Backup: compact hang in volume test

Build 5.0.0-3506
On a volume test, we observed that compact hangs for around 11 hours.
We killed it using command: Kill -SIGQUIT pid and collected stack trace at backup.log.zip
Number of docs present at the time of backup started was around 500M.
This is first time we are running this volume test with compact cmd for 5.0.

Edit:
Note that backup and compact runs fine for first 12 backups and this got hung on 13th time.
About test: Test has 3 iterations. Each iteration takes backup and compact it 6 times, then merge all the backups. We saw this issue in 3rd iteration.