Add Flag to Spawn Child Process with New Process Group ID but the Same Session ID

[groundwater]

I am working on a primitive shell in node that has full-fledged job-control. Proper job control requires being able to spawn child processes with the same session id, but in a new process group. Currently libuv only supports flagging a child for complete detachment via UV_PROCESS_DETACHED.

A bit on TTYs and why this is necessary. I apologize if this is all familiar.

Background

Whenever you log into a shell, such as bash, there is always a controlling TTY for the session. That controlling TTY is just a file descriptor for a character device e.g. /dev/console. The reason you see the output from a command like ls is because after bash forks, it keeps STDOUT/STDERR attached to the controlling TTY before calling exec on ls. Thus the TTY is being shared by almost every process that is a direct or indirect child of your login session.

In order to calm the madness, the TTY drive has the concept of a foreground process group. Only processes in the foreground process group are allowed to read and write to the TTYs character device. If a background process tries to read or write to the TTY it will be sent SIGTTIN or SIGTTOUT. The SIGTTOUT signal can be ignored, but ignoring SIGTTIN will result in an EIO read error.

The foreground process group also lets the line-discipline decide which processes should be sent control signals like SIGINT whenever it processes a ^C character.

Problem

The problem is that the system call (tcsetpgrp) for setting foreground process groups can only be done on process groups in the same session. Thus using UV_PROCESS_DETACHED will cause tcsetpgrp to error.

[Nodejs-Jenkins]

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[bnoordhuis]

Style: space after comma.

[bnoordhuis]

LGTM I think but a regression test would be nice.

[bb010g]

bump

[saghul]

Please reopen at https://github.com/libuv/libuv