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Pardon me, I didn't test this, but I think sendfd's tokio integration is not implemented in a way that is compatible with its internal readiness tracking.

When one is using RecvWithFd, they will probably keep calling recv_with_fd until it returns WouldBlock. However this has an issue that tokio is not aware of this returned WouldBlock: as a result, it will keep assuming that the socket is ready to read, unless a call to tokio's poll_read function is made. If it's an situation where every message can contain an fd, then we can't call tokio's poll_read, because if the read is success, the kernel would close the fds that should have been received, because tokio uses readv().

The tokio APIs uses internal functions that clears readiness flags when WouldBlock is encountered:

https://github.com/tokio-rs/tokio/blob/a08ce0d3e06d650361283dc87c8fe14b146df15d/tokio/src/io/driver/registration.rs#L182

But again, I don't think sendfd is able to call into this API.

The same applies to SendWithFd to some extent, although sendmsg on a Unix socket should rarely block so it might "appear" to work.

So I don't think the tokio integration really works; can anyone confirm that?

It looks like v0.4.1 is ready but not available on crates.io right now.

Would you mind doing a release?

Also, Git tag seems to be missing for v0.4.0 and v0.4.1.