This is another try to fix the flaky tests TestLogsFollowSlowStdoutConsumer and TestRunSlowStdoutConsumer
The previous one is #17537

I noticed another failure in one of the PRs after #17537 was merged but it seems that the test was re-triggered and I couldn't find the exact failed URL in jenkins.

If I add the following debug lines after https://github.com/docker/docker/blob/master/daemon/logger/jsonfilelog/jsonfilelog.go#L370

        buf, _ := ioutil.ReadAll(dec.Buffered())
        logrus.Errorf("failed to decode log entry after %d retries. buffered: |||%s|||, Error: %v", retries, string(buf), err)

when the test fails, I can see the following error log:

time="2015-11-02T08:23:17.008895080Z" level=error msg="failed to decode log entry after 11 retries. buffered: |||\n{\"log\":\"X\\n\",\"stream\":\"stdout\",\"time\":\"2015-11|||, Error: unexpected EOF"
time="2015-11-02T08:23:17.008955856Z" level=error msg="Error streaming logs: unexpected EOF"

After changed maxJSONDecodeRetry to 20000 I run the test for 300 times locally and all succeed.
Before the change it usually fails within 100 times.

/cc @cpuguy83 @LK4D4