTraverseLevel defines the maximum levels to traverse when searching for the head repository.
+const maxForkTraverseLevel = 10
+
+// FindHeadRepo tries to find the head repository based on the base repository and head user ID.
+func FindHeadRepo(ctx context.Context, baseRepo *repo_model.Repository, headUserID int64) (*repo_model.Repository, error) {
+	if baseRepo.IsFork {
+		curRepo := baseRepo
+		for curRepo.OwnerID != headUserID { // We assume the fork deepth is not too deep.
+			if err := curRepo.GetBaseRepo(ctx); err != nil {
+				return nil, err
+			}
+			if curRepo.BaseRepo == nil {
+				return findHeadRepoFromRootBase(ctx, curRepo, headUserID, maxForkTraverseLevel)
+			}
+			curRepo = curRepo.BaseRepo
+		}
+		return curRepo, nil
+	}
+
+	return findHeadRepoFromRootBase(ctx, baseRepo, headUserID, maxForkTraverseLevel)
+}
+
+func findHeadRepoFromRootBase(ctx context.Context, baseRepo *repo_model.Repository, headUserID int64, traverseLevel int) (*repo_model.Repository, error) {
+	if traverseLevel == 0 {
+		return nil, nil
+	}
+	// test if we are lucky
+	repo, err := repo_model.GetUserFork(ctx, baseRepo.ID, headUserID)
+	if err != nil {
+		return nil, err
+	}
+	if repo != nil {
+		return repo, nil
+	}
+
+	firstLevelForkedRepos, err := repo_model.GetRepositoriesByForkID(ctx, baseRepo.ID)
+	if err != nil {
+		return nil, err
+	}
+	for _, repo := range firstLevelForkedRepos {
+		forked, err := findHeadRepoFromRootBase(ctx, repo, headUserID, traverseLevel-1)
+		if err != nil {
+			return nil, err
+		}
+		if forked != nil {
+			return forked, nil
+		}
+	}
+	return nil, nil
+}