daemon/logger/loggerutils: add TestFollowLogsClose

This test case checks that followLogs() exits once the reader is gone.
Currently it does not (i.e. this test is supposed to fail) due to moby#37391.

[kolyshkin@: test case Brian Goff, changelog and all bugs are by me]
Source: https://gist.github.com/cpuguy83/e538793de18c762608358ee0eaddc197

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

daemon/logger/loggerutils/logfile_test.go

@@ -4,6 +4,8 @@ import (
 	"bufio"
 	"context"
 	"io"
+	"io/ioutil"
+	"os"
 	"strings"
 	"testing"
 	"time"
@@ -74,3 +76,44 @@ func TestTailFiles(t *testing.T) {
 		assert.Assert(t, string(msg.Line) == "Where we're going we don't need roads.", string(msg.Line))
 	}
 }
+
+func TestFollowLogsClose(t *testing.T) {
+	lw := logger.NewLogWatcher()
+
+	f, err := ioutil.TempFile("", t.Name())
+	assert.NilError(t, err)
+	defer func() {
+		f.Close()
+		os.Remove(f.Name())
+	}()
+
+	makeDecoder := func(rdr io.Reader) func() (*logger.Message, error) {
+		return func() (*logger.Message, error) {
+			return &logger.Message{}, nil
+		}
+	}
+
+	followLogsDone := make(chan struct{})
+	var since, until time.Time
+	go func() {
+		followLogs(f, lw, make(chan interface{}), makeDecoder, since, until)
+		close(followLogsDone)
+	}()
+
+	select {
+	case <-lw.Msg:
+	case err := <-lw.Err:
+		assert.NilError(t, err)
+	case <-followLogsDone:
+		t.Fatal("follow logs finished unexpectedly")
+	case <-time.After(10 * time.Second):
+		t.Fatal("timeout waiting for log message")
+	}
+
+	lw.Close()
+	select {
+	case <-followLogsDone:
+	case <-time.After(20 * time.Second):
+		t.Fatal("timeout waiting for followLogs() to finish")
+	}
+}