rmem_max: define merge function

          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>

- 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. -->

  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).

      '';

    };

  };

      # 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 {