double forking for process spawning

[pg83]

Hi.

Currently sway uses double-forking for its process spawning, for example, in exec_always.c.

1. Is there any real difference for current sway users, who(sway or init) "parents" launched processes?
2. If no, can this(or sort of) patch be applied - https://github.com/pg83/mix/blob/main/pkgs/bin/sway/supervised/exec_always.c#L48, so one can see perfect pstree listings?

[pg83]

Also this patch removes syncronous calls to read/write/waitpid, increasing responsiveness of process starting(not much, of course).

[pg83]

https://github.com/pg83/sway/commit/d8011541a6a04642a752eb2de719bd10022d90de#diff-71299c6ccbeae979cf588e4d15e1089d7946396ae514c03c6aa003444e29b860L53 - dit diff, for convinience

[ghost]

I am not a sway developer, but I think this feature is absolutely critical.

@pg83

1. Since you are running execlp right after fork(), don't you think that a vfork() would be better?
2. ... or maybe even posix_spawn()?
3. I added a few comments to your pull request, which, it seems to me, have some minor problems.

[pg83]

My patch is VERY preliminary. I will fix all patch problems, but, honestly, first I want to get OK/NOT OK for the idea behind patch, from sway devs :)

[pg83]

posix_spawn() perfectly OK from my point of view, and even better than fork() + exec().

vfork() is a "no go" in new code, actually.

[emersion]

Double-forking is used to avoid all of the SIGCHLD issues. I'd rather not change it. -1 for posix_spawn.

[pg83]

Can you please explain about this issues with SIGCHILD? I have run my patch for weeks now, and does not have any issues...

I'd rather not change it

Can I do anything to change your mind?
May be I can implement such a behaviour under a config option?
Anything I can do to get this feature in mainline sway?

I see this as non-breaking change for most users, but some of us, who wants to see completely supervised tree in pstree(for example) will benefit from this.

[pg83]

posix_spawn

I have no strong opinion here, but you should know, that posix_spawn is technically superior:

• It uses CLONE_VFORK under the hood, safely
• https://www.microsoft.com/en-us/research/uploads/prod/2019/04/fork-hotos19.pdf It is measurable faster, and predictable (yes, I know, from microsoft, about fork(), but it is good writing, worth reading)

[bqv]

+1, im curious for this behaviour.

[pg83]

Any chance to progress here?

[bqv]

Personally i just wrapped sway in a subreaper to get the functionality.
https://github.com/bqv/session

[pg83]

Personally i just wrapped sway in a subreaper to get the functionality. https://github.com/bqv/session

Very interesting, i'll try it.

[pg83]

Yes, this subreaper approach actually works, https://github.com/pg83/mix/blob/main/pkgs/bin/subreaper/mix.sh

But, personally, I will prefer a proper solution in sway, if this is possible at all :)

[pg83]

@emersion any comments, please?

[emersion]

Sorry, I don't have time to look into this atm.

[mstoeckl]

It's quite complicated to implement properly -- the following patch is maybe 30% of the necessary effort -- but in theory it is not required, in the fork-setsid-fork-exec method of process spawning, that the first process (which calls setsid()) must exit immediately. Instead, you can keep the first process alive, and have it call waitpid(...) periodically to reap zombie processes. In fact, it's possible to create a long running server process under Sway which calls setsid(), and performs the final fork-exec steps on being asked to by the main sway process. (This technique also can reduce fork overhead -- see qbs/qbs@557775b .) While the server process is alive, pstree shows all the spawned processes as children of the server/grandchildren of sway; although when sway dies, they are orphaned and no longer are grouped together under pstree; although they will have the same PGID and SID.

0001-Introduce-sway-exec-helper.patch.txt