cups service: Automatically detect Gutenprint in drivers

Additional CUPS drivers can be added via "services.printing.drivers" but
Gutenprint was an exception. It was possible to add a Gutenprint
derivation to that list and it would work at first but unlike the other
drivers Gutenprint requires a script to be run after each update or any
attempt to print something would simply fail and an error would show up
in the jobs queue (http://localhost:631/jobs/):
"The PPD version (5.2.11) is not compatible with Gutenprint 5.2.13.
Please run
`/nix/store/7762kpyhfkcgmr3q81v1bbyy0bjhym80-gutenprint-5.2.13/sbin/cups-genppdupdate'
as administrator."
This is due to state in "/var/lib/cups/ppd" and one would need to run
"/nix/store/.../bin/cups-genppdupdate -p /var/lib/cups/ppd" manually.
The alternative was to enable the following option:
"services.printing.gutenprint" but this had two disadvantages:
1) It is an exception that one could be unaware of or that could
potentially cause some confusion.
2) One couldn't use a customized Gutenprint derivation in
"services.printing.drivers" but would instead have to overwrite
"pkgs.gutenprint".

This new approach simply detects a Gutenprint derivation in
"services.printing.gutenprint" by checking if the meta set of a
derivation contains "isGutenprint = true". Therefore no special
exception for Gutenprint would be required and it could easily be
applied to other drivers if they would require such a script to be run.

1 files changed, 1 insertions, 0 deletions

diff --git a/pkgs/misc/drivers/gutenprint/default.nix b/pkgs/misc/drivers/gutenprint/default.nix
index 0bd40721c4ed..7917d1e36160 100644
--- a/pkgs/misc/drivers/gutenprint/default.nix
+++ b/pkgs/misc/drivers/gutenprint/default.nix
@@ -47,5 +47,6 @@ stdenv.mkDerivation rec {
     homepage = https://sourceforge.net/projects/gimp-print/;
     license = licenses.gpl2;
     platforms = platforms.linux;
+    isGutenprint = true;
   };
 }