add nixops eval subcommand

nix/eval-machine-info.nix

@@ -5,13 +5,15 @@
 , deploymentName
 , args
 , pluginNixExprs
+, evalFile ? null
 }:
 
 with import <nixpkgs> { inherit system; };
 with lib;
 
-
-rec {
+let
+  evaluator = if evalFile != null then (import evalFile) else id;
+in evaluator (rec {
 
   importedPluginNixExprs = map
     (expr: import expr)
@@ -200,4 +202,4 @@ rec {
   getNixOpsArgs = fs: lib.zipAttrs (lib.unique (lib.concatMap fileToArgs (getNixOpsExprs fs)));
 
   nixopsArguments = getNixOpsArgs networkExprs;
-}
+})

nixops/__main__.py

@@ -527,6 +527,22 @@
     help="include the physical specification in the evaluation",
 )
 
+subparser = add_subparser(
+    subparsers,
+    "eval",
+    help="evaluate a Nix expression with the NixOps network as arguments",
+)
+subparser.set_defaults(op=op_eval)
+subparser.add_argument("file", metavar="FILE", help="file containing a Nix expression")
+subparser.add_argument(
+    "--json", action="store_true", help="convert and print the return value as JSON"
+)
+subparser.add_argument(
+    "--strict",
+    action="store_true",
+    help="enable strict evaluation, (use with --json if value is more than a level deep)",
+)
+
 subparser = add_subparser(
     subparsers,
     "list-generations",

nixops/deployment.py

@@ -395,7 +395,7 @@ def _eval_flags(self, exprs: List[str]) -> List[str]:
                 "--arg",
                 "pluginNixExprs",
                 py2nix(extraexprs),
-                "<nixops/eval-machine-info.nix>",
+                self.expr_path + "/eval-machine-info.nix",
             ]
         )
         return flags
@@ -503,6 +503,37 @@ def evaluate(self) -> None:
                 )
                 self.definitions[name] = defn
 
+    def evaluate_code(self, file: str, json: bool = False, strict: bool = False) -> str:
+        """Evaluate nix code in the deployment specification."""
+
+        exprs = self.nix_exprs
+        phys_expr = self.tempdir + "/physical.nix"
+        with open(phys_expr, "w") as f:
+            f.write(self.get_physical_spec())
+        exprs.append(phys_expr)
+
+        try:
+            return subprocess.check_output(
+                ["nix-instantiate"]
+                + self.extra_nix_eval_flags
+                + self._eval_flags(exprs)
+                + [
+                    "--eval-only",
+                    "--arg",
+                    "checkConfigurationOptions",
+                    "false",
+                    "--arg",
+                    "evalFile",
+                    file,
+                ]
+                + (["--strict"] if strict else [])
+                + (["--json"] if json else []),
+                stderr=self.logger.log_file,
+                text=True,
+            )
+        except subprocess.CalledProcessError:
+            raise NixEvalError
+
     def evaluate_option_value(
         self,
         machine_name: str,

nixops/script_defs.py

@@ -893,6 +893,14 @@ def op_show_option(args):
         )
 
 
+def op_eval(args):
+    with deployment(args) as depl:
+        depl.evaluate()
+        sys.stdout.write(
+            depl.evaluate_code(args.file, json=args.json, strict=args.strict)
+        )
+
+
 @contextlib.contextmanager
 def deployment_with_rollback(args):
     with deployment(args) as depl: