Subject: [PATCH v1 1/3] gexp: Allow file-unions with dangling symlinks.

From: Sergey Trofimov <[email-masked]>
Date: Mon, 19 May 2025 11:50:08 +0200

* guix/gexp.scm (file-union): Add #:dangling-symlinks? parameter.

Change-Id: I09d44ec785fd7141b02dee2d8dc23ccc499aa933
---
 doc/guix.texi | 12 +++++++-----
 guix/gexp.scm | 19 ++++++++++---------
 2 files changed, 17 insertions(+), 14 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index fd86551787..ef8504bb3e 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -12673,11 +12673,13 @@ G-Expressions
 This is the declarative counterpart of @code{text-file*}.
 @end deffn
 
-@deffn {Procedure} file-union name files
-Return a @code{<computed-file>} that builds a directory containing all of @var{files}.
-Each item in @var{files} must be a two-element list where the first element is the
-file name to use in the new directory, and the second element is a gexp
-denoting the target file.  Here's an example:
+@deffn {Procedure} file-union name files [#:dangling-symlinks? #f]
+Return a @code{<computed-file>} that builds a directory containing all
+of @var{files}.  Each item in @var{files} must be a two-element list
+where the first element is the file name to use in the new directory,
+and the second element is a gexp denoting the target file.
+@code{#:dangling-symlinks?} controls if gexps must lower to an existing
+file.  Here's an example:
 
 @lisp
 (file-union "etc"
diff --git a/guix/gexp.scm b/guix/gexp.scm
index 8dd746eee0..85d049e26a 100644
--- a/guix/gexp.scm
+++ b/guix/gexp.scm
@@ -2144,7 +2144,7 @@ (define* (mixed-text-file name #:key guile #:rest text)
 
   (computed-file name build #:guile guile))
 
-(define* (file-union name files #:key guile)
+(define* (file-union name files #:key guile (dangling-symlinks? #f))
   "Return a <computed-file> that builds a directory containing all of FILES.
 Each item in FILES must be a two-element list where the first element is the
 file name to use in the new directory, and the second element is a gexp
@@ -2170,14 +2170,15 @@ (define* (file-union name files #:key guile)
                        (map (match-lambda
                               ((target source)
                                (gexp
-                                (begin
-                                  ;; Stat the source to abort early if it does
-                                  ;; not exist.
-                                  (stat (ungexp source))
-
-                                  (mkdir-p (dirname (ungexp target)))
-                                  (symlink (ungexp source)
-                                           (ungexp target))))))
+                                (let ((source (ungexp source))
+                                      (target (ungexp target)))
+                                  (unless (or (ungexp dangling-symlinks?)
+                                              (stat source #f))
+                                    (error (format #f "~a points to inexistent file or dangling symlink ~a"
+                                                   target source)))
+
+                                  (mkdir-p (dirname target))
+                                  (symlink source target)))))
                             files)))))
                  #:guile guile))

base-commit: 450a361532573a02389530a6a80b7821683ed41b
-- 
2.49.0