Fix codeowner detected diff base branch to mergebase (#29783) (#29807)

Fix #29763
Backport #29783 

This PR fixes 2 problems with CodeOwner in the pull request.
- Don't use the pull request base branch but merge-base as a diff base
to detect the code owner.
- CodeOwner detection in fork repositories will be disabled because
almost all the fork repositories will not change CODEOWNERS files but it
should not be used on fork repositories' pull requests.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

tests/integration/pull_review_test.go

@@ -5,9 +5,19 @@ package integration
 
 import (
 	"net/http"
+	"net/url"
+	"strings"
 	"testing"
 
+	"code.gitea.io/gitea/models/db"
+	issues_model "code.gitea.io/gitea/models/issues"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+	repo_service "code.gitea.io/gitea/services/repository"
+	files_service "code.gitea.io/gitea/services/repository/files"
 	"code.gitea.io/gitea/tests"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestPullView_ReviewerMissed(t *testing.T) {
@@ -20,3 +30,115 @@ func TestPullView_ReviewerMissed(t *testing.T) {
 	req = NewRequest(t, "GET", "/user2/repo1/pulls/3")
 	session.MakeRequest(t, req, http.StatusOK)
 }
+
+func TestPullView_CodeOwner(t *testing.T) {
+	onGiteaRun(t, func(t *testing.T, u *url.URL) {
+		user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+
+		// Create the repo.
+		repo, err := repo_service.CreateRepositoryDirectly(db.DefaultContext, user2, user2, repo_service.CreateRepoOptions{
+			Name:          "test_codeowner",
+			Readme:        "Default",
+			AutoInit:      true,
+			DefaultBranch: "master",
+		})
+		assert.NoError(t, err)
+
+		// add CODEOWNERS to default branch
+		_, err = files_service.ChangeRepoFiles(db.DefaultContext, repo, user2, &files_service.ChangeRepoFilesOptions{
+			OldBranch: repo.DefaultBranch,
+			Files: []*files_service.ChangeRepoFile{
+				{
+					Operation:     "create",
+					TreePath:      "CODEOWNERS",
+					ContentReader: strings.NewReader("README.md @user5\n"),
+				},
+			},
+		})
+		assert.NoError(t, err)
+
+		t.Run("First Pull Request", func(t *testing.T) {
+			// create a new branch to prepare for pull request
+			_, err = files_service.ChangeRepoFiles(db.DefaultContext, repo, user2, &files_service.ChangeRepoFilesOptions{
+				NewBranch: "codeowner-basebranch",
+				Files: []*files_service.ChangeRepoFile{
+					{
+						Operation:     "update",
+						TreePath:      "README.md",
+						ContentReader: strings.NewReader("# This is a new project\n"),
+					},
+				},
+			})
+			assert.NoError(t, err)
+
+			// Create a pull request.
+			session := loginUser(t, "user2")
+			createPullRequest(t, session, "user2", "test_codeowner", repo.DefaultBranch, "codeowner-basebranch", "Test Pull Request")
+
+			pr := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{BaseRepoID: repo.ID, HeadBranch: "codeowner-basebranch"})
+			unittest.AssertExistsIf(t, true, &issues_model.Review{IssueID: pr.IssueID, Type: issues_model.ReviewTypeRequest, ReviewerID: 5})
+		})
+
+		// change the default branch CODEOWNERS file to change README.md's codeowner
+		_, err = files_service.ChangeRepoFiles(db.DefaultContext, repo, user2, &files_service.ChangeRepoFilesOptions{
+			Files: []*files_service.ChangeRepoFile{
+				{
+					Operation:     "update",
+					TreePath:      "CODEOWNERS",
+					ContentReader: strings.NewReader("README.md @user8\n"),
+				},
+			},
+		})
+		assert.NoError(t, err)
+
+		t.Run("Second Pull Request", func(t *testing.T) {
+			// create a new branch to prepare for pull request
+			_, err = files_service.ChangeRepoFiles(db.DefaultContext, repo, user2, &files_service.ChangeRepoFilesOptions{
+				NewBranch: "codeowner-basebranch2",
+				Files: []*files_service.ChangeRepoFile{
+					{
+						Operation:     "update",
+						TreePath:      "README.md",
+						ContentReader: strings.NewReader("# This is a new project2\n"),
+					},
+				},
+			})
+			assert.NoError(t, err)
+
+			// Create a pull request.
+			session := loginUser(t, "user2")
+			createPullRequest(t, session, "user2", "test_codeowner", repo.DefaultBranch, "codeowner-basebranch2", "Test Pull Request2")
+
+			pr := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{BaseRepoID: repo.ID, HeadBranch: "codeowner-basebranch2"})
+			unittest.AssertExistsIf(t, true, &issues_model.Review{IssueID: pr.IssueID, Type: issues_model.ReviewTypeRequest, ReviewerID: 8})
+		})
+
+		t.Run("Forked Repo Pull Request", func(t *testing.T) {
+			user5 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 5})
+			forkedRepo, err := repo_service.ForkRepository(db.DefaultContext, user2, user5, repo_service.ForkRepoOptions{
+				BaseRepo: repo,
+				Name:     "test_codeowner",
+			})
+			assert.NoError(t, err)
+
+			// create a new branch to prepare for pull request
+			_, err = files_service.ChangeRepoFiles(db.DefaultContext, forkedRepo, user5, &files_service.ChangeRepoFilesOptions{
+				NewBranch: "codeowner-basebranch-forked",
+				Files: []*files_service.ChangeRepoFile{
+					{
+						Operation:     "update",
+						TreePath:      "README.md",
+						ContentReader: strings.NewReader("# This is a new forked project\n"),
+					},
+				},
+			})
+			assert.NoError(t, err)
+
+			session := loginUser(t, "user5")
+			createPullRequest(t, session, "user5", "test_codeowner", forkedRepo.DefaultBranch, "codeowner-basebranch-forked", "Test Pull Request2")
+
+			pr := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{BaseRepoID: forkedRepo.ID, HeadBranch: "codeowner-basebranch-forked"})
+			unittest.AssertExistsIf(t, false, &issues_model.Review{IssueID: pr.IssueID, Type: issues_model.ReviewTypeRequest, ReviewerID: 8})
+		})
+	})
+}