fix: backup will skip empty databases.

[tangwz]

What problem does this PR solve?

fixed: #337

Backup will skip empty databases.

What is changed and how it works?

add some log for backup empty databases.

Check List

Tests

• Unit test

Code changes

• Add some log.

Side effects

• None

Related changes

• None

Release Note

• No release note

[CLAassistant]

All committers have signed the CLA.

[YuJuncen]

/run-integration-test

Oops... Recently, the TiFlash test case isn't stable enough, don't worry, let's rerun it...

[YuJuncen]

LGTM

[3pointer]

LGTM

[kennytm]

/run-integration-test

https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/br_ghpr_unit_and_integration_test/detail/br_ghpr_unit_and_integration_test/3568/pipeline

[2020-11-01T07:51:19.968Z] ERROR 1105 (HY000) at line 1: the tiflash replica count: 1 should be less than the total tiflash server count: 0

[kennytm]

/run-integration-test

https://internal.pingcap.net/idc-jenkins/blue/rest/organizations/jenkins/pipelines/br_ghpr_unit_and_integration_test/runs/3571/nodes/108/log/?start=0

[2020-11-01T09:11:37.043Z] [2020/11/01 17:11:34.762 +08:00] [ERROR] [push.go:54] ["fail to connect store"] [StoreID=3] [stack="github.com/pingcap/br/pkg/backup.(*pushDown).pushBackup\n\t/home/jenkins/agent/workspace/br_ghpr_unit_and_integration_test/go/src/github.com/pingcap/br/pkg/backup/push.go:54\ngithub.com/pingcap/br/pkg/backup.(*Client).BackupRange\n\t/home/jenkins/agent/workspace/br_ghpr_unit_and_integration_test/go/src/github.com/pingcap/br/pkg/backup/client.go:504\ngithub.com/pingcap/br/pkg/backup.(*Client).BackupRanges.func2.1\n\t/home/jenkins/agent/workspace/br_ghpr_unit_and_integration_test/go/src/github.com/pingcap/br/pkg/backup/client.go:439\ngithub.com/pingcap/br/pkg/utils.(*WorkerPool).ApplyOnErrorGroup.func1\n\t/home/jenkins/agent/workspace/br_ghpr_unit_and_integration_test/go/src/github.com/pingcap/br/pkg/utils/worker.go:62\ngolang.org/x/sync/errgroup.(*Group).Go.func1\n\t/go/pkg/mod/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208/errgroup/errgroup.go:57"]

https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/br_ghpr_unit_and_integration_test/detail/br_ghpr_unit_and_integration_test/3571/pipeline/112

[2020-11-01T09:11:36.598Z] failed to restore, before: 1000; after: 960

[kennytm]

/run-integration-test

https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/br_ghpr_unit_and_integration_test/detail/br_ghpr_unit_and_integration_test/3585/pipeline

[2020-11-02T09:05:55.099Z] failed to restore, before: 1000; after: 960

https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/br_ghpr_unit_and_integration_test/detail/br_ghpr_unit_and_integration_test/3585/pipeline/113

[2020-11-02T09:04:43.139Z] Waiting initializing TiKV...

[2020-11-02T09:04:53.086Z] [2020/11/02 17:04:51.593 +08:00] [WARN] [retry_interceptor.go:61] ["retrying of unary invoker failed"] [target=endpoint://client-5e8b8b6d-aeaa-4df0-8733-66a2f7c33f79/127.0.0.1:2379] [attempt=0] [error="rpc error: code = DeadlineExceeded desc = context deadline exceeded"]

[2020-11-02T09:04:54.455Z] [2020/11/02 17:04:54.313 +08:00] [WARN] [retry_interceptor.go:61] ["retrying of unary invoker failed"] [target=endpoint://client-5e8b8b6d-aeaa-4df0-8733-66a2f7c33f79/127.0.0.1:2379] [attempt=0] [error="rpc error: code = DeadlineExceeded desc = context deadline exceeded"]

[2020-11-02T09:04:55.385Z] {"level":"warn","ts":"2020-11-02T17:04:55.191+0800","caller":"clientv3/retry_interceptor.go:61","msg":"retrying of unary invoker failed","target":"endpoint://client-92948e35-fc23-47b5-88fd-6f54cbf4dfa1/127.0.0.1:2379","attempt":0,"error":"rpc error: code = NotFound desc = etcdserver: requested lease not found"}

[2020-11-02T09:05:51.562Z] Failed to initialize TiKV cluster

[kennytm]

/build