nixos.redis: Fix disabling of RDB persistence.

I was under the impression that setting `services.redis.servers.<name>.save = []` would disable RDB persistence as no schedule would mean no persistence. However since the code did not handle this case specially it actually results in no `save` setting being written and the internal Redis default is used.

This patch handles the empty case to disable RDB persistence.

Disabling RDB persistence is useful in a number of scenarios:

1. Using Redis in a pure-cache mode where persistence is not desired.
2. When using the (generally superior) AOF persistence mode this file is never read so there is little point to writing it.
3. When saving is handled manually

For more information see https://redis.io/docs/manual/persistence/

This is a breaking change as the user may have been relying on `[]` using Redis defaults. However I believe that updating the behaviour for the next release is beneficial as IMHO it is less surprising and does what the user would expect. I have added release notes to warn about this change.

nixos/doc/manual/from_md/release-notes/rl-2211.section.xml

@@ -285,6 +285,13 @@
           <literal>hardware.saleae-logic.package</literal>.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          The Redis module now disables RDB persistence when
+          <literal>services.redis.servers.&lt;name&gt;.save = []</literal>
+          instead of using the Redis default.
+        </para>
+      </listitem>
       <listitem>
         <para>
           Matrix Synapse now requires entries in the

nixos/doc/manual/release-notes/rl-2211.section.md

@@ -110,6 +110,8 @@ Use `configure.packages` instead.
 
 - A new module was added for the Saleae Logic device family, providing the options `hardware.saleae-logic.enable` and `hardware.saleae-logic.package`.
 
+- The Redis module now disables RDB persistence when `services.redis.servers.<name>.save = []` instead of using the Redis default.
+
 - Matrix Synapse now requires entries in the `state_group_edges` table to be unique, in order to prevent accidentally introducing duplicate information (for example, because a database backup was restored multiple times). If your Synapse database already has duplicate rows in this table, this could fail with an error and require manual remediation.
 
 - memtest86+ was updated from 5.00-coreboot-002 to 6.00-beta2. It is now the upstream version from https://www.memtest.org/, as coreboot's fork is no longer available.

nixos/modules/services/databases/redis.nix

@@ -166,7 +166,11 @@ in {
             save = mkOption {
               type = with types; listOf (listOf int);
               default = [ [900 1] [300 10] [60 10000] ];
-              description = "The schedule in which data is persisted to disk, represented as a list of lists where the first element represent the amount of seconds and the second the number of changes.";
+              description = mdDoc ''
+                The schedule in which data is persisted to disk, represented as a list of lists where the first element represent the amount of seconds and the second the number of changes.
+
+                If set to the empty list (`[]`) then RDB persistence will be disabled (useful if you are using AOF or don't want any persistence).
+              '';
             };
 
             slaveOf = mkOption {
@@ -268,7 +272,11 @@ in {
               syslog-enabled = config.syslog;
               databases = config.databases;
               maxclients = config.maxclients;
-              save = map (d: "${toString (builtins.elemAt d 0)} ${toString (builtins.elemAt d 1)}") config.save;
+              save = if config.save == []
+                then ''""'' # Disable saving with `save = ""`
+                else map
+                  (d: "${toString (builtins.elemAt d 0)} ${toString (builtins.elemAt d 1)}")
+                  config.save;
               dbfilename = "dump.rdb";
               dir = "/var/lib/${redisName name}";
               appendOnly = config.appendOnly;