environment: Properly handle SIGINT.

Switching to execlp means that the process spawned in a container is PID
1, which obsoleted one of the 'guix environment --container' tests
because the init process can't be killed in the usual manner.

* guix/scripts/environment.scm (launch-environment/fork): New procedure.
(launch-environment): Switch from system* to execlp.  Add handler for
SIGINT.
(guix-environment): Use launch-environment/fork.
* tests/guix-environment-container.sh: Replace abnormal exit test with
one that works now that the spawned process is PID 1.

guix/scripts/environment.scm

@@ -358,8 +358,22 @@ (define (launch-environment command inputs paths pure?)
   "Run COMMAND in a new environment containing INPUTS, using the native search
 paths defined by the list PATHS.  When PURE?, pre-existing environment
 variables are cleared before setting the new ones."
+  ;; Properly handle SIGINT, so pressing C-c in an interactive terminal
+  ;; application works.
+  (sigaction SIGINT SIG_DFL)
   (create-environment inputs paths pure?)
-  (apply system* command))
+  (match command
+    ((program . args)
+     (apply execlp program program args))))
+
+(define (launch-environment/fork command inputs paths pure?)
+  "Run COMMAND in a new process with an environment containing INPUTS, using
+the native search paths defined by the list PATHS.  When PURE?, pre-existing
+environment variables are cleared before setting the new ones."
+  (match (primitive-fork)
+    (0 (launch-environment command inputs paths pure?))
+    (pid (match (waitpid pid)
+           ((_ . status) status)))))
 
 (define* (launch-environment/container #:key command bash user-mappings
                                        profile paths network?)
@@ -582,4 +596,5 @@ (define (guix-environment . args)
                  (else
                   (return
                    (exit/status
-                    (launch-environment command profile paths pure?)))))))))))))
+                    (launch-environment/fork command profile
+                                             paths pure?)))))))))))))

tests/guix-environment-container.sh

@@ -82,8 +82,13 @@ grep -e "$NIX_STORE_DIR/.*-bash" $tmpdir/mounts # bootstrap bash
 
 rm $tmpdir/mounts
 
+abnormal_exit_code="
+(use-modules (system foreign))
+;; Purposely make Guile crash with a segfault. :)
+(pointer->string (make-pointer 123) 123)"
+
 if guix environment --bootstrap --container \
-	--ad-hoc bootstrap-binaries -- kill -SEGV 2
+	--ad-hoc guile-bootstrap -- guile -c "$abnormal_exit_code"
 then false;
 else
     test $? -gt 127