Fix PR creation on forked repositories (#31863)

Resolves #20475

routers/api/v1/repo/pull.go

@@ -1124,9 +1124,20 @@ func parseCompareInfo(ctx *context.APIContext, form api.CreatePullRequestOption)
 	// Check if current user has fork of repository or in the same repository.
 	headRepo := repo_model.GetForkedRepo(ctx, headUser.ID, baseRepo.ID)
 	if headRepo == nil && !isSameRepo {
-		log.Trace("parseCompareInfo[%d]: does not have fork or in same repository", baseRepo.ID)
-		ctx.NotFound("GetForkedRepo")
-		return nil, nil, nil, "", ""
+		err := baseRepo.GetBaseRepo(ctx)
+		if err != nil {
+			ctx.Error(http.StatusInternalServerError, "GetBaseRepo", err)
+			return nil, nil, nil, "", ""
+		}
+
+		// Check if baseRepo's base repository is the same as headUser's repository.
+		if baseRepo.BaseRepo == nil || baseRepo.BaseRepo.OwnerID != headUser.ID {
+			log.Trace("parseCompareInfo[%d]: does not have fork or in same repository", baseRepo.ID)
+			ctx.NotFound("GetBaseRepo")
+			return nil, nil, nil, "", ""
+		}
+		// Assign headRepo so it can be used below.
+		headRepo = baseRepo.BaseRepo
 	}
 
 	var headGitRepo *git.Repository