etcdctl snapshot mixes stdout and stderr

[hongchaodeng]

etcdctl snapshot restore command mixes stdout and stderr message into stderr. This is due to capnslog sets it by default:
https://github.com/coreos/etcd/blob/0f1ac0cef6834f0927dec74b0f0bf4d0dad9b763/cmd/vendor/github.com/coreos/pkg/capnslog/init.go#L36

It is better to only write error messages into stderr.

[hongchaodeng]

2018-01-19 23:42:54.398661 I | pkg/netutil: resolving test-etcd-backup-restore-0-0000.test-etcd-backup-restore-0.e2e-etcd-operator-flaketest-580.svc:2380 to 10.28.5.36:2380
2018-01-19 23:42:54.402454 I | pkg/netutil: resolving test-etcd-backup-restore-0-0000.test-etcd-backup-restore-0.e2e-etcd-operator-flaketest-580.svc:2380 to 10.28.4.183:2380
Error:  --initial-cluster must include test-etcd-backup-restore-0-0000=https://test-etcd-backup-restore-0-0000.test-etcd-backup-restore-0.e2e-etcd-operator-flaketest-580.svc:2380 given --initial-advertise-peer-urls=https://test-etcd-backup-restore-0-0000.test-etcd-backup-restore-0.e2e-etcd-operator-flaketest-580.svc:2380

From user's perspective, only the error message should go into stderr, i.e. last line with Error. The first two lines look like logging messages, and as a user I would expect them go into stdout.

[xiang90]

what messages did you see went into stdout?

it is wrong that only error should go to stderr. users should expect only result of the command goes (the matched text of grep command for example) into stdout, which is the common practice of unix program. All informational or diagnostic messages go to stderr.

[hongchaodeng]

I agree with that. But is the "resolving ..." messages considered "diagnostic message"? Because the last line should contain enough message to diagnose.

[xiang90]

I agree with that. But is the "resolving ..." messages considered "diagnostic message"? Because the last line should contain enough message to diagnose.

yes. it is informational rather than the executing result of the command. it should go to stderr.

only the result of the command should go to stdout. for restore/bakup, probably we can output succeed to xxx into stdout. but it is OK to not produce anything to stdout and exit 0 to indicate a success.

[xiang90]

closing this out since etcdctl does the right thing.