Async deadlock in Stream implementation

[csarn]

Hi, I want to get a futures::Stream for the output of a std::process::Command.
But my implementation so far blocks after the first call to poll_next() (which returns Poll::Pending).

My (limited) understanding of the async world is that the executor (tokio::main) should retry calling poll_next().

Here is my code:

Rust Playground

Any ideas on

1. how I could achieve what I want, and
2. why exactly my code example deadlocks?

[Heliozoa]

The docs for Stream could be a little clearer imo, you're missing this part of Stream::poll_next:

registering the current task for wakeup if the value is not yet available

The docs for AsyncRead::poll_read are more explicit about what you need to do:

If no data is available for reading, the method returns Poll::Pending and arranges for the current task (via cx.waker().wake_by_ref() ) to receive a notification when the object becomes readable or is closed.

Adding

cx.waker().wake_by_ref();

before you return Poll::Pending makes your code complete running. You can check the async book for more info about wakers and other async stuff.

[alice]

You can't really implement it this way. You should be using tokio::process instead.

[csarn]

Oh, I looked into tokio::process::Command, and apparently didn't look far enough. I was surprised that the "output()" Future returns a Vec instead of something streaming the result. Following the documentation further to tokio::process::ChildStdout, I see the poll_read() method I was missing.
Thanks!