publish: Make the cache eviction policy less aggressive.

Suggested by Mark H Weaver <mhw@netris.org>.

* guix/scripts/publish.scm (nar-expiration-time): New procedure.
(render-narinfo/cached): Use it as the #:entry-expiration passed to
'maybe-remove-expired-cache-entries'.

doc/guix.texi

@@ -6960,7 +6960,8 @@ guarantee that the store items it provides will indeed remain available
 for as long as @var{ttl}.
 
 Additionally, when @option{--cache} is used, cached entries that have
-not been accessed for @var{ttl} may be deleted.
+not been accessed for @var{ttl} and that no longer have a corresponding
+item in the store, may be deleted.
 
 @item --nar-path=@var{path}
 Use @var{path} as the prefix for the URLs of ``nar'' files

guix/scripts/publish.scm

@@ -385,6 +385,24 @@ (define (narinfo-files cache)
                     (string-suffix? ".narinfo" file)))
       '()))
 
+(define (nar-expiration-time ttl)
+  "Return the narinfo expiration time (in seconds since the Epoch).  The
+expiration time is +inf.0 when passed an item that is still in the store; in
+other cases, it is the last-access time of the item plus TTL.
+
+This policy allows us to keep cached nars that correspond to valid store
+items.  Failing that, we could eventually have to recompute them and return
+404 in the meantime."
+  (let ((expiration-time (file-expiration-time ttl)))
+    (lambda (file)
+      (let ((item (string-append (%store-prefix) "/"
+                                 (basename file ".narinfo"))))
+        ;; Note: We don't need to use 'valid-path?' here because FILE would
+        ;; not exist if ITEM were not valid in the first place.
+        (if (file-exists? item)
+            +inf.0
+            (expiration-time file))))))
+
 (define* (render-narinfo/cached store request hash
                                 #:key ttl (compression %no-compression)
                                 (nar-path "nar")
@@ -436,7 +454,7 @@ (define (delete-entry narinfo)
                  (maybe-remove-expired-cache-entries cache
                                                      narinfo-files
                                                      #:entry-expiration
-                                                     (file-expiration-time ttl)
+                                                     (nar-expiration-time ttl)
                                                      #:delete-entry delete-entry
                                                      #:cleanup-period ttl))))
            (not-found request