To record what an application plays, I cannot simply read from it. I add a sink of my own, move the application into it, and record from there. Whatever happens next, that plumbing has to go: leave it behind and the application keeps playing into a sink nobody listens to.

I hand the function that tears it down to clk.atexit.register, and clk calls it when it leaves, whether the command went well or not.

Here the sound server is a couple of files, so that this page can run.

echo speakers > routing.txt

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

from pathlib import Path

import click

from clk.atexit import register
from clk.decorators import command, flag

ROUTING = Path("routing.txt")
PROXY = Path("proxy-sink.txt")


def tear_down(sink):
    ROUTING.write_text(f"{sink}\n")
    PROXY.unlink()


@command()
@flag("--disk-full", help="Pretend the disk fills up while recording")
def record(disk_full):
    "Record what the music player plays"
    sink = ROUTING.read_text().strip()
    PROXY.write_text("loaded\n")
    ROUTING.write_text("proxy\n")
    register(tear_down, sink)
    print(f"recording the music from the {ROUTING.read_text().strip()} sink")
    if disk_full:
        raise click.ClickException("no space left on device")

The recording takes the music through the proxy sink. Once the command is over, the music plays on the speakers again and the proxy sink is gone.

clk record
cat routing.txt
ls proxy-sink.txt 2>&1

recording the music from the proxy sink
speakers
ls: cannot access 'proxy-sink.txt': No such file or directory

The recording may stop in the middle. The plumbing goes all the same.

clk record --disk-full 2>&1
cat routing.txt
ls proxy-sink.txt 2>&1

recording the music from the proxy sink
error: no space left on device
speakers
ls: cannot access 'proxy-sink.txt': No such file or directory