nixos/dhcpcd: assert if privSep && alternative malloc

dhcpcd does not run properly with some of the hardened system mallocs
that are currently available. assert when an incompatible configuration
is detected, as a switch into such a config from eg auto-update can take
hosts offline.

nixos/modules/services/networking/dhcpcd.nix

@@ -183,6 +183,20 @@ in
 
   config = mkIf enableDHCP {
 
+    assertions = [ {
+      # dhcpcd doesn't start properly with malloc ∉ [ libc scudo ]
+      # see https://github.com/NixOS/nixpkgs/issues/151696
+      assertion =
+        dhcpcd.enablePrivSep
+          -> elem config.environment.memoryAllocator.provider [ "libc" "scudo" ];
+      message = ''
+        dhcpcd with privilege separation is incompatible with chosen system malloc.
+          Currently only the `libc` and `scudo` allocators are known to work.
+          To disable dhcpcd's privilege separation, overlay Nixpkgs and override dhcpcd
+          to set `enablePrivSep = false`.
+      '';
+    } ];
+
     systemd.services.dhcpcd = let
       cfgN = config.networking;
       hasDefaultGatewaySet = (cfgN.defaultGateway != null && cfgN.defaultGateway.address != "")