From 5c702852965e6f1f0f88513b4b8c21f144bcb3f8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=A9r=C3=A9my=20Levilain?= <jeremy@jeremylvln.fr>
Date: Fri, 13 Jun 2025 14:58:05 +0200
Subject: [PATCH] feat: properly return an Unprocessable Entity error in case
 of expected handler rejection

---
 src/api/models/controllers.test.ts | 22 ++++++++++++++++++++++
 src/models/handler.test.ts         | 18 +++++++++++++++++-
 src/models/handler.ts              | 17 +++++++++++++++++
 src/setup.ts                       | 16 +++++++++++++---
 test/fixtures/users.ts             |  5 +++++
 5 files changed, 74 insertions(+), 4 deletions(-)

diff --git a/src/api/models/controllers.test.ts b/src/api/models/controllers.test.ts
index 0f3ebdd..0228de0 100644
--- a/src/api/models/controllers.test.ts
+++ b/src/api/models/controllers.test.ts
@@ -967,6 +967,28 @@ describe('controllers/models', () => {
       expect(error.status).toEqual(422);
     });
 
+    it('returns a 422 Unprocessable Entity if the event handler rejects the event', async () => {
+      const controller = apply({ ...services, models });
+
+      const user = models.factory('users');
+
+      await user.create({ firstname: 'John' });
+
+      req.params.model = 'users';
+      req.params.correlation_id = user.state.user_id.toString();
+      req.params.event_type = 'always_unprocessable';
+      req.params.event_version = '0_0_0';
+      req.body = {};
+
+      await controller(req, res, next);
+
+      expect(error).not.toBe(null);
+      expect(error.message).toEqual(
+        'Reducer failed to process event: Always Unprocessable',
+      );
+      expect(error.status).toEqual(422);
+    });
+
     it('returns a 405 `Entity is readonly` in case of an apply on a readonly entity', async () => {
       const controller = apply({ ...services, models });
 
diff --git a/src/models/handler.test.ts b/src/models/handler.test.ts
index b326c74..382b0a6 100644
--- a/src/models/handler.test.ts
+++ b/src/models/handler.test.ts
@@ -1,4 +1,4 @@
-import { handle } from './handler';
+import { handle, UnprocessableReducerError } from './handler';
 
 describe('models/handler', () => {
   const DEFAULT_HANDLER = `
@@ -23,4 +23,20 @@ describe('models/handler', () => {
 
     expect(error.message.startsWith('Cannot read properties')).toEqual(true);
   });
+
+  it('throws an UnprocessableReducerError when a UnprocessableError is thrown by the code', async () => {
+    let error;
+    try {
+      await handle(`throw new UnprocessableError('Forbidden update');`, null, {
+        count: 1,
+      });
+    } catch (err) {
+      error = err;
+    }
+
+    expect(error).toBeInstanceOf(UnprocessableReducerError);
+    expect(error.message).toEqual(
+      'Reducer failed to process event: Forbidden update',
+    );
+  });
 });
diff --git a/src/models/handler.ts b/src/models/handler.ts
index f0e1651..1ac2f1e 100644
--- a/src/models/handler.ts
+++ b/src/models/handler.ts
@@ -1,10 +1,18 @@
 import type FullyHomomorphicEncryptionClient from '../services/fhe';
+import statusCodes from 'http-status-codes';
 
 import path from 'node:path';
 
 import { Worker } from 'worker_threads';
 import { ModelConfig } from '../typings';
 
+export class UnprocessableReducerError extends Error {
+  readonly status = statusCodes.UNPROCESSABLE_ENTITY;
+  constructor(message: string) {
+    super(`Reducer failed to process event: ${message}`);
+  }
+}
+
 export async function handle(
   handler: string | Function,
   state: any,
@@ -103,6 +111,10 @@ export async function thread<T>(
     const workerScript = `
 const { parentPort, workerData } = require('worker_threads');
 
+class UnprocessableError extends Error {
+  type = 'UnprocessableError';
+}
+
 ${script}
 
 async function main() {
@@ -125,6 +137,11 @@ main().catch((err) => {
     });
 
     thread.on('error', (err) => {
+      if ('type' in err && err.type === 'UnprocessableError') {
+        reject(new UnprocessableReducerError(err.message));
+        return;
+      }
+
       reject(err);
     });
     thread.on('exit', () => {
diff --git a/src/setup.ts b/src/setup.ts
index f7dbca6..45f6bf3 100644
--- a/src/setup.ts
+++ b/src/setup.ts
@@ -1,7 +1,17 @@
-process.env.MONGOMS_DOWNLOAD_URL =
-  process.env.MONGOMS_DOWNLOAD_URL ||
-  'https://fastdl.mongodb.org/linux/mongodb-linux-x86_64-debian12-8.0.3.tgz';
+import * as os from 'node:os';
+
+const [SYSTEM_PLATFORM, SYSTEM_MACHINE] = [os.platform(), os.machine()];
+const FALLBACK_MONGOMS_DOWNLOAD_URL =
+  SYSTEM_PLATFORM === 'linux'
+    ? `https://fastdl.mongodb.org/linux/mongodb-linux-${SYSTEM_MACHINE}-debian12-8.0.3.tgz`
+    : SYSTEM_PLATFORM === 'darwin'
+      ? `https://fastdl.mongodb.org/osx/mongodb-macos-${SYSTEM_MACHINE}-8.0.3.tgz`
+      : (function () {
+          throw new Error('Unsupported system architecture');
+        })();
 
+process.env.MONGOMS_DOWNLOAD_URL =
+  process.env.MONGOMS_DOWNLOAD_URL || FALLBACK_MONGOMS_DOWNLOAD_URL;
 process.env.OTEL_PROMETHEUS_EXPORTER_PREVENT_SERVER_START = 'true';
 
 import type { DatastoreConfig, ModelConfig, Services } from './typings';
diff --git a/test/fixtures/users.ts b/test/fixtures/users.ts
index f65f27c..884c8ab 100644
--- a/test/fixtures/users.ts
+++ b/test/fixtures/users.ts
@@ -145,6 +145,11 @@ const modelConfig: ModelConfig = {
           },
         },
       },
+      ALWAYS_UNPROCESSABLE: {
+        '0_0_0': {
+          handler: `throw new UnprocessableError('Always Unprocessable');`,
+        }
+      }
     },
   },
   processings: [
-- 
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