etcdserver: exit on missing backend only if semver is >= 3.0.0

[heyitsanthony]

/cc @xiang90 (is this what you meant?)

[xiang90]

@heyitsanthony Yes. Thanks!

[gyuho]

Confirmed that this fixes the issue from etcd-dev mailing list:

I'm trying to upgrade my cluster from 2.3.7 to 3.0.0. Before the upgrade, etcdctl says that the cluster is healthy, but when I try and start up a node with 3.0.0 I get the following error:

etcdmain: database file (/var/lib/etcd/nms10.dmacc.net/member/snap/db) of the backend is missing

Looking at the data directory when 2.3.7 is running all that I see is this:

[root@nms10 member]# ls -laR
.:
total 16
drwx------. 4 etcd etcd 4096 Jul  1 10:18 .
drwx------. 3 etcd etcd 4096 Jul  1 10:18 ..
drwx------. 2 etcd etcd 4096 Jul  1 10:16 snap
drwx------. 2 etcd etcd 4096 Jul  1 10:18 wal

./snap:
total 2208
drwx------. 2 etcd etcd   4096 Jul  1 10:16 .
drwx------. 4 etcd etcd   4096 Jul  1 10:18 ..
-rw-r--r--. 1 etcd etcd 449368 Jul  1 04:28 0000000000012b16-0000000003c054f0.snap
-rw-r--r--. 1 etcd etcd 449367 Jul  1 05:51 0000000000012b16-0000000003c07c01.snap
-rw-r--r--. 1 etcd etcd 449386 Jul  1 07:15 0000000000012b16-0000000003c0a312.snap
-rw-r--r--. 1 etcd etcd 449400 Jul  1 08:38 0000000000012b16-0000000003c0ca23.snap
-rw-r--r--. 1 etcd etcd 449223 Jul  1 10:01 0000000000012b17-0000000003c0f134.snap

./wal:
total 375048
drwx------. 2 etcd etcd     4096 Jul  1 10:18 .
drwx------. 4 etcd etcd     4096 Jul  1 10:18 ..
-rw-------. 1 etcd etcd 64000083 Jun 19 22:59 0000000000000071-00000000039a8a21.wal
-rw-------. 1 etcd etcd 64000125 Jun 23 01:43 0000000000000072-0000000003a2c10f.wal
-rw-------. 1 etcd etcd 64000053 Jun 26 04:03 0000000000000073-0000000003aaefce.wal
-rw-------. 1 etcd etcd 64000039 Jun 29 06:24 0000000000000074-0000000003b31a71.wal
-rw-------. 1 etcd etcd 44612719 Jul  1 10:25 0000000000000075-0000000003bb4551.wal

https://groups.google.com/forum/#!topic/etcd-dev/LkJkX9ZFiKQ

[xiang90]

lgtm