vfs: fix Create causing windows explorer to truncate files on CTRL-C CTRL-V

Before this fix, doing CTRL-C and CTRL-V on a file in Windows explorer
caused the **source** and the the destination to be truncated to 0.

This is because Windows opens the source file with Create with flags
`O_RDWR|O_CREATE|O_EXCL` but doesn't write to it - it only reads from
it. Rclone was taking the call to Create as a signal to always make a
new file, but this is incorrect.

This fix reads an existing file from the directory if it exists when
Create is called rather than always creating a new one. This fixes the
problem.

Fixes #5181

vfs/dir.go

@@ -843,6 +843,23 @@ func (d *Dir) Open(flags int) (fd Handle, err error) {
 // Create makes a new file node
 func (d *Dir) Create(name string, flags int) (*File, error) {
 	// fs.Debugf(path, "Dir.Create")
+	// Return existing node if one exists
+	node, err := d.stat(name)
+	switch err {
+	case ENOENT:
+		// not found, carry on
+	case nil:
+		// found so check what it is
+		if node.IsFile() {
+			return node.(*File), err
+		}
+		return nil, EEXIST // EISDIR would be better but we don't have that
+	default:
+		// a different error - report
+		fs.Errorf(d, "Dir.Create stat failed: %v", err)
+		return nil, err
+	}
+	// node doesn't exist so create it
 	if d.vfs.Opt.ReadOnly {
 		return nil, EROFS
 	}

vfs/dir_test.go

@@ -375,6 +375,13 @@ func TestDirCreate(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, int64(5), file2.Size())
 
+	// Try creating the file again - make sure we get the same file node
+	file3, err := dir.Create("potato", os.O_RDWR|os.O_CREATE)
+	require.NoError(t, err)
+	assert.Equal(t, int64(5), file3.Size())
+	assert.Equal(t, fmt.Sprintf("%p", file), fmt.Sprintf("%p", file3), "didn't return same node")
+
+	// Test read only fs creating new
 	vfs.Opt.ReadOnly = true
 	_, err = dir.Create("sausage", os.O_WRONLY|os.O_CREATE)
 	assert.Equal(t, EROFS, err)