add patch for radv prefix fade out in systemd as overlay & NixOS module

prematurely fixes https://github.com/systemd/systemd/issues/29651

nix/nixos-modules/default.nix

@@ -6,6 +6,7 @@
   ...
 }@flakeArg:
 let
+  importModuleGroup = lib.importFlakeMod;
   importModule = path: { imports = lib.singleton path; };
 in
 {
@@ -60,4 +61,12 @@ in
       };
     };
 
+  # from sub groups
+  # NOTE: these will change possibly unsensible stuff just by importing them
+
+  inherit (importModuleGroup ./overlays)
+    # (make list commitable)
+    systemd-radv-fadeout
+    ;
+
 }

nix/nixos-modules/overlays/default.nix

@@ -0,0 +1,29 @@
+# This file especially especially includes special overlays
+# where a full include via the nixpkgs.overlays option is not feasible
+# because of a big list of dependencies
+# and where in most cases just setting a certain package option should be enough (e.g. systemd).
+{
+  inputs,
+  lib,
+  outputs,
+  ...
+}@flakeArg:
+let
+  withOverlay =
+    overlay: configFun:
+    { pkgs, ... }:
+    configFun (
+      import inputs.nixpkgs {
+        system = pkgs.system;
+        overlays = lib.singleton overlay;
+      }
+    );
+in
+{
+
+  # TODO until https://github.com/systemd/systemd/issues/29651 is fixed
+  systemd-radv-fadeout = withOverlay outputs.overlays.systemd-radv-fadeout (pkgs: {
+    config.systemd.package = pkgs.systemd;
+  });
+
+}

nix/overlays/default.nix

@@ -11,4 +11,6 @@ in
 
   backports = importOverlay ./backports.nix;
 
+  systemd-radv-fadeout = importOverlay ./systemd-radv-fadeout;
+
 }

nix/overlays/systemd-radv-fadeout/default.nix

@@ -0,0 +1,8 @@
+# TODO until https://github.com/systemd/systemd/issues/29651 is fixed
+{ outputs, ... }@flakeArg:
+{ ... }@systemArg:
+final: prev: {
+  systemd = prev.systemd.overrideAttrs (old: {
+    patches = old.patches ++ [ ./patch.patch ];
+  });
+}

nix/overlays/systemd-radv-fadeout/patch.patch

@@ -0,0 +1,55 @@
+commit b09851c2be354e592802fe9209b4cd6150bd818d
+Author: Felix Stupp <felix.stupp@banananet.work>
+Date:   Tue Sep 3 20:33:03 2024 +0000
+
+    network/radv: fade out announced prefixes rather than just deleting them
+    
+    Fixes https://github.com/systemd/systemd/issues/29651
+
+diff --git a/src/libsystemd-network/sd-radv.c b/src/libsystemd-network/sd-radv.c
+index c384d4e627..537203c13b 100644
+--- a/src/libsystemd-network/sd-radv.c
++++ b/src/libsystemd-network/sd-radv.c
+@@ -679,6 +679,8 @@ void sd_radv_remove_prefix(
+         if (!prefix)
+                 return;
+ 
++        const char *addr_p = IN6_ADDR_PREFIX_TO_STRING(prefix, prefixlen);
++
+         LIST_FOREACH(prefix, cur, ra->prefixes) {
+                 if (prefixlen != cur->opt.prefixlen)
+                         continue;
+@@ -686,9 +688,32 @@ void sd_radv_remove_prefix(
+                 if (!in6_addr_equal(prefix, &cur->opt.in6_addr))
+                         continue;
+ 
++                /* "Fade out" IPv6 prefix, i.e. informing clients about its sudden invalidity.
++                 * Realized by announcing the prefix with preferred=0 & valid=2h.
++                 * This makes clients rotating to a newer prefix if some is already defined.
++                */
++
++                // TODO is copying required here? (& does it do anything at all?)
++                sd_radv_prefix *p = cur;
++
++                // TODO replace hacky way to get current time
++                uint64_t current_time = cur->valid_until - cur->lifetime_valid_usec;
++                // TODO replace constant with setting (valid lifetime) ?
++                uint64_t two_hours_usec = (uint64_t) 2 * 60 * 60 * 1000000;
++                sd_radv_prefix_set_preferred_lifetime(p, 0, current_time);
++                sd_radv_prefix_set_valid_lifetime(p, two_hours_usec, current_time + two_hours_usec);
++
++                // TODO is full replacement procedure required or can we just edit the stored prefix without this?
++                // procedure cloned from sd_radv_add_prefix, if found. I do not call this here because of the duplicated search in the list & because of the different logging message
++                sd_radv_prefix_ref(p);
+                 LIST_REMOVE(prefix, ra->prefixes, cur);
+-                ra->n_prefixes--;
+                 sd_radv_prefix_unref(cur);
++                LIST_APPEND(prefix, ra->prefixes, p);
++
++                log_radv(ra, "Fade out IPv6 prefix %s (preferred: %s, valid: %s)",
++                         addr_p,
++                         FORMAT_TIMESPAN(p->lifetime_preferred_usec, USEC_PER_SEC),
++                         FORMAT_TIMESPAN(p->lifetime_valid_usec, USEC_PER_SEC));
+                 return;
+         }
+ }