fix(external): close stdin properly in shutdown()

[flaneur2020]

it seems that we can not pass EOF to child process's stdin by shutdown().

as the doc from tokio described, shutdown() will fire a shutdown(Write) syscall to the socket. but this syscall does not works for file descriptors other than socket.

        /// Shuts down the output stream, ensuring that the value can be dropped
        /// cleanly.
        ///
        /// Equivalent to:
        ///
        /// ```ignore
        /// async fn shutdown(&mut self) -> io::Result<()>;
        /// ```
        ///
        /// Similar to [`flush`], all intermediately buffered is written to the
        /// underlying stream. Once the operation completes, the caller should
        /// no longer attempt to write to the stream. For example, the
        /// `TcpStream` implementation will issue a `shutdown(Write)` sys call.
        ///
        /// [`flush`]: fn@crate::io::AsyncWriteExt::flush

to real close() the fd to pass the eof properly to the child process, we have to drop() this stdin to close it.

i made an experiment in: https://gist.github.com/flaneur2020/4e74d7c8acc6b8dbe41cacb28c131dba . this program will keep hangs with shutdown(), and will runs as expected with drop().

this pr fixed the shutdown() to drop() to close the file descriptor properly.

[xxchan]

Thanks for the fix! Just curious where are you using the external engine?

[xxchan]

Do you want to bump version (and CHANGELOG) now? I can publish a new release for you.

[flaneur2020]

Thanks for the fix! Just curious where are you using the external engine?

i am trying to maintain a small set of a smoke test suite for my cloud service, this test suite is not planned to be open sourced and won't become big, so i guess it might be a good choice to use the external engine for this use case.

[flaneur2020]

Do you want to bump version (and CHANGELOG) now? I can publish a new release for you.

thank you a lot! let me update it : )

[xxchan]

I just found the implementation of shutdown for ChildStdio is just Poll::Ready(Ok(())). So the problem is drop not called, not because of shutdown called.

[BugenZhao]

Shall we directly panic if it's already shut down, just like

sqllogictest-rs/sqllogictest-engines/src/postgres.rs

Line 50 in 3a603d7

&self.conn.as_ref().expect("connection is shutdown").0

[flaneur2020]

make sense, updated it in 6a05348, PTAL

[xxchan]

0.27.2 published