nixos/sysctl: use highest value on conflict for net.core.wmem_max

we previously defined a custom type for `boot.kernel.sysctl."net.core.rmem_max"`
to resolve to the highest value set. this patch adds the same behavior to
`"net.core.wmem_max"`.

as this changes the type from a string to an integer, which is a breaking
change this patch also includes a release note and updates the transmission
module to use a number for `wmem_max`.

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -67,6 +67,8 @@ The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been m
   `globalRedirect` can now have redirect codes other than 301 through
   `redirectCode`.
 
+- [](#opt-boot.kernel.sysctl._net.core.wmem_max_) changed from a string to an integer because of the addition of a custom merge option (taking the highest value defined to avoid conflicts between 2 services trying to set that value), just as [](#opt-boot.kernel.sysctl._net.core.rmem_max_) since 22.11.
+
 - Gitea 1.21 upgrade has several breaking changes, including:
   - Custom themes and other assets that were previously stored in `custom/public/*` now belong in `custom/public/assets/*`
   - New instances of Gitea using MySQL now ignore the `[database].CHARSET` config option and always use the `utf8mb4` charset, existing instances should migrate via the `gitea doctor convert` CLI command.

nixos/modules/config/sysctl.nix

@@ -21,19 +21,27 @@ in
   options = {
 
     boot.kernel.sysctl = mkOption {
-      type = types.submodule {
+      type = let
+        highestValueType = types.ints.unsigned // {
+          merge = loc: defs:
+            foldl
+              (a: b: if b.value == null then null else lib.max a b.value)
+              0
+              (filterOverrides defs);
+        };
+      in 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);
-          };
+          type = types.nullOr highestValueType;
           default = null;
           description = lib.mdDoc "The maximum socket receive buffer size. In case of conflicting values, the highest will be used.";
         };
+
+        options."net.core.wmem_max" = mkOption {
+          type = types.nullOr highestValueType;
+          default = null;
+          description = lib.mdDoc "The maximum socket send buffer size. In case of conflicting values, the highest will be used.";
+        };
       };
       default = {};
       example = literalExpression ''

nixos/modules/services/torrent/transmission.nix

@@ -434,7 +434,7 @@ in
       # at least up to the values hardcoded here:
       (mkIf cfg.settings.utp-enabled {
         "net.core.rmem_max" = mkDefault 4194304; # 4MB
-        "net.core.wmem_max" = mkDefault "1048576"; # 1MB
+        "net.core.wmem_max" = mkDefault 1048576; # 1MB
       })
       (mkIf cfg.performanceNetParameters {
         # Increase the number of available source (local) TCP and UDP ports to 49151.