Don't enable cron by default

The rationale for disabling this is: 1) systemd timers are better; 2)
it gets rid of one usually unnecessary process, which makes containers
more light-weight.

Note that cron is still enabled if services.cron.systemCronJobs is
non-empty, so this only matters if you have no declarative cron jobs
but do have user cron jobs.

nixos/doc/manual/release-notes/rl-unstable.xml

@@ -36,7 +36,7 @@
 
 </para>
 
-<para>Following new services were added since the last release:
+<para>The following new services were added since the last release:
 
 <itemizedlist>
 <listitem><para><literal>brltty</literal></para></listitem>
@@ -50,6 +50,12 @@ following incompatible changes:
 
 <itemizedlist>
 
+<listitem><para><command>cron</command> is no longer enabled by
+default, unless you have a non-empty
+<option>services.cron.systemCronJobs</option>. To force
+<command>cron</command> to be enabled, set
+<option>services.cron.enable = true</option>.</para></listitem>
+
 <listitem><para>Steam now doesn't need root rights to work. Instead of using
 <literal>*-steam-chrootenv</literal>, you should now just run <literal>steam</literal>.
 <literal>steamChrootEnv</literal> package was renamed to <literal>steam</literal>,

nixos/modules/services/scheduling/cron.nix

@@ -4,8 +4,6 @@ with lib;
 
 let
 
-  inherit (config.services) jobsTags;
-
   # Put all the system cronjobs together.
   systemCronJobsFile = pkgs.writeText "system-crontab"
     ''
@@ -25,9 +23,9 @@ let
     sendmailPath = "/var/setuid-wrappers/sendmail";
   };
 
-  allFiles = map (f: "\"${f}\"") (
-    [ "${systemCronJobsFile}" ] ++ config.services.cron.cronFiles
-  );
+  allFiles =
+    optional (config.services.cron.systemCronJobs != []) systemCronJobsFile
+    ++ config.services.cron.cronFiles;
 
 in
 
@@ -91,7 +89,13 @@ in
 
   ###### implementation
 
-  config = mkIf (config.services.cron.enable && allFiles != []) {
+  config = mkMerge [
+
+    { services.cron.enable = mkDefault (allFiles != []);
+
+    }
+
+    (mkIf (config.services.cron.enable && allFiles != []) {
 
       security.setuidPrograms = [ "crontab" ];
 
@@ -105,7 +109,7 @@ in
           preStart =
             ''
               rm -f /etc/crontab
-            cat ${toString allFiles} > /etc/crontab
+              cat ${concatMapStrings (f: "\"${f}\" ") allFiles} > /etc/crontab
               chmod 0600 /etc/crontab
 
               mkdir -m 710 -p /var/cron
@@ -121,6 +125,8 @@ in
           serviceConfig.ExecStart = "${cronNixosPkg}/bin/cron -n";
         };
 
-  };
+    })
+
+  ];
 
 }