knot-dns: add a test for version-sensitive dependencies

pkgs/servers/dns/knot-dns/default.nix

@@ -1,7 +1,8 @@
 { lib, stdenv, fetchurl, pkg-config, gnutls, liburcu, lmdb, libcap_ng, libidn2, libunistring
 , systemd, nettle, libedit, zlib, libiconv, libintl, libmaxminddb, libbpf, nghttp2, libmnl
 , ngtcp2-gnutls
-, autoreconfHook, nixosTests, knot-resolver
+, autoreconfHook
+, nixosTests, knot-resolver, knot-dns, runCommandLocal
 , fetchpatch
 }:
 
@@ -65,6 +66,19 @@ stdenv.mkDerivation rec {
     inherit knot-resolver;
   } // lib.optionalAttrs stdenv.isLinux {
     inherit (nixosTests) knot;
+    # Some dependencies are very version-sensitive, so the might get dropped
+    # or embedded after some update, even if the nixPackagers didn't intend to.
+    # For non-linux I don't know a good replacement for `ldd`.
+    deps = runCommandLocal "knot-deps-test"
+      { nativeBuildInputs = [ (lib.getBin stdenv.cc.libc) ]; }
+      ''
+        for libname in libngtcp2 libbpf; do
+          echo "Checking for $libname:"
+          ldd '${knot-dns.bin}/bin/knotd' | grep -F "$libname"
+          echo "OK"
+        done
+        touch "$out"
+      '';
   };
 
   meta = with lib; {