local: retry remove on Windows sharing violation error #2202

Before this change asynchronous closes in cmount could cause sharing
violations under Windows on Remove which manifest themselves
frequently as test failures.

This change lets the Remove be retried on a sharing violation under
Windows.

backend/local/remove_test.go

@@ -0,0 +1,50 @@
+package local
+
+import (
+	"io/ioutil"
+	"os"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// Check we can remove an open file
+func TestRemove(t *testing.T) {
+	fd, err := ioutil.TempFile("", "rclone-remove-test")
+	require.NoError(t, err)
+	name := fd.Name()
+	defer func() {
+		_ = os.Remove(name)
+	}()
+
+	exists := func() bool {
+		_, err := os.Stat(name)
+		if err == nil {
+			return true
+		} else if os.IsNotExist(err) {
+			return false
+		}
+		require.NoError(t, err)
+		return false
+	}
+
+	assert.True(t, exists())
+	// close the file in the background
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		time.Sleep(250 * time.Millisecond)
+		require.NoError(t, fd.Close())
+	}()
+	// delete the open file
+	err = remove(name)
+	require.NoError(t, err)
+	// check it no longer exists
+	assert.False(t, exists())
+	// wait for background close
+	wg.Wait()
+}