Decouple removing the fileWatcher from reading

Fixes #27779

Currently `followLogs` can get into a deadlock if we receive an inotify
IN_MODIFY event while we are trying to close the `fileWatcher`. This is
due to the fact that closing the `fileWatcher` happens in the same block
as consumes events from the `fileWatcher`. We are trying to run
`fileWatcher.Close`, which is waiting for an IN_IGNORE event to come in
over inotify to confirm the watch was been removed. But, because an
IN_MODIFY event has appeared after `Close` was entered but before the
IN_IGNORE, the broadcast never comes. The IN_MODIFY cannot be consumed
as the events channel is unbuffered and the only `select` that reads
from it is busy waiting for the IN_IGNORE event.

In order to try and fix this race condition I've moved the removal of
the `fileWatcher` out to a separate go block that waits for a signal to
close, removes the watcher and then signals to the previous selects on
the close signal.

This has introduced a `fileWatcher.Remove` in the final case, but if we
try and remove a watcher that does not exist it will just return an
error saying so. We are not doing any checking on the return of `Remove`
so this shouldn't cause any side-effects.

Signed-off-by: Tom Booth <tombooth@gmail.com>

daemon/logger/jsonfilelog/read.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"time"
 
+	"golang.org/x/net/context"
 	"gopkg.in/fsnotify.v1"
 
 	"github.com/Sirupsen/logrus"
@@ -172,9 +173,22 @@ func followLogs(f *os.File, logWatcher *logger.LogWatcher, notifyRotate chan int
 	}
 	defer func() {
 		f.Close()
+		fileWatcher.Remove(name)
 		fileWatcher.Close()
 	}()
 
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	go func() {
+		select {
+		case <-logWatcher.WatchClose():
+			fileWatcher.Remove(name)
+			cancel()
+		case <-ctx.Done():
+			return
+		}
+	}()
+
 	var retries int
 	handleRotate := func() error {
 		f.Close()
@@ -209,8 +223,7 @@ func followLogs(f *os.File, logWatcher *logger.LogWatcher, notifyRotate chan int
 			case fsnotify.Rename, fsnotify.Remove:
 				select {
 				case <-notifyRotate:
-				case <-logWatcher.WatchClose():
+				case <-ctx.Done():
-					fileWatcher.Remove(name)
 					return errDone
 				}
 				if err := handleRotate(); err != nil {
@@ -232,8 +245,7 @@ func followLogs(f *os.File, logWatcher *logger.LogWatcher, notifyRotate chan int
 				return errRetry
 			}
 			return err
-		case <-logWatcher.WatchClose():
+		case <-ctx.Done():
-			fileWatcher.Remove(name)
 			return errDone
 		}
 	}
@@ -290,7 +302,7 @@ func followLogs(f *os.File, logWatcher *logger.LogWatcher, notifyRotate chan int
 		}
 		select {
 		case logWatcher.Msg <- msg:
-		case <-logWatcher.WatchClose():
+		case <-ctx.Done():
 			logWatcher.Msg <- msg
 			for {
 				msg, err := decodeLogLine(dec, l)