distro/nix: add

The generated default.nix exactly corresponds to what's on
https://github.com/NixOS/nixpkgs/blob/master/pkgs/servers/dns/knot-dns/default.nix
(except for tiny differences directly due to 3.0.8 vs. 3.1.0)

distro/pkg/nix/default.nix

+{ lib, stdenv, fetchurl, pkg-config, gnutls, liburcu, lmdb, libcap_ng, libidn2, libunistring
+, systemd, nettle, libedit, zlib, libiconv, libintl, libmaxminddb, libbpf, nghttp2, libmnl
+, autoreconfHook, nixosTests
+}:
+
+let inherit (lib) optional optionals; in
+
+stdenv.mkDerivation rec {
+  pname = "knot-dns";
+  version = "{{ version }}";
+
+  src = fetchurl {
+    url = "https://secure.nic.cz/files/knot-dns/knot-${version}.tar.xz";
+    sha256 = "{{ src_hash }}";
+  };
+
+  outputs = [ "bin" "out" "dev" ];
+
+  configureFlags = [
+    "--with-configdir=/etc/knot"
+    "--with-rundir=/run/knot"
+    "--with-storage=/var/lib/knot"
+  ];
+
+  patches = [
+    # Don't try to create directories like /var/lib/knot at build time.
+    # They are later created from NixOS itself.
+    ./dont-create-run-time-dirs.patch
+    ./runtime-deps.patch
+  ];
+
+  nativeBuildInputs = [ pkg-config autoreconfHook ];
+  buildInputs = [
+    gnutls liburcu libidn2 libunistring
+    nettle libedit
+    libiconv lmdb libintl
+    nghttp2 # DoH support in kdig
+    libmaxminddb # optional for geoip module (it's tiny)
+    libmnl # required for knot >= 3.1
+    # without sphinx &al. for developer documentation
+    # TODO: add dnstap support?
+  ]
+    ++ optionals stdenv.isLinux [
+      libcap_ng systemd
+      libbpf # XDP support
+    ]
+    ++ optional stdenv.isDarwin zlib; # perhaps due to gnutls
+
+  enableParallelBuilding = true;
+
+  CFLAGS = [ "-O2" "-DNDEBUG" ];
+
+  doCheck = true;
+  checkFlags = "V=1"; # verbose output in case some test fails
+  doInstallCheck = true;
+
+  postInstall = ''
+    rm -r "$out"/lib/*.la
+  '';
+
+  passthru.tests = { inherit (nixosTests) knot; };
+
+  meta = with lib; {
+    description = "Authoritative-only DNS server from .cz domain registry";
+    homepage = "https://knot-dns.cz";
+    license = licenses.gpl3Plus;
+    platforms = platforms.unix;
+    maintainers = [ maintainers.vcunat ];
+  };
+}

distro/pkg/nix/dont-create-run-time-dirs.patch

+diff --git a/samples/Makefile.am b/samples/Makefile.am
+index c253c91..107401d 100644
+--- a/samples/Makefile.am
++++ b/samples/Makefile.am
+@@ -19,11 +19,6 @@ EXTRA_DIST = knot.sample.conf.in example.com.zone
+ 
+ if HAVE_DAEMON
+ 
+-install-data-local: knot.sample.conf
+-	if [ \! -f $(DESTDIR)/$(config_dir)/knot.sample.conf ]; then \
+-	  $(INSTALL) -d $(DESTDIR)/$(config_dir); \
+-	  $(INSTALL_DATA) knot.sample.conf $(srcdir)/example.com.zone $(DESTDIR)/$(config_dir); \
+-	fi
+ uninstall-local:
+ 	-rm -rf $(DESTDIR)/$(config_dir)/knot.sample.conf \
+ 	        $(DESTDIR)/$(config_dir)/example.com.zone
+diff --git a/src/utils/Makefile.inc b/src/utils/Makefile.inc
+index e6765d9..d859d23 100644
+--- a/src/utils/Makefile.inc
++++ b/src/utils/Makefile.inc
+@@ -79,11 +79,6 @@ endif HAVE_DNSTAP
+ endif HAVE_UTILS
+ 
+ if HAVE_DAEMON
+-# Create storage and run-time directories
+-install-data-hook:
+-	$(INSTALL) -d $(DESTDIR)/@config_dir@
+-	$(INSTALL) -d $(DESTDIR)/@run_dir@
+-	$(INSTALL) -d $(DESTDIR)/@storage_dir@
+ 
+ sbin_PROGRAMS = knotc knotd
+ 

distro/pkg/nix/runtime-deps.patch

+Remove unnecessary runtime dependencies.
+
+`knotc status configure` shows summary from the configure script,
+but that contains also references like include paths.
+Filter these at least in a crude way (whole lines).
+--- a/configure.ac
++++ b/configure.ac
+@@ -766,5 +766,5 @@ result_msg_base="  Knot DNS $VERSION
+ 
+-result_msg_esc=$(echo -n "$result_msg_base" | sed '$!s/$/\\n/' | tr -d '\n')
++result_msg_esc=$(echo -n "$result_msg_base" | grep -Fv "$NIX_STORE" | sed '$!s/$/\\n/' | tr -d '\n')
+ 
+ AC_DEFINE_UNQUOTED([CONFIGURE_SUMMARY],["$result_msg_esc"],[Configure summary])
+ 

distro/pkg/nix/top-level.nix

+
+with import <nixpkgs> {};
+
+(callPackage ./. {
+}).overrideAttrs (attrs: {
+  src = ./knot-{{ version }}.tar.xz;
+})
+