Technical Tip: How to fix upgrade failure due to an etcd service issue

'etcd service' was first introduced in v7.3.0:

What's New in 7.3.0
https://etcd.io/docs/v3.3/faq/

The upgrade can fail because the 'etcd service' is not able to start. It could not fall back to the latest snapshot (snapshot file is missing or corrupt):

Jul 22 11:41:45 fortisiem bash[911808]: {"level":"warn","ts":"2025-07-22T11:41:45.034526+0200","caller":"snap/db.go:88","msg":"failed to find [SNAPSHOT-INDEX].snap.db","snapshot-index":7900079,"snapshot-file-path":"/var/lib/etcd/default.etcd/member/snap/0000000000788baf.snap.db","error":"snap: snapshot file doesn't exist"}
Jul 22 11:41:45 fortisiem bash[911808]: {"level":"panic","ts":"2025-07-22T11:41:45.034572+0200","caller":"etcdserver/server.go:518","msg":"failed to recover v3 backend from snapshot","error":"failed to find database snapshot file (snap: snapshot file doesn't exist)","stacktrace":"go.etcd.io/etcd/server/v3/etcdserver.NewServer\n\tgo.etcd.io/etcd/server/v3/etcdserver/server.go:518\ngo.etcd.io/etcd/server/v3/embed.StartEtcd\n\tgo.etcd.io/etcd/server/v3/embed/etcd.go:245\ngo.etcd.io/etcd/server/v3/etcdmain.startEtcd\n\tgo.etcd.io/etcd/server/v3/etcdmain/etcd.go:228\ngo.etcd.io/etcd/server/v3/etcdmain.startEtcdOrProxyV2\n\tgo.etcd.io/etcd/server/v3/etcdmain/etcd.go:123\ngo.etcd.io/etcd/server/v3/etcdmain.Main\n\tgo.etcd.io/etcd/server/v3/etcdmain/main.go:40\nmain.main\n\tgo.etcd.io/etcd/server/v3/main.go:31\nruntime.main\n\truntime/proc.go:267"}
Jul 22 11:41:45 fortisiem bash[911808]: panic: failed to recover v3 backend from snapshot

Run the commands below to recover the 'etcd service':

sudo systemctl stop etcd
sudo rm -rf /var/lib/etcd/default.etcd
sudo systemctl start etcd

Run the upgrade again.