Protected tag is no internal server error (#30962)

Fixes #30959

Adds an API test for protected tags.
Fix existing tag in combination with fixtures.

tests/integration/api_releases_test.go

@@ -154,6 +154,31 @@ func TestAPICreateAndUpdateRelease(t *testing.T) {
 	assert.EqualValues(t, rel.Note, newRelease.Note)
 }
 
+func TestAPICreateProtectedTagRelease(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 4})
+	writer := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 4})
+	session := loginUser(t, writer.LowerName)
+	token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeWriteRepository)
+
+	gitRepo, err := gitrepo.OpenRepository(git.DefaultContext, repo)
+	assert.NoError(t, err)
+	defer gitRepo.Close()
+
+	commit, err := gitRepo.GetBranchCommit("master")
+	assert.NoError(t, err)
+
+	req := NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/%s/releases", repo.OwnerName, repo.Name), &api.CreateReleaseOption{
+		TagName:      "v0.0.1",
+		Title:        "v0.0.1",
+		IsDraft:      false,
+		IsPrerelease: false,
+		Target:       commit.ID.String(),
+	}).AddTokenAuth(token)
+	MakeRequest(t, req, http.StatusUnprocessableEntity)
+}
+
 func TestAPICreateReleaseToDefaultBranch(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()