In Trio, how do you write data to a socket without waiting?

Question

In Trio, if you want to write some data to a TCP socket then the obvious choice is send_all:

my_stream = await trio.open_tcp_stream("localhost", 1234)
await my_stream.send_all(b"some data")

Note that this both sends that data over the socket and waits for it to be written. But what if you just want to queue up the data to be sent, but not wait for it to be written (at least, you don't want to wait in the same coroutine)?

In asyncio this is straightforward because the two parts are separate functions: write() and drain(). For example:

writer.write(data)
await writer.drain()

So of course if you just want to write the data and not wait for it you can just call write() without awaiting drain(). Is there equivalent functionality in Trio? I know this two-function design is controversial because it makes it hard to properly apply backpressure, but in my application I need them separated.

For now I've worked around it by creating a dedicated writer coroutine for each connection and having a memory channel to send data to that coroutine, but it's quite a lot of faff compared to choosing between calling one function or two, and it seems a bit wasteful (presumably there's still a send buffer under the hood, and my memory channel is like a buffer on top of that buffer).

Answer 1

I posted this on the Trio chat and Nathaniel J. Smith, the creator of Trio, replied with this:

Trio doesn't maintain a buffer "under the hood", no. There's just the kernel's send buffer, but the kernel will apply backpressure whether you want it to or not, so that doesn't help you.

Using a background writer task + an unbounded memory channel is basically what asyncio does for you implicitly.

The other option, if you're putting together a message in multiple pieces and then want to send it when you're done would be to append them into a bytearray and then call send_all once at the end, at the same place where you'd call drain in asyncio

(but obviously that only works if you're calling drain after every logical message; if you're just calling write and letting asyncio drain it in the background then that doesn't help)

So the question was based on a misconception: I wanted to write into Trio's hidden send buffer, but no such thing exists! Using a separate coroutine that waits on a stream and calls send_all() makes more sense than I had thought.

I ended up using a hybrid of the two ideas (using separate coroutine with a memory channel vs using bytearray): save the data to a bytearray, then use a condition variable ParkingLot to signal to the other coroutine that it's ready to be written. That lets me coalesce writes, and also manually check if the buffer's getting too large.