bug#55913: run-guix-command catches too much, causing bogus "guix: ~a: command not found" error messages

bug#55913: run-guix-command catches too much, causing bogus "guix: ~a: command not found" error messages

[Maxime Devos]

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Hi,

Someone on IRC (#guix) didn't have guile-json in their environment or
something.  When they did "./pre-inst-env guix lint", that failed with
"guix: lint: command not found", which is bogus, since
guix/scripts/lint.{scm,go} existed.

The issue was tracked down to the 'run-guix-command' procedure, in
particular the definition of module:

  (define module
    ;; Check if there is a matching extension.
    (match [something that returns #false in this situation]
      (#f
       (catch 'misc-error
         (lambda ()
           (resolve-interface `(guix scripts ,command)))
         (lambda _
           (let ((hint (command-hint command (commands))))
             (format (current-error-port)
                     (G_ "guix: ~a: command not found~%") command)
             (when hint
               (display-hint (format #f (G_ "Did you mean @code{~a}?")
                                     hint)))
             (show-guix-usage)))))
      (file [a case that doesn't apply here])))

To test the hypothesis that exception handling went wrong, the
exception handling was modified to do some 'pk':

       (catch 'misc-error
         (lambda ()
           (resolve-interface `(guix scripts ,command)))
         (lambda _
           (pk 'what _) ; <---- new line!
           [old code]))

which resulted in:

;;; (what (misc-error #f "no code for module ~S" ((json)) #f))

My proposal to avoid the wrong error message is (untested):

;; #:ensure #false: don't create an empty module when not found
;; (why is #:ensure #true even the default?).
(let ((module (resolve-module `(guix scripts ,command)
                              #:ensure #false)))
  (if module
      (module-public-interface module) ; script module was found
      [display hints & error message & guix-usage etc.]))

(Reported-By: foobarxyz on #guix)

Greetings,
Maxime

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