utils: Define ‘search-input-file’ procedure.

The procedure ‘which’ from (guix build utils)
is used for two different purposes:

  1. for finding the absolute file name of a binary
     that needs to run during the build process

  2. for finding the absolute file name of a binary,
     for the target system (as in --target=TARGET),
     e.g. for substituting sh->/gnu/store/.../bin/sh,
     python->/gnu/store/.../bin/python.

When compiling natively (target=#f in Guix parlance),
this is perfectly fine.

However, when cross-compiling, there is a problem.
"which" looks in $PATH for binaries.  That's good for purpose (1),
but incorrect for (2), as the $PATH contains binaries from native-inputs
instead of inputs.

This commit defines a ‘search-input-file’ procedure. It functions
like 'which', but instead of searching in $PATH, it searches in
the 'inputs' of the build phase, which must be passed to
‘search-input-file’ as an argument. Also, the file name must
include "bin/" or "sbin/" as appropriate.

* guix/build/utils.scm (search-input-file): New procedure.
* tests/build-utils.scm
  ("search-input-file: exception if not found")
  ("search-input-file: can find if existent"): Test it.
* doc/guix.texi (File Search): Document it.

Partially-Fixes: <https://issues.guix.gnu.org/47869>
Co-Authored-By: Ludovic Courtès <ludo@gnu.org>
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

1 files changed, 20 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 535e7614fd..3557c977e1 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -8661,6 +8661,26 @@ Return the complete file name for @var{program} as found in
 @code{$PATH}, or @code{#f} if @var{program} could not be found.
 @end deffn
 
+@deffn {Scheme Procedure} search-input-file @var{inputs} @var{name}
+Return the complete file name for @var{name} as found in @var{inputs}.
+If @var{name} could not be found, an exception is raised instead.
+Here, @var{inputs} is an association list like @var{inputs} and
+@var{native-inputs} as available to build phases.
+@end deffn
+
+Here is a (simplified) example of how @code{search-input-file} is used
+in a build phase of the @code{wireguard-tools} package:
+
+@lisp
+(add-after 'install 'wrap-wg-quick
+  (lambda* (#:key inputs outputs #:allow-other-keys)
+    (let ((coreutils (string-append (assoc-ref inputs "coreutils")
+                                    "/bin")))
+      (wrap-program (search-input-file outputs "bin/wg-quick")
+        #:sh (search-input-file inputs "bin/bash")
+        `("PATH" ":" prefix ,(list coreutils))))))
+@end lisp
+
 @subsection Build Phases
 
 @cindex build phases