Consider the case when log stream has no log events

[yutachaos]

Issue #, if available:

Description of changes:

• If the log stream has no log events, describe-log-stream output does not include LastIngestionTime, so it becomes panic due to nil pointer dereference。

Reproduce

# NG pattern
aws logs create-log-group --log-group-name test-log-group
aws logs create-log-stream --log-group-name test-log-group --log-stream-name test-log-stream
aws logs describe-log-streams --log-group-name test-log-group --log-stream-name test-log-stream
{
    "logStreams": [
        {
            "logStreamName": "test-log-stream",
            "creationTime": 1585056526451,
            "arn": "arn:aws:logs:ap-northeast-1::log-group:test-log-group:log-stream:test-log-stream",
            "storedBytes": 0
        }
    ]
}
aws logs get-log-events --log-group-name test-log-group --log-stream-name test-log-stream
{
    "events": [],
    "nextForwardToken": "hogehoge",
    "nextBackwardToken": fugafuga"
}
utern test-log-group
+ test-log-group
⠋ Fetching log streams... panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x14f82d6]

goroutine 1 [running]:
github.com/knqyf263/utern/cloudwatch.(*Client).ListStreams.func1(0xc000332cc0, 0xc0004c4c01, 0x10000c0004073e0)
	/home/travis/gopath/src/github.com/knqyf263/utern/cloudwatch/stream.go:32 +0xf6
github.com/aws/aws-sdk-go/service/cloudwatchlogs.(*CloudWatchLogs).DescribeLogStreamsPagesWithContext(0xc00000e040, 0x17b4da0, 0xc000089b40, 0xc0004896b0, 0xc0002cd4a8, 0x0, 0x0, 0x0, 0xc0002cd4c8, 0x14853f4)
	/home/travis/gopath/pkg/mod/github.com/aws/[email protected]/service/cloudwatchlogs/api.go:1666 +0x163
github.com/knqyf263/utern/cloudwatch.(*Client).ListStreams(0xc0000680b0, 0x17b4da0, 0xc000089b40, 0xc000374ff0, 0xe, 0x1710cb72770, 0x1b48838, 0x0, 0x0, 0x0, ...)
	/home/travis/gopath/src/github.com/knqyf263/utern/cloudwatch/stream.go:66 +0x1d2
github.com/knqyf263/utern/cloudwatch.(*Client).Tail(0xc0000680b0, 0x17b4da0, 0xc000089b40, 0xc000089b40, 0xc000200580)
	/home/travis/gopath/src/github.com/knqyf263/utern/cloudwatch/cloudwatch.go:100 +0x782
main.run(0xc0001f4e70, 0xc0001f4e70, 0x0)
	/home/travis/gopath/src/github.com/knqyf263/utern/main.go:121 +0x8f
main.main.func1(0xc0000d86e0, 0x0, 0x0)
	/home/travis/gopath/src/github.com/knqyf263/utern/main.go:90 +0x8d
github.com/urfave/cli.HandleAction(0x1557380, 0x1631ed8, 0xc0000d86e0, 0x0, 0x0)
	/home/travis/gopath/pkg/mod/github.com/urfave/[email protected]/app.go:523 +0xbe
github.com/urfave/cli.(*App).Run(0xc0001fe000, 0xc0000aa040, 0x2, 0x2, 0x0, 0x0)
	/home/travis/gopath/pkg/mod/github.com/urfave/[email protected]/app.go:285 +0x5df
main.main()
	/home/travis/gopath/src/github.com/knqyf263/utern/main.go:93 +0x907

# OK pattern

TIMESTAMP=$(node -e 'console.log(Date.now())')
aws logs put-log-events --log-group-name test-log-group --log-stream-name test-log-stream --log-events timestamp=$TIMESTAMP,message='test'
aws logs describe-log-streams --log-group-name test-log-group --log-stream-name test-log-stream
utern test-log-group
aws logs put-log-events --log-gm --log-events timestamp=$TIMESTAMP,message='test'g-strea
{
    "nextSequenceToken": "fuga"
}
aws logs describe-log-streams --log-group-name test-log-group --log-stream-name test-log-stream
{
    "logStreams": [
        {
            "logStreamName": "test-log-stream",
            "creationTime": 1585056526451,
            "firstEventTimestamp": 1585056572981,
            "lastEventTimestamp": 1585056572981,
            "lastIngestionTime": 1585056573954,
            "uploadSequenceToken": "mogamoga",
            "arn": "arn:aws:logs:ap-northeast-1:xxxxxxx:log-group:test-log-group:log-stream:test-log-stream",
            "storedBytes": 0
        }
    ]
}

utern test-log-group
+ test-log-group
+ test-log-group › test-log-stream (2020-03-24T22:29:33+09:00)
test-log-group test-log-stream test

[knqyf263]

Thank you for fixing the bug! I'd like to make sure uploadSequenceToken is always nil if a log stream is empty and the opposite as well. Also, I'm wondering if lastIngestionTime is always not nil with non-nil uploadSequenceToken.

[yutachaos]

Thank you for fixing the bug! I'd like to make sure uploadSequenceToken is always nil if a log stream is empty and the opposite as well. Also, I'm wondering if lastIngestionTime is always not nil with non-nil uploadSequenceToken.

Thank for your comment.
If there is no log event in the log stream, FirstEventTimestamp, LastEventTimestamp, LastIngestionTime, and UploadSequenceToken will be nil.
This activity is not officially documented.
However, it is confirmed by a support ticket on AWS.

Added FirstEventTimestamp, LastEventTimestamp, LastIngestionTime, and UploadSequenceToken nil check.

[knqyf263]

Thanks a lot!