fix gcc warning about dangling-reference in backup_engine_test

[raffertyyu]

gcc 14.1 reports some warnings about dangling-reference occured in backup_engine_test.

/data/rocksdb/utilities/backup/backup_engine_test.cc: In member function 'virtual void rocksdb::{anonymous}::BackupEngineTest_ExcludeFiles_Test::TestBody()':
/data/rocksdb/utilities/backup/backup_engine_test.cc:4411:64: error: possibly dangling reference to a temporary [-Werror=dangling-reference]
 4411 |         std::make_pair(alt_backup_engine, backup_engine_.get())}) {
      |                                                                ^
/data/rocksdb/utilities/backup/backup_engine_test.cc:4410:23: note: the temporary was destroyed at the end of the full expression 'std::make_pair<rocksdb::BackupEngine*, rocksdb::BackupEngine*&>(((rocksdb::{anonymous}::BackupEngineTest_ExcludeFiles_Test*)this)->rocksdb::{anonymous}::BackupEngineTest_ExcludeFiles_Test::rocksdb::{anonymous}::BackupEngineTest.rocksdb::{anonymous}::BackupEngineTest::backup_engine_.std::unique_ptr<rocksdb::BackupEngine>::get(), alt_backup_engine)'
 4410 |        {std::make_pair(backup_engine_.get(), alt_backup_engine),
      |         ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/data/rocksdb/utilities/backup/backup_engine_test.cc:4411:64: error: possibly dangling reference to a temporary [-Werror=dangling-reference]
 4411 |         std::make_pair(alt_backup_engine, backup_engine_.get())}) {
      |                                                                ^
/data/rocksdb/utilities/backup/backup_engine_test.cc:4411:23: note: the temporary was destroyed at the end of the full expression 'std::make_pair<rocksdb::BackupEngine*&, rocksdb::BackupEngine*>(alt_backup_engine, ((rocksdb::{anonymous}::BackupEngineTest_ExcludeFiles_Test*)this)->rocksdb::{anonymous}::BackupEngineTest_ExcludeFiles_Test::rocksdb::{anonymous}::BackupEngineTest.rocksdb::{anonymous}::BackupEngineTest::backup_engine_.std::unique_ptr<rocksdb::BackupEngine>::get())'
 4411 |         std::make_pair(alt_backup_engine, backup_engine_.get())}) {
      |         ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It seems to be related to this update in gcc:
https://gcc.gnu.org/gcc-14/changes.html#:~:text=%2DWdangling%2Dreference%20false%20positives%20have%20been%20reduced.%20The%20warning%20does%20not%20warn%20about%20std%3A%3Aspan%2Dlike%20classes%3B%20there%20is%20also%20a%20new%20attribute%20gnu%3A%3Ano_dangling%20to%20suppress%20the%20warning.%20See%20the%20manual%20for%20more%20info.

[ajkr]

Can backup_engine become a dangling pointer here? I am wondering if this caused a subsequent use of backup_engine to segfault in https://github.com/facebook/rocksdb/actions/runs/9033386298/job/24823630007?pr=12637

[raffertyyu]

You're right. I ignored the change of backup_engine_ before. I've modified code to assign backup_engine_.get() to backup_engine when backup_engine_ changed.

[facebook-github-bot]

@ajkr has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@ajkr merged this pull request in 4dd084f.