launcher: When terminated, ensure child processes are cleaned up

[rvl]

Relates to #8.

Overview

The child process(es) were cleaned up in the event of Ctrl-C (SIGINT), but not when the launcher was killed by the kill command (SIGTERM).

[KtorZ]

@rvl Have you tested this 🤔 ?
When I was finalizing the PR on the launcher, I spent quite some time trying to install some signal handlers without much success for the SIG_KILL. Others, and in particular SIG_INT are handled properly by the process library and the bracket we use, making sure to clean up children as the main launcher exits.

However, I doesn't seem possible to catch a SIG_KILL and do anything with it. I believe that's part of the semantic of SIG_KILL actually, it just terminates the process, immediately.

cf:

$ ps -ef | grep cardano
ktorz     7021  5284  0 08:35 pts/17   00:00:00 /home/ktorz/Documents/IOHK/cardano-wallet/.stack-work/install/x86_64-linux/lts-13.8/8.6.3/bin/cardano-wallet-launcher
ktorz     7053  7021 67 08:35 pts/17   00:00:12 cardano-http-bridge start --port 8080
ktorz     7121  7021 93 08:35 pts/17   00:00:16 cardano-wallet-server --wallet-server-port 8090 --http-bridge-port 8080 --network mainnet

$ kill 7021 // or kill -9 7021

$ ps -ef | grep cardano
ktorz     7053  2225 85 08:35 pts/17   00:00:58 cardano-http-bridge start --port 8080
ktorz     7121  2225 94 08:35 pts/17   00:01:04 cardano-wallet-server --wallet-server-port 8090 --http-bridge-port 8080 --network mainnet

[rvl]

Yes I've tested it. You can't do much about SIGKILL (kill -9).

But SIGTERM (kill with the default signal) was not working until I made this change. SIGTERM will be a common way of stopping the wallet.

[KtorZ]

@rvl May you specify the manual steps you use to test this? It doesn't seem to work for me using the default signal for kill

My bad. I didn't check the branch out actually.