deprecation: Refine the old daemon deprecation warning message.

* guix/deprecation.scm (warn-about-old-daemon): Mention a direct consequence
of failing to upgrade the daemon.
* guix/store.scm (build-things): Add a comment explaining the rationale to
emit a deprecation warning for a daemon older than version 0x163.

Suggested-by: Ricardo Wurmus <rekado@elephly.net>

2 files changed, 8 insertions, 3 deletions

diff --git a/guix/deprecation.scm b/guix/deprecation.scm
index 666e178d75..09a27789c9 100644
--- a/guix/deprecation.scm
+++ b/guix/deprecation.scm
@@ -36,8 +36,9 @@
 ;;; Code:
 
 (define (warn-about-old-daemon)
-  (warning (G_ "Your Guix daemon is seriously outdated, please consider
- updating it by following the 'Upgrading Guix' documentation section.~%")))
+  (warning (G_ "Your Guix daemon is severely outdated, and will soon cease to
+be able to download binary substitutes.  To upgrade it, refer to the
+'Upgrading Guix' section in the manual.~%")))
 
 (define* (warn-about-deprecation variable properties
                                  #:key replacement)
diff --git a/guix/store.scm b/guix/store.scm
index 11dfc095b1..1d176fb99d 100644
--- a/guix/store.scm
+++ b/guix/store.scm
@@ -1442,7 +1442,11 @@ When a handler is installed with 'with-build-handler', it is called any time
                              things)))
             (parameterize ((current-store-protocol-version
                             (store-connection-version store)))
-              (when (< (current-store-protocol-version) 355) ;0x163
+              (when (< (current-store-protocol-version) #x163)
+                ;; This corresponds to the first version bump of the daemon
+                ;; since the introduction of lzip compression support.  The
+                ;; version change happened with commit 6ef61cc4c30 on the
+                ;; 2018/10/15).
                 (warn-about-old-daemon))
               (if (>= (store-connection-minor-version store) 15)
                   (build store things mode)