rmem_max: define merge function

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

···
           the npm install step prunes dev dependencies.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          boot.kernel.sysctl is defined as a freeformType and adds a
+          custom merge option for <quote>net.core.rmem_max</quote>
+          (taking the highest value defined to avoid conflicts between 2
+          services trying to set that value)
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
 </section>

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

···
 
 - The `nodePackages` package set now defaults to the LTS release in the `nodejs` package again, instead of being pinned to `nodejs-14_x`. Several updates to node2nix have been made for compatibility with newer Node.js and npm versions and a new `postRebuild` hook has been added for packages to perform extra build steps before the npm install step prunes dev dependencies.
 
+- boot.kernel.sysctl is defined as a freeformType and adds a custom merge option for "net.core.rmem_max" (taking the highest value defined to avoid conflicts between 2 services trying to set that value)
+
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

nixos/modules/config/sysctl.nix

···
   options = {
 
     boot.kernel.sysctl = mkOption {
+      type = types.submodule {
+        freeformType = types.attrsOf sysctlOption;
+        options."net.core.rmem_max" = mkOption {
+          type = types.nullOr types.ints.unsigned // {
+            merge = loc: defs:
+              foldl
+                (a: b: if b.value == null then null else lib.max a b.value)
+                0
+                (filterOverrides defs);
+          };
+          default = null;
+          description = lib.mdDoc "The maximum socket receive buffer size. In case of conflicting values, the highest will be used.";
+        };
+      };
       default = {};
       example = literalExpression ''
         { "net.ipv4.tcp_syncookies" = false; "vm.swappiness" = 60; }
       '';
-      type = types.attrsOf sysctlOption;
       description = lib.mdDoc ''
         Runtime parameters of the Linux kernel, as set by
         {manpage}`sysctl(8)`.  Note that sysctl
···
         parameter may be a string, integer, boolean, or null
         (signifying the option will not appear at all).
       '';
+
     };
 
   };

nixos/modules/services/torrent/transmission.nix

···
       # https://trac.transmissionbt.com/browser/trunk/libtransmission/tr-udp.c?rev=11956.
       # at least up to the values hardcoded here:
       (mkIf cfg.settings.utp-enabled {
-        "net.core.rmem_max" = mkDefault "4194304"; # 4MB
+        "net.core.rmem_max" = mkDefault 4194304; # 4MB
         "net.core.wmem_max" = mkDefault "1048576"; # 1MB
       })
       (mkIf cfg.performanceNetParameters {