vfs: move writeback of dirty data out of close() method into its own method (FlushWrites) and remove close() call from Flush()

If a file handle is duplicated with dup() and the duplicate handle is
flushed, rclone will go ahead and close the file, making the original
file handle stale. This change removes the close() call from Flush() and
replaces it with FlushWrites() so that the file only gets closed when
Release() is called. The new FlushWrites method takes care of actually
writing the file back to the underlying storage.

Fixes #3381

cmd/mountlib/mounttest/write.go

@@ -1,10 +1,13 @@
 package mounttest
 
 import (
+	"os"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+
+	"github.com/rclone/rclone/vfs"
 )
 
 // TestWriteFileNoWrite tests writing a file with no write()'s to it
@@ -82,3 +85,48 @@ func TestWriteFileFsync(t *testing.T) {
 	run.waitForWriters()
 	run.rm(t, "to be synced")
 }
+
+// TestWriteFileDup tests behavior of mmap() in Python by using dup() on a file handle
+func TestWriteFileDup(t *testing.T) {
+	run.skipIfNoFUSE(t)
+
+	if run.vfs.Opt.CacheMode < vfs.CacheModeWrites {
+		t.Skip("not supported on vfs-cache-mode < writes")
+		return
+	}
+
+	filepath := run.path("to be synced")
+	fh, err := osCreate(filepath)
+	require.NoError(t, err)
+
+	testData := []byte("0123456789")
+
+	err = fh.Truncate(int64(len(testData) + 2))
+	require.NoError(t, err)
+
+	err = fh.Sync()
+	require.NoError(t, err)
+
+	var dupFd uintptr
+	dupFd, err = writeTestDup(fh.Fd())
+	require.NoError(t, err)
+
+	dupFile := os.NewFile(dupFd, fh.Name())
+	_, err = dupFile.Write(testData)
+	require.NoError(t, err)
+
+	err = dupFile.Close()
+	require.NoError(t, err)
+
+	_, err = fh.Seek(int64(len(testData)), 0)
+	require.NoError(t, err)
+
+	_, err = fh.Write([]byte("10"))
+	require.NoError(t, err)
+
+	err = fh.Close()
+	require.NoError(t, err)
+
+	run.waitForWriters()
+	run.rm(t, "to be synced")
+}