search-paths: Define $SSL_CERT_DIR and $SSL_CERT_FILE.

For the ‘why’, see the docstring next to $SSL_CERT_DIR.  In later commits,
packages will be changed to use these variables and the variables will be
added to more packages.

* guix/search-paths.scm ($SSL_CERT_DIR, $SSL_CERT_FILE): New variables.
* doc/guix.texi (Search Paths): Document them.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -88,7 +88,7 @@ Copyright @copyright{} 2020 Daniel Brooks@*
 Copyright @copyright{} 2020 John Soo@*
 Copyright @copyright{} 2020 Jonathan Brielmaier@*
 Copyright @copyright{} 2020 Edgar Vincent@*
-Copyright @copyright{} 2021 Maxime Devos@*
+Copyright @copyright{} 2021, 2022 Maxime Devos@*
 Copyright @copyright{} 2021 B. Wilson@*
 Copyright @copyright{} 2021 Xinglu Chen@*
 Copyright @copyright{} 2021 Raghav Gururajan@*
@@ -10127,6 +10127,25 @@ Again, the libxml2 example shows a situation where this is needed.
 @end table
 @end deftp
 
+Some search paths are not tied by a single package but to many packages.
+To reduce duplications, some of them are pre-defined in @code{(guix
+search-paths)}.
+
+@defvr {Scheme Variable} $SSL_CERT_DIR
+@defvrx {Scheme Variable} $SSL_CERT_FILE
+These two search paths indicate where X.509 certificates can be found
+(@pxref{X.509 Certificates}).
+@end defvr
+
+These pre-defined search paths can be used as in the following example:
+
+@lisp
+(package
+  (name "curl")
+  ;; some fields omitted ...
+  (native-search-paths (list $SSL_CERT_DIR $SSL_CERT_FILE)))
+@end lisp
+
 How do you turn search path specifications on one hand and a bunch of
 directories on the other hand in a set of environment variable
 definitions?  That's the job of @code{evaluate-search-paths}.

guix/search-paths.scm

@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2013, 2014, 2015, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2022 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -32,6 +33,8 @@
             search-path-specification-file-pattern
 
             $PATH
+            $SSL_CERT_DIR
+            $SSL_CERT_FILE
 
             search-path-specification->sexp
             sexp->search-path-specification
@@ -70,6 +73,29 @@
    (variable "PATH")
    (files '("bin" "sbin"))))
 
+;; Two variables for certificates (see (guix)X.509 Certificates),
+;; respected by 'openssl', possibly GnuTLS in the future
+;; (https://gitlab.com/gnutls/gnutls/-/merge_requests/1541)
+;; and many of their dependents -- even some GnuTLS depepdents
+;; like Guile.  As they are not tied to a single package, define
+;; them here to avoid duplication.
+;;
+;; Additionally, the 'native-search-paths' field is not thunked,
+;; so doing (package-native-search-paths openssl)
+;; could cause import cycle issues.
+(define-public $SSL_CERT_DIR
+  (search-path-specification
+   (variable "SSL_CERT_DIR")
+   (separator #f)              ;single entry
+   (files '("etc/ssl/certs"))))
+
+(define-public $SSL_CERT_FILE
+  (search-path-specification
+   (variable "SSL_CERT_FILE")
+   (file-type 'regular)
+   (separator #f)              ;single entry
+   (files '("etc/ssl/certs/ca-certificates.crt"))))
+
 (define (search-path-specification->sexp spec)
   "Return an sexp representing SPEC, a <search-path-specification>.  The sexp
 corresponds to the arguments expected by `set-path-environment-variable'."