GNU bug report logs - #77707
[shepherd] chdir and invoke do not work as intended in services

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Ludovic Courtès <ludo <at> chbouib.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#77707: closed ([shepherd] chdir and invoke do not work as
 intended in services)
Date: Sun, 13 Apr 2025 19:53:01 +0000

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Your message dated Sun, 13 Apr 2025 21:52:29 +0200
with message-id <87mscjdeki.fsf <at> gnu.org>
and subject line Re: bug#77707: [shepherd] chdir and invoke do not work as intended in services
has caused the debbugs.gnu.org bug report #77707,
regarding [shepherd] chdir and invoke do not work as intended in services
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


-- 
77707: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=77707
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Maxim Cournoyer <maxim.cournoyer <at> gmail.com>
To: bug-guix <bug-guix <at> gnu.org>
Subject: [shepherd] chdir and invoke do not work as intended in services
Date: Thu, 10 Apr 2025 23:44:04 +0900

Hi,

It appears that when changing directory and calling a program with
'invoke', the program still has its current working directory set to
'/'.

I'm betting this may be some kind of odd interaction between the
Shepherd service threads and forking a process?

Can we improve that?  If it's a limitation of the implementation model,
than I think it's a surprising enough gotcha to warrant documenting it
in Shepherd's manual.

Here's a simple reproducer:

--8<---------------cut here---------------start------------->8---
modified   gnu/system/examples/bare-bones.tmpl
@@ -3,8 +3,8 @@
 ;; for a "bare bones" setup, with no X11 display server.
 
 (use-modules (gnu))
-(use-service-modules networking ssh)
-(use-package-modules screen ssh)
+(use-service-modules networking shepherd ssh)
+(use-package-modules base screen ssh)
 
 (operating-system
   (host-name "komputilo")
@@ -47,7 +47,20 @@
 
   ;; Add services to the baseline: a DHCP client and an SSH
   ;; server.  You may wish to add an NTP service here.
-  (services (append (list (service dhcp-client-service-type)
+  (services (append (list (simple-service
+                           'forked-processes-cwd-wrong
+                           shepherd-root-service-type
+                           (list
+                            (shepherd-service
+                             (provision '(bug-77115))
+                             (start #~(lambda _
+                                        (mkdir "/etc/testdir")
+                                        (chdir "/etc/testdir")
+                                        (invoke #$(file-append coreutils "/bin/touch")
+                                                "new-file.txt")
+                                        #t))
+                             (one-shot? #t))))
+                          (service dhcp-client-service-type)
                           (service openssh-service-type
                                    (openssh-configuration
                                     (openssh openssh-sans-x)
--8<---------------cut here---------------end--------------->8---

Then run it from your tree using something like:

--8<---------------cut here---------------start------------->8---
$(./pre-inst-env guix system vm gnu/system/examples/bare-bones.tmpl --no-graphic)
--8<---------------cut here---------------end--------------->8---

You should see upon inspection that:

--8<---------------cut here---------------start------------->8---
root <at> komputilo ~# ls /new-file.txt 
/new-file.txt                        # wrong place
root <at> komputilo ~# ls /etc/testdir/   # empty
--8<---------------cut here---------------end--------------->8---

-- 
Thanks,
Maxim

[Message part 3 (message/rfc822, inline)]

From: Ludovic Courtès <ludo <at> chbouib.org>
To: Maxim Cournoyer <maxim.cournoyer <at> gmail.com>
Cc: 77707-done <at> debbugs.gnu.org
Subject: Re: bug#77707: [shepherd] chdir and invoke do not work as intended
 in services
Date: Sun, 13 Apr 2025 21:52:29 +0200

Hi,

Maxim Cournoyer <maxim.cournoyer <at> gmail.com> skribis:

> +                             (start #~(lambda _
> +                                        (mkdir "/etc/testdir")
> +                                        (chdir "/etc/testdir")
> +                                        (invoke #$(file-append coreutils "/bin/touch")
> +                                                "new-file.txt")

[...]

> You should see upon inspection that:
>
> root <at> komputilo ~# ls /new-file.txt 
> /new-file.txt                        # wrong place
> root <at> komputilo ~# ls /etc/testdir/   # empty

That’s because ‘invoke’ uses ‘system*’, which is in fact within the
‘shepherd’ process an alias for ‘spawn-command’.

This is fixed in 8d88024e0898dd7d4581c4e977c9786af734a855, which will be
in 1.0.4.

However generally speaking it’s a good idea to leave the current
directory of the ‘shepherd’ process unchanged as bad things could happen
otherwise (though ‘with-directory-excursion’ from (shepherd support)
should be safe.)

Thanks,
Ludo’.

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