gnu: sane-backends-minimal: Respect backend search paths.

SANE's dynamic loading mechanism looks for configs in SANE_CONFIG_DIR,
and the libraries are expected to be either in LD_LIBRARY_PATH or in the
LIBDIR, which is set during the build to $PREFIX/lib/sane. A small patch
is necessary to make SANE respect backend search paths set by Guix.

* gnu/local.mk: Register sane-look-for-plugins-in-SANE_BACKEND_LIB_PATH.patch
* gnu/packages/scanner.scm (sane-backends-minimal): Patch dynamic
loading mechanism to look for libraries in SANE_BACKEND_LIB_PATH.

Change-Id: I132a709d171331d249acfe2fd1242dae43867ae8

gnu/local.mk

@@ -2266,6 +2266,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/sage-safeguard-sage-getargspec-cython.patch \
   %D%/packages/patches/sajson-for-gemmi-numbers-as-strings.patch	\
   %D%/packages/patches/sajson-build-with-gcc10.patch		\
+  %D%/packages/patches/sane-look-for-plugins-in-SANE_BACKEND_LIB_PATH.patch	\
   %D%/packages/patches/sbc-fix-build-non-x86.patch		\
   %D%/packages/patches/sbcl-fix-arm64-shared-lib.patch		\
   %D%/packages/patches/sbcl-aserve-add-HTML-5-elements.patch	\

gnu/packages/patches/sane-look-for-plugins-in-SANE_BACKEND_LIB_PATH.patch

@@ -0,0 +1,25 @@
+From 324ad2e5b5d4b08fb682fa05d5baaaf087d13f5c Mon Sep 17 00:00:00 2001
+From: Sergey Trofimov <sarg@sarg.org.ru>
+Date: Sat, 20 Sep 2025 18:26:44 +0200
+Subject: [PATCH] Look for plugins in SANE_BACKEND_LIB_PATH.
+
+---
+ backend/dll.c | 2 ++
+ 1 file changed, 2 insertions(+)
+
+diff --git a/backend/dll.c b/backend/dll.c
+index bf34c4f6d..d52e81206 100644
+--- a/backend/dll.c
++++ b/backend/dll.c
+@@ -461,6 +461,8 @@ load (struct backend *be)
+     be->op[i] = op_unsupported;
+ 
+   path = getenv ("LD_LIBRARY_PATH");
++  if (!path)
++    path = getenv ("SANE_BACKEND_LIB_PATH");	/* for guix */
+   if (!path)
+     path = getenv ("SHLIB_PATH");	/* for HP-UX */
+   if (!path)
+-- 
+2.51.0
+

gnu/packages/scanner.scm

@@ -48,6 +48,7 @@
   #:use-module ((guix licenses)
                 #:prefix license:)
   #:use-module (guix packages)
+  #:use-module (guix search-paths)
   #:use-module (guix utils))
 
 (define-public sane-airscan
@@ -115,6 +116,8 @@ both WSD and eSCL.")
              (file-name (git-file-name name version))
              (sha256
               (base32 "1fb6shx9bz0svcyasmyqs93rbbwq7kzg6l0h1zh3kjvcwhchyv72"))
+             (patches (search-patches
+                       "sane-look-for-plugins-in-SANE_BACKEND_LIB_PATH.patch"))
              (modules '((guix build utils)))
              (snippet
               ;; Generated HTML files and udev rules normally embed a
@@ -186,6 +189,11 @@ both WSD and eSCL.")
                           (string-append out
                                          "/lib/udev/rules.d/"
                                          "60-libsane.rules")))))
+         (add-after 'install 'remove-dll.conf
+           (lambda _
+             ;; dll.conf lists enabled backends, so it should be removed as
+             ;; there are none in this package
+             (delete-file (string-append %output "/etc/sane.d/dll.conf"))))
          (add-after 'install 'make-reproducible
            ;; XXX Work around an old bug <https://issues.guix.gnu.org/26247>.
            ;; Then work around "Throw to key `decoding-error' ..." by using sed.
@@ -197,6 +205,14 @@ both WSD and eSCL.")
                              (invoke "sed" "-i" "/^PO-Revision-Date:/d" file))
                            (list "en@boldquot/LC_MESSAGES/sane-backends.mo"
                                  "en@quot/LC_MESSAGES/sane-backends.mo")))))))))
+    (native-search-paths
+     (list
+      (search-path-specification
+        (variable "SANE_CONFIG_DIR")
+        (files '("etc/sane.d")))
+      (search-path-specification
+        (variable "SANE_BACKEND_LIB_PATH")
+        (files '("lib/sane")))))
     (home-page "http://www.sane-project.org")
     (synopsis
      "Raster image scanner library and drivers, without scanner support")