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_unix.go

@@ -4,10 +4,12 @@ package mounttest
 
 import (
 	"runtime"
-	"syscall"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"golang.org/x/sys/unix"
+
+	"github.com/rclone/rclone/vfs"
 )
 
 // TestWriteFileDoubleClose tests double close on write
@@ -21,14 +23,14 @@ func TestWriteFileDoubleClose(t *testing.T) {
 	assert.NoError(t, err)
 	fd := out.Fd()
 
-	fd1, err := syscall.Dup(int(fd))
+	fd1, err := unix.Dup(int(fd))
 	assert.NoError(t, err)
 
-	fd2, err := syscall.Dup(int(fd))
+	fd2, err := unix.Dup(int(fd))
 	assert.NoError(t, err)
 
 	// close one of the dups - should produce no error
-	err = syscall.Close(fd1)
+	err = unix.Close(fd1)
 	assert.NoError(t, err)
 
 	// write to the file
@@ -41,14 +43,26 @@ func TestWriteFileDoubleClose(t *testing.T) {
 	err = out.Close()
 	assert.NoError(t, err)
 
-	// write to the other dup - should produce an error
-	_, err = syscall.Write(fd2, buf)
-	assert.Error(t, err, "input/output error")
+	// write to the other dup
+	_, err = unix.Write(fd2, buf)
+	if run.vfs.Opt.CacheMode < vfs.CacheModeWrites {
+		// produces an error if cache mode < writes
+		assert.Error(t, err, "input/output error")
+	} else {
+		// otherwise does not produce an error
+		assert.NoError(t, err)
+	}
 
 	// close the dup - should not produce an error
-	err = syscall.Close(fd2)
+	err = unix.Close(fd2)
 	assert.NoError(t, err)
 
 	run.waitForWriters()
 	run.rm(t, "testdoubleclose")
 }
+
+// writeTestDup performs the platform-specific implementation of the dup() unix
+func writeTestDup(oldfd uintptr) (uintptr, error) {
+	newfd, err := unix.Dup(int(oldfd))
+	return uintptr(newfd), err
+}