Ensure Nix expressions are up to date in CI

- Adds a step to the Buildkite CI to check that the Nix expressions of
the project are up-to-date. If they are not, it provides a patch as an
artifact that the user can download and apply with `patch -p1 <
nix-expr.patch`. This was done to make it easier for non-Nix users to
update the dependencies of the project.

Author: sevanspowell

.buildkite/pipeline.nix

@@ -11,6 +11,34 @@ in
 
 {
   steps.commands = {
+    nixExpr = commonAttrs // {
+      label = "ensure Nix expressions are up-to-date";
+      command = ''
+        echo "Checking if Nix expressions are up-to-date..."
+
+        nix-shell --run 'sbtix-gen-all2'
+
+        set +e
+        git diff --exit-code > nix-expr.patch
+        if [ "$?" -eq "1" ]; then
+          set -e
+          echo "Nix expressions not up-to-date."
+          echo "Download and apply the patch available in the artifact paths of this step:"
+          echo "  patch -p1 < nix-expr.patch"
+          echo "Aborting."
+          exit 1
+        else
+          set -e
+          echo "Nix expressions up-to-date!"
+          exit 0
+        fi
+      '';
+      retry.automatic = false;
+      artifactPaths = [
+        "nix-expr.patch"
+      ];
+    };
+
     compile = commonAttrs // {
       label = "compile everything";
       command = ''

.buildkite/shell.nix

@@ -1,4 +1,4 @@
-{ pkgs }:
+{ sources,  pkgs }:
 
 let
 
@@ -18,13 +18,16 @@ let
     exec ${pkgs.protobuf}/bin/protoc "$@"
   '';
 
+  sbtix = pkgs.callPackage sources.Sbtix { };
+
 in
 
   with pkgs;
 
   mkShell {
 
-    buildInputs = [ sbt solc jdk8 protoc-wrapper ];
+    buildInputs = [ sbt solc jdk8 protoc-wrapper sbtix ];
     # SBT = "sbt -v -mem 2048 -J-Xmx4g -Dsbt.ivy.home=/cache/ivy2 -Dsbt.boot.directory=/cache/sbt -Dmaven.repo.local=/cache/maven -Dnix=true";
     SBT = "sbt -v -mem 2048 -J-Xmx4g -Dnix=true";
+    SBTIX_GEN = "true";
   }