[bug#63263] [PATCH] gexp: Stop generating unreadable builder scripts.

[Christopher Baines <mail@cbaines.net>]

In Guile, it's possible to produce output from write that can't be read, and
this applies to the code staged through g-expressions for derivations.  This
commit detects this early when the derivation is being created, rather than
leaving the error to happen when the derivation is built.

This is important as it means that tools like guix lint will indicate that
there's a problem, hopefully reducing the number of broken derivations in
Guix.

* guix/gexp.scm (gexp->derivation): Check that the builder script can be read.
---
 guix/gexp.scm | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/guix/gexp.scm b/guix/gexp.scm
index 0fe4f1c98a..7af9302ccf 100644
--- a/guix/gexp.scm
+++ b/guix/gexp.scm
@@ -1215,9 +1215,18 @@ (define (add-modules exp modules)
                                                          #:target target)
                                        (return #f)))
                        (guile -> (lowered-gexp-guile lowered))
-                       (builder  (text-file script-name
-                                            (sexp->string
-                                             (lowered-gexp-sexp lowered)))))
+                       (builder  (text-file
+                                  script-name
+                                  (let ((builder-string
+                                         (sexp->string
+                                          (lowered-gexp-sexp lowered))))
+                                    (catch 'read-error
+                                      (lambda ()
+                                        (call-with-input-string builder-string
+                                          read)
+                                        builder-string)
+                                      (lambda (key . args)
+                                        (error "invalid gexp" name exp  args)))))))
     (mbegin %store-monad
       (set-grafting graft?)                       ;restore the initial setting
       (raw-derivation name
-- 
2.39.1