guix gc: Add '--verify'.

* guix/scripts/gc.scm (show-help, %options): Add --verify.
  (guix-gc): Handle it.
* doc/guix.texi (Invoking guix gc): Document --verify, and move --optimize
  description right below it.

doc/guix.texi

@@ -1541,8 +1541,9 @@ is achieved by running @code{guix package --delete-generations}
 
 The @command{guix gc} command has three modes of operation: it can be
 used to garbage-collect any dead files (the default), to delete specific
-files (the @code{--delete} option), or to print garbage-collector
+files (the @code{--delete} option), to print garbage-collector
-information.  The available options are listed below:
+information, or for more advanced queries.  The garbage collection
+options are as follows:
 
 @table @code
 @item --collect-garbage[=@var{min}]
@@ -1564,17 +1565,6 @@ Attempt to delete all the store files and directories specified as
 arguments.  This fails if some of the files are not in the store, or if
 they are still live.
 
-@item --optimize
-@cindex deduplication
-Optimize the store by hard-linking identical files---this is
-@dfn{deduplication}.
-
-The daemon performs deduplication after each successful build or archive
-import, unless it was started with @code{--disable-deduplication}
-(@pxref{Invoking guix-daemon, @code{--disable-deduplication}}).  Thus,
-this option is primarily useful when the daemon was running with
-@code{--disable-deduplication}.
-
 @item --list-dead
 Show the list of dead files and directories still present in the
 store---i.e., files and directories no longer reachable from any root.
@@ -1602,6 +1592,47 @@ of these, recursively.  In other words, the returned list is the
 
 @end table
 
+Lastly, the following options allow you to check the integrity of the
+store and to control disk usage.
+
+@table @option
+
+@item --verify[=@var{options}]
+@cindex integrity, of the store
+@cindex integrity checking
+Verify the integrity of the store.
+
+By default, make sure that all the store items marked as valid in the
+daemon's database actually exist in @file{/gnu/store}.
+
+When provided, @var{options} must a comma-separated list containing one
+or more of @code{contents} and @code{repair}.
+
+When passing @option{--verify=contents}, the daemon will compute the
+content hash of each store item and compare it against its hash in the
+database.  Hash mismatches are reported as data corruptions.  Because it
+traverses @emph{all the files in the store}, this command can take a
+long time, especially on systems with a slow disk drive.
+
+@cindex repairing the store
+Using @option{--verify=repair} or @option{--verify=contents,repair}
+causes the daemon to try to repair corrupt store items by fetching
+substitutes for them (@pxref{Substitutes}).  Because repairing is not
+atomic, and thus potentially dangerous, it is available only to the
+system administrator.
+
+@item --optimize
+@cindex deduplication
+Optimize the store by hard-linking identical files---this is
+@dfn{deduplication}.
+
+The daemon performs deduplication after each successful build or archive
+import, unless it was started with @code{--disable-deduplication}
+(@pxref{Invoking guix-daemon, @code{--disable-deduplication}}).  Thus,
+this option is primarily useful when the daemon was running with
+@code{--disable-deduplication}.
+
+@end table
 
 @node Invoking guix pull
 @section Invoking @command{guix pull}

guix/scripts/gc.scm

@@ -57,6 +57,11 @@ (define (show-help)
   (display (_ "
       --referrers        list the referrers of PATHS"))
   (newline)
+  (display (_ "
+      --verify[=OPTS]    verify the integrity of the store; OPTS is a
+                         comma-separated combination of 'repair' and
+                         'contents'"))
+  (newline)
   (display (_ "
   -h, --help             display this help and exit"))
   (display (_ "
@@ -94,6 +99,17 @@ (define %options
                 (lambda (opt name arg result)
                   (alist-cons 'action 'optimize
                               (alist-delete 'action result))))
+        (option '("verify") #f #t
+                (let ((not-comma (char-set-complement (char-set #\,))))
+                  (lambda (opt name arg result)
+                    (let ((options (if arg
+                                       (map string->symbol
+                                            (string-tokenize arg not-comma))
+                                       '())))
+                      (alist-cons 'action 'verify
+                                  (alist-cons 'verify-options options
+                                              (alist-delete 'action
+                                                            result)))))))
         (option '("list-dead") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'action 'list-dead
@@ -177,6 +193,12 @@ (define (list-relatives relatives)
          (list-relatives referrers))
         ((optimize)
          (optimize-store store))
+        ((verify)
+         (let ((options (assoc-ref opts 'verify-options)))
+           (exit
+            (verify-store store
+                          #:check-contents? (memq 'contents options)
+                          #:repair? (memq 'repair options)))))
         ((list-dead)
          (for-each (cut simple-format #t "~a~%" <>)
                    (dead-paths store)))