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Add terminate-on-parent-exit subprocess option #111

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46 changes: 36 additions & 10 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -143,6 +143,42 @@ Note that you still can call `subprocess_destroy`, and `subprocess_join` after
calling `subprocess_terminate`, and that the return code filled by
`subprocess_join(&process, &process_return)` is then guaranteed to be _non zero_.

### Terminating a Process When Its Parent Exits

Specify `subprocess_option_terminate_on_parent_exit` when creating a process to
terminate the child and its descendants if the parent process exits:

```c
const char *command_line[] = {"worker", NULL};
struct subprocess_s subprocess;
int result = subprocess_create(
command_line,
subprocess_option_search_user_path |
subprocess_option_terminate_on_parent_exit,
&subprocess);
```

On Windows 10 and later this is implemented with a kill-on-close Job Object.
The process is associated with the job atomically during creation, before it
can execute or spawn descendants. Creation with this option fails if the OS
does not support process-creation job lists. On POSIX platforms the child is
placed in a dedicated process group monitored through a pipe held by the
parent. This deliberately keeps the parent outside the group, so terminating
the child group cannot accidentally terminate the parent or unrelated
processes. A POSIX descendant which explicitly starts a new session or moves
to another process group escapes this guarantee.

The grouping is intentionally opt-in. This follows the lesson from Apache
Commons Daemon [DAEMON-232](https://issues.apache.org/jira/browse/DAEMON-232),
which removed jsvc's unconditional `setpgrp`/`setpgid` because it broke
supervisors that signal a logical process group. The watchdog also closes its
own pipe writer before waiting for EOF, avoiding the parent-lifeline failure
fixed by OpenJDK [JDK-8307990](https://bugs.openjdk.org/browse/JDK-8307990).

Destroying a running subprocess created with this option also terminates its
process group. This differs from the default behavior, where destroying a
subprocess releases its resources without terminating it.

### Reading Asynchronously

If you want to be able to read from a process _before_ calling `subprocess_join`
Expand Down Expand Up @@ -232,16 +268,6 @@ has to inherit the environment of the parent because the environment implicitly
contains the privileges of the parent process (accessing the internet) that the
child requires.

## Todo

The current list of todos:

* Add the ability to
[set environment variables of the child process](https://github.com/sheredom/subprocess.h/issues/1)
as suggested by [@graphitemaster](https://github.com/graphitemaster).
* Add the ability to specify if a child process should die if the parent process
is terminated.

## AI Usage

AI tool use is explicitly permitted in commits to this repository. There is a
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