weather: Report unauthorized substitute servers.

The goal is to make it easier to diagnose substitute
misconfiguration (where we’re passing a substitute URL whose
corresponding key is not authorized).

Suggested by Emmanuel Agullo.

* guix/scripts/weather.scm (check-narinfo-authorization): New procedure.
(report-server-coverage): Use it.
* doc/guix.texi (Invoking guix weather): Document it.
(Getting Substitutes from Other Servers): Add “Troubleshooting” frame.

Change-Id: I0a049c39eefb10d6a06634c8b16aa86902769791

doc/guix.texi

@@ -4059,6 +4059,7 @@ guix-daemon}).  It can also be disabled temporarily by passing the
 
 @node Getting Substitutes from Other Servers
 @subsection Getting Substitutes from Other Servers
+@c Note: This section name appears in a hint printed by 'guix weather'.
 
 @cindex substitute servers, adding more
 Guix can look up and fetch substitutes from several servers.  This is
@@ -4158,6 +4159,21 @@ can list as many substitute servers as you like, with the caveat that
 substitute lookup can be slowed down if too many servers need to be
 contacted.
 
+@quotation Troubleshooting
+To diagnose problems, you can run @command{guix weather}.  For example,
+running:
+
+@example
+guix weather coreutils
+@end example
+
+@noindent
+not only tells you which of the currently-configured servers has
+substitutes for the @code{coreutils} package, it also reports whether
+one of these servers is unauthorized.  @xref{Invoking guix weather}, for
+more information.
+@end quotation
+
 Note that there are also situations where one may want to add the URL of
 a substitute server @emph{without} authorizing its key.
 @xref{Substitute Authentication}, to understand this fine point.
@@ -16499,7 +16515,10 @@ up building packages by yourself (@pxref{Substitutes}).  The
 specified servers so you can have an idea of whether you'll be grumpy
 today.  It can sometimes be useful info as a user, but it is primarily
 useful to people running @command{guix publish} (@pxref{Invoking guix
-publish}).
+publish}).  Sometimes substitutes @emph{are} available but they are not
+authorized on your system; @command{guix weather} reports it so you can
+authorize them if you want (@pxref{Getting Substitutes from Other
+Servers}).
 
 @cindex statistics, for substitutes
 @cindex availability of substitutes

guix/scripts/weather.scm

@@ -35,6 +35,8 @@
   #:use-module ((guix build utils) #:select (every*))
   #:use-module (guix substitutes)
   #:use-module (guix narinfo)
+  #:use-module (guix pki)
+  #:autoload   (gcrypt pk-crypto) (canonical-sexp->string)
   #:use-module (guix http-client)
   #:use-module (guix ci)
   #:use-module (guix sets)
@@ -185,6 +187,44 @@ or #f if it could not be determined."
     (()
      #f)))
 
+(define (check-narinfo-authorization narinfo)
+  "Print a warning when NARINFO is not signed by an authorized key."
+  (define acl
+    (catch 'system-error
+      (lambda ()
+        (current-acl))
+      (lambda args
+        (warning (G_ "could not read '~a': ~a~%")
+                 %acl-file (strerror (system-error-errno args)))
+        (warning (G_ "'~a' is unreadable, cannot determine whether \
+substitutes are authorized~%")
+                 %acl-file)
+        #f)))
+
+  (unless (or (not acl) (valid-narinfo? narinfo acl))
+    (warning (G_ "substitutes from '~a' are unauthorized~%")
+             (narinfo-uri-base narinfo))
+    ;; The "all substitutes" below reflects the fact that, in reality, it *is*
+    ;; possible to download "unauthorized" substitutes, as long as they match
+    ;; authorized substitutes.
+    (display-hint (G_ "To authorize all substitutes from @uref{~a} to be
+downloaded, the following command needs to be run as root:
+
+@example
+guix archive --authorize <<EOF
+~a
+EOF
+@end example
+
+Alternatively, on Guix System, you can add the signing key above to the
+@code{authorized-keys} field of @code{guix-configuration}.
+
+See \"Getting Substitutes from Other Servers\" in the manual for more
+information.")
+                  (narinfo-uri-base narinfo)
+                  (canonical-sexp->string
+                   (signature-subject (narinfo-signature narinfo))))))
+
 (define* (report-server-coverage server items
                                  #:key display-missing?)
   "Report the subset of ITEMS available as substitutes on SERVER.
@@ -204,6 +244,12 @@ In case ITEMS is an empty list, return 1 instead."
                     #:make-progress-reporter
                     (lambda* (total #:key url #:allow-other-keys)
                       (progress-reporter/bar total)))))
+    (match narinfos
+      (() #f)
+      ((narinfo . _)
+       ;; Help diagnose missing substitute authorizations.
+       (check-narinfo-authorization narinfo)))
+
     (let ((obtained  (length narinfos))
           (requested (length items))
           (missing   (lset-difference string=?
@@ -586,8 +632,11 @@ SERVER.  Display information for packages with at least THRESHOLD dependents."
                            (with-store store
                              (substitute-urls store))
                            (begin
-                             (warning (G_ "could not determine current \
-substitute URLs; using defaults~%"))
+                             ;; Could not determine the daemon's current
+                             ;; substitute URLs, presumably because it's too
+                             ;; old.
+                             (warning (G_ "using default \
+substitute URLs~%"))
                              %default-substitute-urls)))
              (systems  (match (filter-map (match-lambda
                                             (('system . system) system)