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_windows.go

@@ -0,0 +1,38 @@
+//+build windows
+
+package local
+
+import (
+	"os"
+	"syscall"
+	"time"
+
+	"github.com/ncw/rclone/fs"
+)
+
+const (
+	ERROR_SHARING_VIOLATION syscall.Errno = 32
+)
+
+// Removes name, retrying on a sharing violation
+func remove(name string) (err error) {
+	const maxTries = 10
+	var sleepTime = 1 * time.Millisecond
+	for i := 0; i < maxTries; i++ {
+		err = os.Remove(name)
+		if err == nil {
+			break
+		}
+		pathErr, ok := err.(*os.PathError)
+		if !ok {
+			break
+		}
+		if pathErr.Err != ERROR_SHARING_VIOLATION {
+			break
+		}
+		fs.Logf(name, "Remove detected sharing violation - retry %d/%d sleeping %v", i+1, maxTries, sleepTime)
+		time.Sleep(sleepTime)
+		sleepTime <<= 1
+	}
+	return err
+}