bug#35701: guix size should account for hard links in its disk space calculation

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

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Consider the following operating system declaration:


[-- Attachment #2: robot-framework-system.scm --]
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;; This is an operating system configuration template for a bare-bone,
;; containerization-friendly setup, with no X11 display server and
;; no Guix daemon / client.

(use-modules (gnu)
	     (gnu packages bash)
	     (gnu packages python)
	     (gnu packages python-xyz)
	     (gnu packages xml)
	     (guix packages))

(operating-system
  (host-name "robot-framework")
  (timezone "America/Montreal")

  ;; Boot in "legacy" BIOS mode, assuming /dev/sdX is the
  ;; target hard disk, and "my-root" is the label of the target
  ;; root file system.
  (bootloader (bootloader-configuration
               (bootloader grub-bootloader)
               (target "/dev/sda")))
  (file-systems (cons (file-system
                        (device (file-system-label "my-root"))
                        (mount-point "/")
                        (type "ext4"))
                      %base-file-systems))

  (users (cons (user-account
                (name "builder")
                (group "users")
		(supplementary-groups '("wheel"))
		(home-directory "/home/builder"))
	       %base-user-accounts))

  ;; Globally-installed packages.
  (packages (cons* python-wrapper
		   (list python "tk")
		   python-robotframework
		   python-robotframework-sshlibrary
		   python-robotframework-lint
		   python-xmltodict
		   %base-packages))

  (services (list
	     ;; Enable #!/bin/sh and #!/bin/bash shebangs.
	     (service special-files-service-type
		      `(("/bin/bash" ,(file-append (canonical-package bash)
						   "/bin/bash"))))
	     (service special-files-service-type
                      `(("/bin/sh" ,(file-append (canonical-package bash)
						 "/bin/sh"))))
	     ;; The following is a very small subset extracted of
	     ;; %base-services.
	     (service login-service-type)
	     (service udev-service-type (udev-configuration))
	     (syslog-service)))

  ;; When using sudo, by default some environment variables such as
  ;; PYTHONPATH are dropped.  Make it so that any environment
  ;; variables are honored.  This is important so that the Guix system
  ;; profile can work correctly for any user.
  (sudoers-file (plain-file "sudoers" "\
root ALL=(ALL) ALL
%wheel ALL=(ALL) ALL
Defaults !env_reset,!env_delete\n")))

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$ guix size `guix system build robot-framework-system.scm`

gives:

store item                                                       total    self
/gnu/store/f9fy1b2yf1qxy523732r7z34ymwqz64b-linux-libre-5.1        241.5   241.5  21.8%
/gnu/store/1lcniyxkxkh8g73zvh2gpbccvl6ggna7-locale-2.28             91.8    91.8   8.3%
/gnu/store/zm3188ipzi262s0m8bxm24br77yh9pd8-python-3.7.0           183.0    74.6   6.7%
/gnu/store/h8l1pby3cm6b4fxsfwwr65b4d1hyh6cs-python-3.7.0           183.0    74.6   6.7%
/gnu/store/wqr8rwnwpmag01i6kb0laf62vsqnhrxy-python-3.7.0           183.0    74.6   6.7%
/gnu/store/b7fqhszxl02g6pfm3vw6b3cjz472qrly-python-3.7.0           182.8    74.6   6.7%
[...]

total: 1106.3 MiB

But,

$ guix size `guix system --no-grafts build robot-framework-system.scm`

store item                                                       total    self
/gnu/store/f9fy1b2yf1qxy523732r7z34ymwqz64b-linux-libre-5.1        241.5   241.5  28.4%
/gnu/store/1lcniyxkxkh8g73zvh2gpbccvl6ggna7-locale-2.28             91.8    91.8  10.8%
/gnu/store/b7fqhszxl02g6pfm3vw6b3cjz472qrly-python-3.7.0           182.8    74.6   8.8%
/gnu/store/ybglr7nfs8v9kpnm8vf4drg3gafnvd15-guile-static-stripped-2.2.4    45.9    45.9   5.4%
total: 851.2 MiB

Conclusion: the size of grafts is added multiple times even though they
should be very light on disk usage given that they are mostly hard links
to the original, ungrafted output.