guix system: Add '--on-error'.

* guix/ui.scm (load*): Add #:on-error parameter.
  [tag, error-string]: New variables.
  Wrap 'load' call in 'call-with-prompt'.  Pass TAG to 'make-stack'.  Honor
  ON-ERROR after 'report-load-error' call.
  (report-load-error): Change to not exit on error.  Make private.
* guix/scripts/system.scm (show-help, %options): Add --on-error.
  (guix-system): Use 'load*' and pass it #:on-error.

doc/guix.texi

@@ -5995,6 +5995,25 @@ For the @code{vm-image} and @code{disk-image} actions, create an image
 of the given @var{size}.  @var{size} may be a number of bytes, or it may
 include a unit as a suffix (@pxref{Block size, size specifications,,
 coreutils, GNU Coreutils}).
+
+@item --on-error=@var{strategy}
+Apply @var{strategy} when an error occurs when reading @var{file}.
+@var{strategy} may be one of the following:
+
+@table @code
+@item nothing-special
+Report the error concisely and exit.  This is the default strategy.
+
+@item backtrace
+Likewise, but also display a backtrace.
+
+@item debug
+Report the error and enter Guile's debugger.  From there, you can run
+commands such as @code{,bt} to get a backtrace, @code{,locals} to
+display local variable values, and more generally inspect the program's
+state.  @xref{Debug Commands,,, guile, GNU Guile Reference Manual}, for
+a list of available debugging commands.
+@end table
 @end table
 
 Note that all the actions above, except @code{build} and @code{init},

guix/scripts/system.scm

@@ -382,6 +382,9 @@ Build the operating system declared in FILE according to ACTION.\n"))
   - 'init', initialize a root file system to run GNU.\n"))
 
   (show-build-options-help)
+  (display (_ "
+      --on-error=STRATEGY
+                         apply STRATEGY when an error occurs while reading FILE"))
   (display (_ "
       --image-size=SIZE  for 'vm-image', produce an image of SIZE"))
   (display (_ "
@@ -422,6 +425,10 @@ Build the operating system declared in FILE according to ACTION.\n"))
          (option '(#\V "version") #f #f
                  (lambda args
                    (show-version-and-exit "guix system")))
+         (option '("on-error") #t #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'on-error (string->symbol arg)
+                               result)))
          (option '("image-size") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'image-size (size->number arg)
@@ -514,7 +521,8 @@ Build the operating system declared in FILE according to ACTION.\n"))
            (action   (assoc-ref opts 'action))
            (system   (assoc-ref opts 'system))
            (os       (if file
-                         (read-operating-system file)
+                         (load* file %user-module
+                                #:on-error (assoc-ref opts 'on-error))
                          (leave (_ "no configuration file specified~%"))))
 
            (dry?     (assoc-ref opts 'dry-run?))

guix/ui.scm

@@ -43,6 +43,8 @@
   #:use-module (ice-9 match)
   #:use-module (ice-9 format)
   #:use-module (ice-9 regex)
+  #:autoload   (system repl repl)  (start-repl)
+  #:autoload   (system repl debug) (make-debug stack->vector)
   #:replace (symlink)
   #:export (_
             N_
@@ -51,7 +53,6 @@
             leave
             make-user-module
             load*
-            report-load-error
             warn-about-load-error
             show-version-and-exit
             show-bug-report-information
@@ -146,7 +147,8 @@ messages."
               modules)
     module))
 
-(define (load* file user-module)
+(define* (load* file user-module
+                #:key (on-error 'nothing-special))
   "Load the user provided Scheme source code FILE."
   (define (frame-with-source frame)
     ;; Walk from FRAME upwards until source location information is found.
@@ -158,6 +160,14 @@ messages."
               frame
               (loop (frame-previous frame) frame)))))
 
+  (define (error-string frame args)
+    (call-with-output-string
+     (lambda (port)
+       (apply display-error frame port (cdr args)))))
+
+  (define tag
+    (make-prompt-tag "user-code"))
+
   (catch #t
     (lambda ()
       ;; XXX: Force a recompilation to avoid ABI issues.
@@ -170,11 +180,14 @@ messages."
 
          ;; Hide the "auto-compiling" messages.
          (parameterize ((current-warning-port (%make-void-port "w")))
+           (call-with-prompt tag
+             (lambda ()
                ;; Give 'load' an absolute file name so that it doesn't try to
                ;; search for FILE in %LOAD-PATH.  Note: use 'load', not
                ;; 'primitive-load', so that FILE is compiled, which then allows us
                ;; to provide better error reporting with source line numbers.
-           (load (canonicalize-path file))))))
+               (load (canonicalize-path file)))
+             (const #f))))))
     (lambda _
       ;; XXX: Errors are reported from the pre-unwind handler below, but
       ;; calling 'exit' from there has no effect, so we call it here.
@@ -182,31 +195,43 @@ messages."
     (rec (handle-error . args)
          ;; Capture the stack up to this procedure call, excluded, and pass
          ;; the faulty stack frame to 'report-load-error'.
-         (let* ((stack (make-stack #t handle-error))
+         (let* ((stack (make-stack #t handle-error tag))
                 (depth (stack-length stack))
                 (last  (and (> depth 0) (stack-ref stack 0)))
                 (frame (frame-with-source
                         (if (> depth 1)
                             (stack-ref stack 1)   ;skip the 'throw' frame
                             last))))
-           (report-load-error file args frame)))))
+
+           (report-load-error file args frame)
+
+           (case on-error
+             ((debug)
+              (newline)
+              (display (_ "entering debugger; type ',bt' for a backtrace\n"))
+              (start-repl #:debug (make-debug (stack->vector stack) 0
+                                              (error-string frame args)
+                                              #f)))
+             ((backtrace)
+              (newline (current-error-port))
+              (display-backtrace stack (current-error-port)))
+             (else
+              #t))))))
 
 (define* (report-load-error file args #:optional frame)
-  "Report the failure to load FILE, a user-provided Scheme file, and exit.
+  "Report the failure to load FILE, a user-provided Scheme file.
 ARGS is the list of arguments received by the 'throw' handler."
   (match args
     (('system-error . _)
      (let ((err (system-error-errno args)))
-       (leave (_ "failed to load '~a': ~a~%") file (strerror err))))
+       (report-error (_ "failed to load '~a': ~a~%") file (strerror err))))
     (('syntax-error proc message properties form . rest)
      (let ((loc (source-properties->location properties)))
        (format (current-error-port) (_ "~a: error: ~a~%")
-               (location->string loc) message)
+               (location->string loc) message)))
-       (exit 1)))
     ((error args ...)
      (report-error (_ "failed to load '~a':~%") file)
-     (apply display-error frame (current-error-port) args)
+     (apply display-error frame (current-error-port) args))))
-     (exit 1))))
 
 (define (warn-about-load-error file args)         ;FIXME: factorize with ↑
   "Report the failure to load FILE, a user-provided Scheme file, without