Fix PR creation on forked repositories (#31863)

Resolves #20475

routers/api/v1/repo/pull.go

@@ -1124,11 +1124,22 @@ 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")
+		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
 	if isSameRepo {
 		headRepo = ctx.Repo.Repository

tests/integration/pull_create_test.go

@@ -199,3 +199,30 @@ func TestPullBranchDelete(t *testing.T) {
 		session.MakeRequest(t, req, http.StatusOK)
 	})
 }
+
+/*
+Setup:
+The base repository is: user2/repo1
+Fork repository to: user1/repo1
+Push extra commit to: user2/repo1, which changes README.md
+Create a PR on user1/repo1
+
+Test checks:
+Check if pull request can be created from base to the fork repository.
+*/
+func TestPullCreatePrFromBaseToFork(t *testing.T) {
+	onGiteaRun(t, func(t *testing.T, u *url.URL) {
+		sessionFork := loginUser(t, "user1")
+		testRepoFork(t, sessionFork, "user2", "repo1", "user1", "repo1", "")
+
+		// Edit base repository
+		sessionBase := loginUser(t, "user2")
+		testEditFile(t, sessionBase, "user2", "repo1", "master", "README.md", "Hello, World (Edited)\n")
+
+		// Create a PR
+		resp := testPullCreateDirectly(t, sessionFork, "user1", "repo1", "master", "user2", "repo1", "master", "This is a pull title")
+		// check the redirected URL
+		url := test.RedirectURL(resp)
+		assert.Regexp(t, "^/user1/repo1/pulls/[0-9]*$", url)
+	})
+}