Add support for writing the build revision in built environment

Author: arnarg

modules/build.nix

@@ -112,11 +112,20 @@ in {
 
         mkdir -p "\$dest"
 
-        echo "switching manifests"
-
-        ${pkgs.rsync}/bin/rsync --recursive --delete -L "${config.build.environmentPackage}/" "\$dest"
-
-        echo "done!"
+        # We need to check if there is a difference between
+        # the newly built environment and the destination
+        # excluding `.revision` because that will most likely
+        # always change when going through CI, avoiding infinite
+        # loop.
+        if ! ${pkgs.diffutils}/bin/diff -q -r --exclude .revision "${config.build.environmentPackage}" "\$dest" &>/dev/null; then
+          echo "switching manifests"
+
+          ${pkgs.rsync}/bin/rsync --recursive --delete -L "${config.build.environmentPackage}/" "\$dest"
+
+          echo "done!"
+        else
+          echo "no changes!"
+        fi
         EOF
 
         chmod +x $out/activate

modules/nixidy.nix

@@ -62,6 +62,17 @@ in {
       };
     };
 
+    build = {
+      revision = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        example = literalExpression ''
+          if (self ? rev) then self.rev else self.dirtyRev
+        '';
+        description = "The revision being built. Will be written to `.revision` in the environment destination directory.";
+      };
+    };
+
     extraFiles = mkOption {
       type = types.attrsOf (types.submodule extraFilesOpts);
       default = {};
@@ -181,5 +192,9 @@ in {
 
     _module.args.charts = config.nixidy.charts;
     nixidy.charts = lib.optionalAttrs (cfg.chartsDir != null) (mkChartAttrs cfg.chartsDir);
+
+    nixidy.extraFiles = lib.optionalAttrs (cfg.build.revision != null) {
+      ".revision".text = cfg.build.revision;
+    };
   };
 }