From 0f7ba79166b96141104eb98edfddea4172f69da0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rub=C3=A9n=20Norte?= <rubennorte@meta.com>
Date: Fri, 25 Jul 2025 05:47:35 -0700
Subject: [PATCH] Avoid creating too many unnecessary build directories for
 Fantom bundles (#52788)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52788

Changelog: [internal]

Minor change to just create a single output directory for generated JS code in Fantom per Jest run.

Reviewed By: lenaic

Differential Revision: D78808564

fbshipit-source-id: 70e1a60dfcdcc3fc6ee5f08ced1e9c8f8cab2782
---
 .../runner/global-setup/globalSetup.js             |  2 ++
 private/react-native-fantom/runner/runner.js       | 14 ++++++++++----
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/private/react-native-fantom/runner/global-setup/globalSetup.js b/private/react-native-fantom/runner/global-setup/globalSetup.js
index be1c78b2318..bd6b84bacc4 100644
--- a/private/react-native-fantom/runner/global-setup/globalSetup.js
+++ b/private/react-native-fantom/runner/global-setup/globalSetup.js
@@ -15,6 +15,8 @@ export default async function globalSetup(
   globalConfig: {...},
   projectConfig: {...},
 ): Promise<void> {
+  process.env.__FANTOM_RUN_ID__ ??= `run-${Date.now()}`;
+
   validateEnvironmentVariables();
 
   if (!isOSS) {
diff --git a/private/react-native-fantom/runner/runner.js b/private/react-native-fantom/runner/runner.js
index ec1f7a46da4..453beb28202 100644
--- a/private/react-native-fantom/runner/runner.js
+++ b/private/react-native-fantom/runner/runner.js
@@ -51,11 +51,17 @@ import nullthrows from 'nullthrows';
 import path from 'path';
 import readline from 'readline';
 
+const fantomRunID = process.env.__FANTOM_RUN_ID__;
+if (fantomRunID == null) {
+  throw new Error(
+    'Expected Fantom run ID to be set by global setup, but it was not (process.env.__FANTOM_RUN_ID__ is null)',
+  );
+}
+
 const BUILD_OUTPUT_ROOT = path.resolve(__dirname, '..', 'build', 'js');
-fs.mkdirSync(BUILD_OUTPUT_ROOT, {recursive: true});
-const BUILD_OUTPUT_PATH = fs.mkdtempSync(
-  path.join(BUILD_OUTPUT_ROOT, `run-${Date.now()}-`),
-);
+const BUILD_OUTPUT_PATH = path.join(BUILD_OUTPUT_ROOT, fantomRunID);
+
+fs.mkdirSync(BUILD_OUTPUT_PATH, {recursive: true});
 
 function buildError(
   failureDetail: FailureDetail,