How to use &mut File.set_len(), if OpenOptions::append(true)

help
1

my code:

 match File::create_new(&fp) {
            Ok(f) => Ok(f),
            Err(x) => match x {
                y if y.kind() == ErrorKind::AlreadyExists => {
                    File::options().read(true).append(true).open(&fp)
                }

                other => Err(other),
            },
        }

    pub fn format(&self) -> io::Result<()> {
        let mut data = Vec::new();
        let mut fl = self.file.borrow_mut(); 
        fl.read.rewind()?;
        fl.read.read_from_file_until(&mut data, 4000)?;
        let mut data = String::from_utf8_lossy(&data).into_owned();
        let ind = data
            .match_indices(".")
            .map(|x| {
                x.0
            })
            .collect::<Vec<usize>>();
        Self::_add_char(&mut data, &ind, '\n');
        println!("{} {:?}", data, ind);
       dbg!(fl.write.get_mut().set_len(0))?;
        dbg!(fl.write.rewind())?;
        dbg!(fl.write.get_mut().write_all(data.as_bytes()))?;
        Ok(())
    }

error:

fl.write.get_mut().set_len(0)=Err(
    Os {
        code: 5,
        kind: PermissionDenied,
        message: "...",
    },
)
2

Does the file you use for testing already exist or not? create_new does not set the append option to true, maybe that is the reason why an error is given?

3

Yes, this file already exists

4

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