From e3183739bbe85772c195877fff66b377bb2265b5 Mon Sep 17 00:00:00 2001
From: Ruslan Shestopalyuk <rshest@meta.com>
Date: Fri, 15 Aug 2025 08:12:04 -0700
Subject: [PATCH] Options to run Fantom with address and thread sanitizers
 enabled (#53300)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53300

# Changelog:
[Internal] -

This adds two new modes to Fantom, allowing to run the native (C++) side with enabling either:
* Address sanitizer, which would detect memory overwrites
* Thread sanitizer, which can detect potential threading issues, such  as race conditions

This are opt-in for now.

Currently, both modes already detect different errors, which have a high chance to be real issues and have to be fixed.

Reviewed By: lenaic

Differential Revision: D80339524

fbshipit-source-id: 784ddb9f0af79a04b074e107e4955724d54d5685
---
 .../react-native-fantom/__docs__/README.md    | 18 +++++++++++++
 .../runner/EnvironmentOptions.js              | 12 +++++++++
 private/react-native-fantom/runner/utils.js   | 27 ++++++++++++++++++-
 3 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/private/react-native-fantom/__docs__/README.md b/private/react-native-fantom/__docs__/README.md
index 60e9c90b243..f14e152bc7c 100644
--- a/private/react-native-fantom/__docs__/README.md
+++ b/private/react-native-fantom/__docs__/README.md
@@ -220,6 +220,24 @@ FANTOM_DEBUG_CPP=1 yarn fantom <regexForTestFiles>
 This would start a debugging session in VS Code with an initial breakpoint in
 the Fantom CLI binary.
 
+#### Address and thread sanitizer for C++
+
+It's also possible to run the C++ side with the thread or address sanitizer
+enabled, which can help with debugging memory and threading issues.
+
+To enable the address sanitizer, run your fantom test with the flag
+`FANTOM_ENABLE_ASAN`:
+
+```shell
+FANTOM_ENABLE_ASAN=1 yarn fantom <regexForTestFiles>
+```
+
+For thread sanitizer, correspondingly, use flag `FANTOM_ENABLE_TSAN`:
+
+```shell
+FANTOM_ENABLE_TSAN=1 yarn fantom <regexForTestFiles>
+```
+
 ### Profiling
 
 #### JS sampling profiler
diff --git a/private/react-native-fantom/runner/EnvironmentOptions.js b/private/react-native-fantom/runner/EnvironmentOptions.js
index bf9ac5e3704..553049aacaa 100644
--- a/private/react-native-fantom/runner/EnvironmentOptions.js
+++ b/private/react-native-fantom/runner/EnvironmentOptions.js
@@ -10,6 +10,8 @@
 
 const VALID_ENVIRONMENT_VARIABLES = [
   'FANTOM_DEBUG_CPP',
+  'FANTOM_ENABLE_ASAN',
+  'FANTOM_ENABLE_TSAN',
   'FANTOM_ENABLE_CPP_DEBUGGING',
   'FANTOM_FORCE_CI_MODE',
   'FANTOM_FORCE_OSS_BUILD',
@@ -67,6 +69,16 @@ export const debugJS: boolean = Boolean(process.env.FANTOM_DEBUG_JS);
 
 export const profileJS: boolean = Boolean(process.env.FANTOM_PROFILE_JS);
 
+/**
+ * Enables address sanitizer (ASAN) build mode for the C++ side.
+ */
+export const enableASAN: boolean = Boolean(process.env.FANTOM_ENABLE_ASAN);
+
+/**
+ * Enables thread sanitizer (TSAN) build mode for the C++ side.
+ */
+export const enableTSAN: boolean = Boolean(process.env.FANTOM_ENABLE_TSAN);
+
 export const enableJSMemoryInstrumentation: boolean = Boolean(
   process.env.FANTOM_ENABLE_JS_MEMORY_INSTRUMENTATION,
 );
diff --git a/private/react-native-fantom/runner/utils.js b/private/react-native-fantom/runner/utils.js
index eefc4d3cb99..1dcdc26774e 100644
--- a/private/react-native-fantom/runner/utils.js
+++ b/private/react-native-fantom/runner/utils.js
@@ -54,7 +54,32 @@ export function getHermesCompilerTarget(variant: HermesVariant): string {
 export function getBuckModesForPlatform(
   enableRelease: boolean = false,
 ): $ReadOnlyArray<string> {
-  const mode = enableRelease ? 'opt' : 'dev';
+  let mode = enableRelease ? 'opt' : 'dev';
+
+  if (enableRelease) {
+    if (EnvironmentOptions.enableASAN || EnvironmentOptions.enableTSAN) {
+      printConsoleLog({
+        type: 'console-log',
+        level: 'warn',
+        message:
+          'ASAN and TSAN are not supported in release mode. Use dev mode instead.',
+      });
+    }
+  } else {
+    if (EnvironmentOptions.enableASAN) {
+      printConsoleLog({
+        type: 'console-log',
+        level: 'warn',
+        message:
+          'ASAN and TSAN modes cannot be used together. Using ASAN mode as a fallback.',
+      });
+      mode = 'dev-asan';
+    } else if (EnvironmentOptions.enableASAN) {
+      mode = 'dev-asan';
+    } else if (EnvironmentOptions.enableTSAN) {
+      mode = 'dev-tsan';
+    }
+  }
 
   let osPlatform;
   switch (os.platform()) {