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>

services/issue/pull.go

@@ -0,0 +1,121 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package issue
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	issues_model "code.gitea.io/gitea/models/issues"
+	org_model "code.gitea.io/gitea/models/organization"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+func getMergeBase(repo *git.Repository, pr *issues_model.PullRequest, baseBranch, headBranch string) (string, error) {
+	// Add a temporary remote
+	tmpRemote := fmt.Sprintf("mergebase-%d-%d", pr.ID, time.Now().UnixNano())
+	if err := repo.AddRemote(tmpRemote, repo.Path, false); err != nil {
+		return "", fmt.Errorf("AddRemote: %w", err)
+	}
+	defer func() {
+		if err := repo.RemoveRemote(tmpRemote); err != nil {
+			log.Error("getMergeBase: RemoveRemote: %v", err)
+		}
+	}()
+
+	mergeBase, _, err := repo.GetMergeBase(tmpRemote, baseBranch, headBranch)
+	return mergeBase, err
+}
+
+func PullRequestCodeOwnersReview(ctx context.Context, pull *issues_model.Issue, pr *issues_model.PullRequest) error {
+	files := []string{"CODEOWNERS", "docs/CODEOWNERS", ".gitea/CODEOWNERS"}
+
+	if pr.IsWorkInProgress(ctx) {
+		return nil
+	}
+
+	if err := pr.LoadHeadRepo(ctx); err != nil {
+		return err
+	}
+
+	if pr.HeadRepo.IsFork {
+		return nil
+	}
+
+	if err := pr.LoadBaseRepo(ctx); err != nil {
+		return err
+	}
+
+	repo, err := git.OpenRepository(ctx, pr.BaseRepo.RepoPath())
+	if err != nil {
+		return err
+	}
+	defer repo.Close()
+
+	commit, err := repo.GetBranchCommit(pr.BaseRepo.DefaultBranch)
+	if err != nil {
+		return err
+	}
+
+	var data string
+	for _, file := range files {
+		if blob, err := commit.GetBlobByPath(file); err == nil {
+			data, err = blob.GetBlobContent(setting.UI.MaxDisplayFileSize)
+			if err == nil {
+				break
+			}
+		}
+	}
+
+	rules, _ := issues_model.GetCodeOwnersFromContent(ctx, data)
+
+	// get the mergebase
+	mergeBase, err := getMergeBase(repo, pr, git.BranchPrefix+pr.BaseBranch, pr.GetGitRefName())
+	if err != nil {
+		return err
+	}
+
+	// https://github.com/go-gitea/gitea/issues/29763, we need to get the files changed
+	// between the merge base and the head commit but not the base branch and the head commit
+	changedFiles, err := repo.GetFilesChangedBetween(mergeBase, pr.HeadCommitID)
+	if err != nil {
+		return err
+	}
+
+	uniqUsers := make(map[int64]*user_model.User)
+	uniqTeams := make(map[string]*org_model.Team)
+	for _, rule := range rules {
+		for _, f := range changedFiles {
+			if (rule.Rule.MatchString(f) && !rule.Negative) || (!rule.Rule.MatchString(f) && rule.Negative) {
+				for _, u := range rule.Users {
+					uniqUsers[u.ID] = u
+				}
+				for _, t := range rule.Teams {
+					uniqTeams[fmt.Sprintf("%d/%d", t.OrgID, t.ID)] = t
+				}
+			}
+		}
+	}
+
+	for _, u := range uniqUsers {
+		if u.ID != pull.Poster.ID {
+			if _, err := issues_model.AddReviewRequest(ctx, pull, u, pull.Poster); err != nil {
+				log.Warn("Failed add assignee user: %s to PR review: %s#%d, error: %s", u.Name, pr.BaseRepo.Name, pr.ID, err)
+				return err
+			}
+		}
+	}
+	for _, t := range uniqTeams {
+		if _, err := issues_model.AddTeamReviewRequest(ctx, pull, t, pull.Poster); err != nil {
+			log.Warn("Failed add assignee team: %s to PR review: %s#%d, error: %s", t.Name, pr.BaseRepo.Name, pr.ID, err)
+			return err
+		}
+	}
+
+	return nil
+}