build, package: Add `--fallback' option.

* guix/scripts/build.scm (%options, show-help): Add `--fallback'.
  (guix-build): Call `set-build-options' with #:fallback?.
* guix/scripts/package.scm (%options, show-help): Add `--fallback'.
  (guix-package): Call `set-build-options' with #:fallback?.
* doc/guix.texi (Invoking guix package, Invoking guix build): Document
  `--fallback'.

doc/guix.texi

@@ -595,6 +595,10 @@ Use @var{profile} instead of the user's default profile.
 @itemx -n
 Show what would be done without actually doing it.
 
+@item --fallback
+When substituting a pre-built binary fails, fall back to building
+packages locally.
+
 @item --no-substitutes
 @itemx --max-silent-time=@var{seconds}
 Same as for @command{guix build} (@pxref{Invoking guix build}).
@@ -1219,6 +1223,10 @@ the end of the build log.  This is useful when debugging build issues.
 @itemx -n
 Do not build the derivations.
 
+@item --fallback
+When substituting a pre-built binary fails, fall back to building
+packages locally.
+
 @item --no-substitutes
 Build instead of resorting to pre-built substitutes.
 

guix/scripts/build.scm

@@ -81,6 +81,8 @@ (define (show-help)
   -K, --keep-failed      keep build tree of failed builds"))
   (display (_ "
   -n, --dry-run          do not build the derivations"))
+  (display (_ "
+      --fallback         fall back to building when the substituter fails"))
   (display (_ "
       --no-substitutes   build instead of resorting to pre-built substitutes"))
   (display (_ "
@@ -140,6 +142,10 @@ (define %options
         (option '(#\n "dry-run") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'dry-run? #t result)))
+        (option '("fallback") #f #f
+                (lambda (opt name arg result)
+                  (alist-cons 'fallback? #t
+                              (alist-delete 'fallback? result))))
         (option '("no-substitutes") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'substitutes? #f
@@ -267,6 +273,7 @@ (define package->derivation
           (set-build-options (%store)
                              #:keep-failed? (assoc-ref opts 'keep-failed?)
                              #:build-cores (or (assoc-ref opts 'cores) 0)
+                             #:fallback? (assoc-ref opts 'fallback?)
                              #:use-substitutes? (assoc-ref opts 'substitutes?)
                              #:max-silent-time (assoc-ref opts 'max-silent-time)
                              #:verbosity (assoc-ref opts 'verbosity))

guix/scripts/package.scm

@@ -437,6 +437,8 @@ (define (show-help)
   -p, --profile=PROFILE  use PROFILE instead of the user's default profile"))
   (display (_ "
   -n, --dry-run          show what would be done without actually doing it"))
+  (display (_ "
+      --fallback         fall back to building when the substituter fails"))
   (display (_ "
       --no-substitutes   build instead of resorting to pre-built substitutes"))
   (display (_ "
@@ -499,6 +501,10 @@ (define %options
         (option '(#\n "dry-run") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'dry-run? #t result)))
+        (option '("fallback") #f #f
+                (lambda (opt name arg result)
+                  (alist-cons 'fallback? #t
+                              (alist-delete 'fallback? result))))
         (option '("no-substitutes") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'substitutes? #f
@@ -909,6 +915,7 @@ (define (process-query opts)
         (with-error-handling
           (parameterize ((%store (open-connection)))
             (set-build-options (%store)
+                               #:fallback? (assoc-ref opts 'fallback?)
                                #:use-substitutes?
                                (assoc-ref opts 'substitutes?)
                                #:max-silent-time