From 1483f9eec0a1c104eb8aa85282121f7aa371d05a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rub=C3=A9n=20Norte?= <rubennorte@meta.com>
Date: Tue, 12 Aug 2025 03:40:34 -0700
Subject: [PATCH] Refactor logic to find available port for Metro in Fantom
 (#53204)

Summary:

Changelog: [internal]

This changes the logic to find an available port for Metro on Fantom to do this outside Metro. Before, we'd set `0` as the port for Metro to find an available port, but in a following change we'll need to know the port before calling into Metro. This allows that.

Reviewed By: rshest

Differential Revision: D79804005
---
 .../runner/global-setup/globalSetup.js        | 35 +++++++++++++------
 1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/private/react-native-fantom/runner/global-setup/globalSetup.js b/private/react-native-fantom/runner/global-setup/globalSetup.js
index f87eec9848a0..af8871e74d97 100644
--- a/private/react-native-fantom/runner/global-setup/globalSetup.js
+++ b/private/react-native-fantom/runner/global-setup/globalSetup.js
@@ -11,6 +11,7 @@
 import {isOSS, validateEnvironmentVariables} from '../EnvironmentOptions';
 import build from './build';
 import Metro from 'metro';
+import {Server} from 'net';
 import path from 'path';
 
 export default async function globalSetup(
@@ -33,26 +34,38 @@ async function startMetroServer() {
     config: path.resolve(__dirname, '..', '..', 'config', 'metro.config.js'),
   });
 
+  if (process.env.__FANTOM_METRO_PORT__ == null) {
+    const availablePort = await findAvailablePort();
+    process.env.__FANTOM_METRO_PORT__ = String(availablePort);
+  }
+
   // We need to reuse the same port across runs because can only set environment
   // variables for workers in the first one.
   // $FlowExpectedError[cannot-write]
-  metroConfig.server.port =
-    process.env.__FANTOM_METRO_PORT__ != null
-      ? Number(process.env.__FANTOM_METRO_PORT__)
-      : // Any available port
-        0;
+  metroConfig.server.port = Number(process.env.__FANTOM_METRO_PORT__);
 
   const server = await Metro.runServer(metroConfig, {
     waitForBundler: true,
     watch: true,
   });
 
-  if (process.env.__FANTOM_METRO_PORT__ == null) {
-    process.env.__FANTOM_METRO_PORT__ = String(
-      server.httpServer.address().port,
-    );
-  }
-
   // $FlowExpectedError[prop-missing]
   globalThis.__METRO_SERVER__ = server;
 }
+
+async function findAvailablePort(): Promise<number> {
+  return new Promise((resolve, reject) => {
+    const server = new Server();
+    server.listen(0, 'localhost', undefined, () => {
+      const port = server.address().port;
+      server.close(error => {
+        if (error != null) {
+          reject(error);
+        } else {
+          resolve(port);
+        }
+      });
+    });
+    server.on('error', reject);
+  });
+}