bug#49771: conflicting pam-limits-service and pam-mount-service-type

[muradm <mail@muradm.net>]

pam-limits-service and pam-mount-service-type are working when 
used only one of them. When both are present in list of (services, 
conflict hapens when guix system reconfigure is invoked. Digging 
the problem led to use of etc-service-type.

pam-limits-service defines /etc/security/limits.conf in 
gnu/services/base.scm:

(define pam-limits-service-type
  (let ((security-limits
         ;; Create /etc/security containing the provided 
         "limits.conf" file.
         (lambda (limits-file)
           `(("security"
              ,(computed-file
                "security"
                #~(begin
                    (mkdir #$output)
                    (stat #$limits-file)
                    (symlink #$limits-file
                             (string-append #$output "/limits.conf"))))))))
        (pam-extension
         (lambda (pam)

Basically, it says to etc-service-type i need "security" under 
"/etc" and uses mkdir to create it.

pam-mount-service-type asks "security/pam_mount.conf.xml" from 
etc-service-type.

(define (pam-mount-etc-service config)
  `(("security/pam_mount.conf.xml"
     ,(make-pam-mount-configuration-file config))))

When both pam-mount-service-type and pam-limits-service are 
defined in (services ...), if pam-mount-service-type is before 
pam-limits, guix system reconfigure fails with "Permission 
denied", if pam-limits is before then it is "File exists".

I would suggest to fix gnu/services/base.scm so that 
pam-limits-services-type ask for "security/limits.conf" just like 
pam-mount-services-type does in order to avoid conflict.

Currently, both pam-limits-service and pam-mount-service-type are 
not usable at the same time.