introduce .models files, crash when we can't parse them as JSON

Summary:
Mariana Trench has multiple semantically different files it uses as part of its configuration, which all happen to be communicated as JSON. We previously didn't distinguish between these, so we try to parse and silently fail for a lot of our
model JSON files. Let's introduce a new extension (copying other static analysis tool conventions here) that handles model generators and error out if they're invalid.

Reviewed By: anwesht

Differential Revision: D47552197

fbshipit-source-id: 5833bdaa83fe5cf784cb6c1d774f903ca6b449ca

source/ModelGeneration.cpp

@@ -44,9 +44,11 @@ make_model_generators(Context& context) {
   for (const auto& path : context.options->model_generator_search_paths()) {
     LOG(3, "Searching for model generators in `{}`...", path);
     for (auto& entry : boost::filesystem::recursive_directory_iterator(path)) {
-      if (entry.path().extension() != ".json") {
+      if (entry.path().extension() != ".json" &&
+          entry.path().extension() != ".models") {
         continue;
       }
+      bool always_validate_models = entry.path().extension() == ".models";
 
       auto path_copy = entry.path();
       auto identifier = path_copy.replace_extension("").filename().string();
@@ -57,11 +59,16 @@ make_model_generators(Context& context) {
         Json::Value json = JsonValidation::parse_json_file(entry.path());
 
         if (!json.isObject()) {
-          // TODO(T153463464): This means it is most likely not a model
-          // generator (it could be a rule definition file, lifecycle
-          // definition file, etc.). Ignore it silently for now. In the future,
-          // we should use the extension to differentiate between file types.
-          continue;
+          // TODO(T153463464): We always validate .models files, but still
+          // accept legacy .json files that may not parse. (it could be a rule
+          // definition file, lifecycle definition file, etc.). Ignore it
+          // silently for now. In the future, we should use the extension to
+          // differentiate between file types.
+          if (!always_validate_models) {
+            continue;
+          }
+          throw ModelGeneratorError(fmt::format(
+              "Unable to parse `{}` as a valid models JSON.", entry.path()));
         }
 
         auto [_, inserted] = generators.emplace(