automate nixpkgs update

krebs/krops.nix

@@ -12,7 +12,7 @@
 
   krebs-source = {
     nixpkgs.git = {
-      ref = "e42c07d78a1fefc2d91f11f00e938253b3618a3b";
+      ref = (lib.importJSON ./nixpkgs.json).rev;
       url = https://github.com/NixOS/nixpkgs;
     };
     stockholm.file = toString ../.;

krebs/nixpkgs.json

@@ -0,0 +1,7 @@
+{
+  "url": "https://github.com/NixOS/nixpkgs-channels",
+  "rev": "4df3426f5a5e78cef4835897a43abd9e2a092b74",
+  "date": "2018-08-19T09:20:40+02:00",
+  "sha256": "05k5mssiqxffxi45mss9wjns6k76i248rpasa48akdcriry1mp63",
+  "fetchSubmodules": true
+}

krebs/update-channel.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+dir=$(dirname $0)
+oldrev=$(cat $dir/nixpkgs.json | jq -r .rev | sed 's/\(.\{7\}\).*/\1/')
+nix-shell -p nix-prefetch-git --run 'nix-prefetch-git \
+  --url https://github.com/NixOS/nixpkgs-channels \
+  --rev refs/heads/nixos-18.03' \
+> $dir/nixpkgs.json
+newrev=$(cat $dir/nixpkgs.json | jq -r .rev | sed 's/\(.\{7\}\).*/\1/')
+git commit $dir/nixpkgs.json -m "nixpkgs: $oldrev -> $newrev"