Restore deleted branches when syncing (#29898)

Regression of #29493. If a branch has been deleted, repushing it won't
restore it.

Lunny may have noticed that, but I didn't delve into the comment then
overlooked it:
https://github.com/go-gitea/gitea/pull/29493#discussion_r1509046867

The additional comments added are to explain the issue I found during
testing, which are unrelated to the fixes.

routers/private/hook_post_receive.go

@@ -75,6 +75,10 @@ func HookPostReceive(ctx *gitea_context.PrivateContext) {
 			updates = append(updates, option)
 			if repo.IsEmpty && (refFullName.BranchName() == "master" || refFullName.BranchName() == "main") {
 				// put the master/main branch first
+				// FIXME: It doesn't always work, since the master/main branch may not be the first batch of updates.
+				//        If the user pushes many branches at once, the Git hook will call the internal API in batches, rather than all at once.
+				//        See https://github.com/go-gitea/gitea/blob/cb52b17f92e2d2293f7c003649743464492bca48/cmd/hook.go#L27
+				//        If the user executes `git push origin --all` and pushes more than 30 branches, the master/main may not be the default branch.
 				copy(updates[1:], updates)
 				updates[0] = option
 			}
@@ -129,9 +133,7 @@ func HookPostReceive(ctx *gitea_context.PrivateContext) {
 				commitIDs = append(commitIDs, update.NewCommitID)
 			}
 
-			if err := repo_service.SyncBranchesToDB(ctx, repo.ID, opts.UserID, branchNames, commitIDs, func(commitID string) (*git.Commit, error) {
+			if err := repo_service.SyncBranchesToDB(ctx, repo.ID, opts.UserID, branchNames, commitIDs, gitRepo.GetCommit); err != nil {
-				return gitRepo.GetCommit(commitID)
-			}); err != nil {
 				ctx.JSON(http.StatusInternalServerError, private.HookPostReceiveResult{
 					Err: fmt.Sprintf("Failed to sync branch to DB in repository: %s/%s Error: %v", ownerName, repoName, err),
 				})

services/repository/branch.go

@@ -318,11 +318,11 @@ func SyncBranchesToDB(ctx context.Context, repoID, pusherID int64, branchNames,
 		for i, branchName := range branchNames {
 			commitID := commitIDs[i]
 			branch, exist := branchMap[branchName]
-			if exist && branch.CommitID == commitID {
+			if exist && branch.CommitID == commitID && !branch.IsDeleted {
 				continue
 			}
 
-			commit, err := getCommit(branchName)
+			commit, err := getCommit(commitID)
 			if err != nil {
 				return fmt.Errorf("get commit of %s failed: %v", branchName, err)
 			}

tests/integration/git_push_test.go

@@ -69,6 +69,23 @@ func testGitPush(t *testing.T, u *url.URL) {
 			return pushed, deleted
 		})
 	})
+
+	t.Run("Push to deleted branch", func(t *testing.T) {
+		runTestGitPush(t, u, func(t *testing.T, gitPath string) (pushed, deleted []string) {
+			doGitPushTestRepository(gitPath, "origin", "master")(t) // make sure master is the default branch instead of a branch we are going to delete
+			pushed = append(pushed, "master")
+
+			doGitCreateBranch(gitPath, "branch-1")(t)
+			doGitPushTestRepository(gitPath, "origin", "branch-1")(t)
+			pushed = append(pushed, "branch-1")
+
+			// delete and restore
+			doGitPushTestRepository(gitPath, "origin", "--delete", "branch-1")(t)
+			doGitPushTestRepository(gitPath, "origin", "branch-1")(t)
+
+			return pushed, deleted
+		})
+	})
 }
 
 func runTestGitPush(t *testing.T, u *url.URL, gitOperation func(t *testing.T, gitPath string) (pushed, deleted []string)) {