Allow specifying R/W flags for reads and writes on ```tokio_uring::fs::File```

[ChrysoliteAzalea]

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Hello everyone! I propose adding methods to tokio_uring::fs::File that would allow passing RWF_ flags to read and write operations.

Explanation

Along with ordinary I/O system calls (read()/write()), vectored I/O (readv()/writev()), positional I/O (pread()/pwrite()) and vectored positional I/O (preadv()/pwritev()), Linux also has file I/O system calls preadv2() and pwritev2() that, in addition to preadv()/pwritev(), allow passing read/write flags, that affect individual I/O operations. Examples of such flags are RWF_NOWAIT (for reading, which means if it has to perform some possibly-blocking operations, it instead should fail with EAGAIN) and RWF_APPEND (perform an atomic append even if the file is opened without O_APPEND flag). io_uring also allows passing these flags in submissions.

Possible benefits

This would be useful for implementing writing to stdout and stderr using io_uring. Stdout and/or stderr may point to regular files, but don't have the O_APPEND flag set, which means, if several processes have their stdout or stderr redirected to such file, they would have to specify RWF_APPEND when writing to such file to avoid overwriting each other's data.

[ChrysoliteAzalea]

Example of writing to stdout with io_uring:

async fn write_to_stdout(data: &str) -> std::io::Result<()>
{
   let b_fd = std::io::stdout().as_fd().try_clone_to_owned()?;
   let file_object = std::fs::File::from(b_fd);
   let iouring_object = tokio_uring::fs::File::from_std(file_object);
   let heapcopy = data.to_owned().into_bytes();
   Ok(iouring_object.write_all_at_with_flags(heapcopy, 0, libc::RWF_APPEND).await.0?)
}

[ileixe]

It looks too destructive to change all the public API and I believe tokio-uring has similar mental model not to expose I/O flag but provides higher abstraction like Stream.

[ChrysoliteAzalea]

It looks too destructive to change all the public API and I believe tokio-uring has similar mental model not to expose I/O flag but provides higher abstraction like Stream.

It does not change already-present pubic API, it adds new methods to tokio_uring::fs::File.