[bug#27553] [PATCH shepherd] Register SIGCHLD handler after primitive fork

[Jelle Licht <jlicht@fsfe.org>]

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Hi all,

I am not sure if this is also the proper ML for the GNU Shepherd, but
looking in the archives lead me to believe it actually is. If not, I
suggest the gnu.org page for shepherd be updated with the correct info.

I recently starting playing around with user shepherd, and found out that
when running a shepherd 0.3.2 daemonized as non-init process (via "(action
'shepherd 'daemonize)"), zombie processes are created whenever you start
and subsequently stop any service.

Thinking I did something wrong, I asked lfam on #guix to share his (very
helpful) init.scm for user shepherd, yet I still noticed the same behaviour.

I believe commit `efa2f45c5f7dc735407381b7b8a83d6c37f828db' inadvertently
introduced an ordering issue, where the SIGCHLD handler is registered
/before/ shepherd has the chance to daemonize. I believe the following
trivial patch addresses this snafu.

Regards,
Jelle

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From 2603a1f4201668c13f992dbde3dd46be4dc2c31d Mon Sep 17 00:00:00 2001
From: Jelle Licht <jlicht@fsfe.org>
Date: Sun, 2 Jul 2017 02:58:36 +0200
Subject: [PATCH] Register SIGCHLD handler after primitive-fork.

* modules/shepherd.scm (main): Move call to 'sigaction' so daemonized process
  can handle SIGCHLD signal.
---
 modules/shepherd.scm | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/modules/shepherd.scm b/modules/shepherd.scm
index f7c169d..dfae7d6 100644
--- a/modules/shepherd.scm
+++ b/modules/shepherd.scm
@@ -142,9 +142,6 @@
     ;; Start the 'root' service.
     (start root-service)
 
-    ;; Install the SIGCHLD handler.
-    (sigaction SIGCHLD respawn-service SA_NOCLDSTOP)
-
     ;; This _must_ succeed.  (We could also put the `catch' around
     ;; `main', but it is often useful to get the backtrace, and
     ;; `caught-error' does not do this yet.)
@@ -164,6 +161,9 @@
 	     (apply format #f (gettext (cadr args)) (caddr args))
 	     (quit 1))))
 
+    ;; Install the SIGCHLD handler.
+    (sigaction SIGCHLD respawn-service SA_NOCLDSTOP)
+
     (when (provided? 'threads)
       ;; XXX: This terrible hack allows us to make sure that signal handlers
       ;; get a chance to run in a timely fashion.  Without it, after an EINTR,
-- 
2.13.2