time-machine: Make target commit check cheaper.

Commit 79ec651a286c71a3d4c72be33a1f80e76a560031 introduced a check to
error out when attempting to use ‘time-machine’ to travel to a commit
before ‘v1.0.0’.

This commit fixes a performance issue with the strategy used in
79ec651a286c71a3d4c72be33a1f80e76a560031 (the repository was opened,
updated, and traversed a second time by ‘validate-guix-channel’) as well
as a user interface issue (“Updating channel” messages would be printed
too late).

This patch reimplements the check in terms of the existing #:validate-pull
mechanism, which is designed to avoid extra repository operations.

Fixes <https://issues.guix.gnu.org/65788>.

* guix/inferior.scm (cached-channel-instance): Change default value
of #:validate-channels.  Remove call to VALIDATE-CHANNELS; pass it
as #:validate-pull to ‘latest-channel-instances’.
* guix/scripts/time-machine.scm (%reference-channels): New variable.
(validate-guix-channel): New procedure, written as a simplification of…
(guix-time-machine)[validate-guix-channel]: … this.  Remove.
Pass #:reference-channels to ‘cached-channel-instance’.

Reported-by: Simon Tournier <zimon.toutoune@gmail.com>
Change-Id: I9b0ec61fba7354fe08b04a91f4bd32b72a35460c

2 files changed, 59 insertions, 59 deletions

diff --git a/guix/inferior.scm b/guix/inferior.scm
index fca6fb4b22..190ba01b3c 100644
--- a/guix/inferior.scm
+++ b/guix/inferior.scm
@@ -872,14 +872,17 @@ prefix, resolve it; and if 'commit' is unset, fetch CHANNEL's branch tip."
                                   (authenticate? #t)
                                   (cache-directory (%inferior-cache-directory))
                                   (ttl (* 3600 24 30))
-                                  validate-channels)
+                                  (reference-channels '())
+                                  (validate-channels (const #t)))
   "Return a directory containing a guix filetree defined by CHANNELS, a list of channels.
 The directory is a subdirectory of CACHE-DIRECTORY, where entries can be
 reclaimed after TTL seconds.  This procedure opens a new connection to the
 build daemon.  AUTHENTICATE? determines whether CHANNELS are authenticated.
-VALIDATE-CHANNELS, if specified, must be a one argument procedure accepting a
-list of channels that can be used to validate the channels; it should raise an
-exception in case of problems."
+
+VALIDATE-CHANNELS must be a four-argument procedure used to validate channel
+instances against REFERENCE-CHANNELS; it is passed as #:validate-pull to
+'latest-channel-instances' and should raise an exception in case a target
+channel commit is deemed \"invalid\"."
   (define commits
     ;; Since computing the instances of CHANNELS is I/O-intensive, use a
     ;; cheaper way to get the commit list of CHANNELS.  This limits overhead
@@ -927,30 +930,31 @@ exception in case of problems."
 
   (if (file-exists? cached)
       cached
-      (begin
-        (when (procedure? validate-channels)
-          (validate-channels channels))
-        (run-with-store store
-          (mlet* %store-monad ((instances
-                                -> (latest-channel-instances store channels
-                                                             #:authenticate?
-                                                             authenticate?))
-                               (profile
-                                (channel-instances->derivation instances)))
-            (mbegin %store-monad
-              ;; It's up to the caller to install a build handler to report
-              ;; what's going to be built.
-              (built-derivations (list profile))
-
-              ;; Cache if and only if AUTHENTICATE? is true.
-              (if authenticate?
-                  (mbegin %store-monad
-                    (symlink* (derivation->output-path profile) cached)
-                    (add-indirect-root* cached)
-                    (return cached))
-                  (mbegin %store-monad
-                    (add-temp-root* (derivation->output-path profile))
-                    (return (derivation->output-path profile))))))))))
+      (run-with-store store
+        (mlet* %store-monad ((instances
+                              -> (latest-channel-instances store channels
+                                                           #:authenticate?
+                                                           authenticate?
+                                                           #:current-channels
+                                                           reference-channels
+                                                           #:validate-pull
+                                                           validate-channels))
+                             (profile
+                              (channel-instances->derivation instances)))
+          (mbegin %store-monad
+            ;; It's up to the caller to install a build handler to report
+            ;; what's going to be built.
+            (built-derivations (list profile))
+
+            ;; Cache if and only if AUTHENTICATE? is true.
+            (if authenticate?
+                (mbegin %store-monad
+                  (symlink* (derivation->output-path profile) cached)
+                  (add-indirect-root* cached)
+                  (return cached))
+                (mbegin %store-monad
+                  (add-temp-root* (derivation->output-path profile))
+                  (return (derivation->output-path profile)))))))))
 
 (define* (inferior-for-channels channels
                                 #:key
diff --git a/guix/scripts/time-machine.scm b/guix/scripts/time-machine.scm
index f31fae7435..bd64364fa2 100644
--- a/guix/scripts/time-machine.scm
+++ b/guix/scripts/time-machine.scm
@@ -46,12 +46,6 @@
   #:use-module (srfi srfi-71)
   #:export (guix-time-machine))
 
-;;; The required inferiors mechanism relied on by 'guix time-machine' was
-;;; firmed up in v1.0.0; it is the oldest, safest commit that can be travelled
-;;; to.
-(define %oldest-possible-commit
-  "6298c3ffd9654d3231a6f25390b056483e8f407c") ;v1.0.0
-
 
 ;;;
 ;;; Command-line options.
@@ -146,6 +140,31 @@ Execute COMMAND ARGS... in an older version of Guix.\n"))
 
 
 ;;;
+;;; Avoiding traveling too far back.
+;;;
+
+;;; The required inferiors mechanism relied on by 'guix time-machine' was
+;;; firmed up in v1.0.0; it is the oldest, safest commit that can be travelled
+;;; to.
+(define %oldest-possible-commit
+  "6298c3ffd9654d3231a6f25390b056483e8f407c") ;v1.0.0
+
+(define %reference-channels
+  (list (channel (inherit %default-guix-channel)
+                 (commit %oldest-possible-commit))))
+
+(define (validate-guix-channel channel start commit relation)
+  "Raise an error if CHANNEL is the 'guix' channel and the RELATION of COMMIT
+to %OLDEST-POSSIBLE-COMMIT is not that of an ancestor."
+  (unless (or (not (guix-channel? channel))
+              (memq relation '(ancestor self)))
+    (raise (formatted-message
+            (G_ "cannot travel past commit `~a' from May 1st, 2019")
+            (string-take %oldest-possible-commit 12)))))
+
+
+
+;;;
 ;;; Entry point.
 ;;;
 
@@ -160,31 +179,6 @@ Execute COMMAND ARGS... in an older version of Guix.\n"))
             (ref          (assoc-ref opts 'ref))
             (substitutes?  (assoc-ref opts 'substitutes?))
             (authenticate? (assoc-ref opts 'authenticate-channels?)))
-
-       (define (validate-guix-channel channels)
-         "Finds the Guix channel among CHANNELS, and validates that REF as
-captured from the closure, a git reference specification such as a commit hash
-or tag associated to the channel, is valid and new enough to satisfy the 'guix
-time-machine' requirements.  If the captured REF variable is #f, the reference
-validate is the one of the Guix channel found in CHANNELS.  A
-`formatted-message' condition is raised otherwise."
-         (let* ((guix-channel (find guix-channel? channels))
-                (guix-channel-commit (channel-commit guix-channel))
-                (guix-channel-branch (channel-branch guix-channel))
-                (guix-channel-ref (if guix-channel-commit
-                                      `(tag-or-commit . ,guix-channel-commit)
-                                      `(branch . ,guix-channel-branch)))
-                (reference (or ref guix-channel-ref))
-                (checkout commit relation (update-cached-checkout
-                                           (channel-url guix-channel)
-                                           #:ref reference
-                                           #:starting-commit
-                                           %oldest-possible-commit)))
-           (unless (memq relation '(ancestor self))
-             (raise (formatted-message
-                     (G_ "cannot travel past commit `~a' from May 1st, 2019")
-                     (string-take %oldest-possible-commit 12))))))
-
        (when command-line
          (let* ((directory
                  (with-store store
@@ -197,6 +191,8 @@ validate is the one of the Guix channel found in CHANNELS.  A
                        (set-build-options-from-command-line store opts)
                        (cached-channel-instance store channels
                                                 #:authenticate? authenticate?
+                                                #:reference-channels
+                                                %reference-channels
                                                 #:validate-channels
                                                 validate-guix-channel)))))
                 (executable (string-append directory "/bin/guix")))