Use NSData for the JS->ObjC ArrayBuffer argument path

packages/react-native-codegen/src/generators/modules/GenerateModuleObjCpp/serializeMethod.js

@@ -221,7 +221,7 @@ function getParamObjCType(
       return notStruct(wrapOptional('NSArray *', !nullable));
     }
     case 'ArrayBufferTypeAnnotation': {
-      return notStruct(wrapOptional('NSMutableData *', !nullable));
+      return notStruct(wrapOptional('NSData *', !nullable));
     }
   }
 

packages/react-native-codegen/src/generators/modules/__tests__/__snapshots__/GenerateModuleHObjCpp-test.js.snap

@@ -103,8 +103,8 @@ Map {
 @protocol NativeSampleTurboModuleSpec <RCTBridgeModule, RCTTurboModule>
 
 - (NSMutableData *)getArrayBuffer;
-- (void)voidArrayBuffer:(NSMutableData *)arg;
-- (void)voidNullableArrayBuffer:(NSMutableData * _Nullable)arg;
+- (void)voidArrayBuffer:(NSData *)arg;
+- (void)voidNullableArrayBuffer:(NSData * _Nullable)arg;
 - (void)promiseArrayBuffer:(RCTPromiseResolveBlock)resolve
                     reject:(RCTPromiseRejectBlock)reject;
 

packages/react-native/ReactCommon/react/nativemodule/core/iostests/RCTTurboModuleTests.mm

@@ -123,12 +123,11 @@ - (void)testInvokeTurboModuleWithNull
   OCMVerify(OCMTimes(1), [instance_ testMethodWhichTakesObject:nil]);
 }
 
-// A JS ArrayBuffer converts to an NSMutableData that owns an independent copy of
-// the bytes — NSMutableData cannot alias a foreign buffer, so the result stays
-// valid and is safe to mutate after the source buffer is gone. This covers the
-// ArrayBuffer-backed-by-native-MutableBuffer case, which is the one that could in
-// principle have been aliased zero-copy.
-- (void)testArrayBufferConvertsToIndependentNSMutableData
+// A JS ArrayBuffer converts (on the argument path) to an immutable NSData that
+// owns an independent copy of the bytes, so the result stays valid after the
+// source buffer is gone. This covers the ArrayBuffer-backed-by-native-MutableBuffer
+// case, which is the one that could in principle have been aliased zero-copy.
+- (void)testArrayBufferConvertsToIndependentNSData
 {
   constexpr size_t kBufferSize = 64 * 1024;
 
@@ -142,15 +141,15 @@ - (void)testArrayBufferConvertsToIndependentNSMutableData
   id converted =
       TurboModuleConvertUtils::convertJSIValueToObjCObject(*rt, facebook::jsi::Value(*rt, arrayBuffer), nullptr);
 
-  XCTAssertTrue([converted isKindOfClass:[NSMutableData class]]);
-  NSMutableData *data = (NSMutableData *)converted;
+  XCTAssertTrue([converted isKindOfClass:[NSData class]]);
+  NSData *data = (NSData *)converted;
   XCTAssertEqual(data.length, (NSUInteger)kBufferSize);
   XCTAssertEqual(*static_cast<const uint8_t *>(data.bytes), 0xAB);
 
-  // Independent copy: mutating the NSMutableData must not write through to the
-  // source MutableBuffer.
-  *static_cast<uint8_t *>(data.mutableBytes) = 0xCD;
-  XCTAssertEqual(*buffer->data(), 0xAB, @"NSMutableData must not alias the source buffer");
+  // Independent copy: mutating the source MutableBuffer must not write through to
+  // the NSData.
+  *buffer->data() = 0xCD;
+  XCTAssertEqual(*static_cast<const uint8_t *>(data.bytes), 0xAB, @"NSData must not alias the source buffer");
 }
 
 @end

packages/react-native/ReactCommon/react/nativemodule/core/platform/ios/ReactCommon/RCTTurboModule.mm

@@ -199,16 +199,15 @@ size_t size() const override
   };
 }
 
-// Copy the ArrayBuffer's bytes into an NSMutableData. A zero-copy wrap is not
-// possible here: NSMutableData needs its own resizable backing store and cannot
-// alias a foreign buffer (even via initWithBytesNoCopy:length:deallocator:, which
-// copies eagerly for the mutable subclass). Copying also makes the result safe to
-// retain in a block, store, or dispatch to another thread, regardless of whether
-// the bytes were owned by JS (valid only for this callstack) or by a native
-// MutableBuffer (which the JS ArrayBuffer may GC concurrently).
-static NSMutableData *convertJSIArrayBufferToNSMutableData(jsi::Runtime &rt, const jsi::ArrayBuffer &value)
+// Copy the ArrayBuffer's bytes into an immutable NSData. An inbound buffer is
+// owned by the caller, not the native module, so NSData (not NSMutableData) is
+// the correct read-only contract. Copying makes the NSData self-contained and
+// safe to retain in a block, store, or dispatch to another thread, regardless of
+// whether the bytes were owned by JS (valid only for this callstack) or by a
+// native MutableBuffer (which the JS ArrayBuffer may GC concurrently).
+static NSData *convertJSIArrayBufferToNSData(jsi::Runtime &rt, const jsi::ArrayBuffer &value)
 {
-  return [NSMutableData dataWithBytes:value.data(rt) length:value.size(rt)];
+  return [NSData dataWithBytes:value.data(rt) length:value.size(rt)];
 }
 
 id convertJSIValueToObjCObject(
@@ -241,7 +240,7 @@ id convertJSIValueToObjCObject(
       return convertJSIFunctionToCallback(runtime, o.getFunction(runtime), jsInvoker);
     }
     if (o.isArrayBuffer(runtime)) {
-      return convertJSIArrayBufferToNSMutableData(runtime, o.getArrayBuffer(runtime));
+      return convertJSIArrayBufferToNSData(runtime, o.getArrayBuffer(runtime));
     }
     return convertJSIObjectToNSDictionary(runtime, o, jsInvoker, useNSNull);
   }

packages/rn-tester/RNTesterUnitTests/RCTTurboModuleArrayBufferTests.mm

@@ -105,16 +105,17 @@ @implementation RCTTestArrayBufferTurboModule
 
 RCT_EXPORT_MODULE()
 
-RCT_EXPORT_SYNCHRONOUS_TYPED_METHOD(NSMutableData *, testMethodWhichMutatesArrayBuffer : (NSMutableData *)buffer)
+RCT_EXPORT_SYNCHRONOUS_TYPED_METHOD(NSMutableData *, testMethodWhichTransformsArrayBuffer : (NSData *)buffer)
 {
-  auto *bytes = static_cast<uint8_t *>(buffer.mutableBytes);
-  for (NSUInteger i = 0; i < buffer.length; ++i) {
+  NSMutableData *result = [buffer mutableCopy];
+  auto *bytes = static_cast<uint8_t *>(result.mutableBytes);
+  for (NSUInteger i = 0; i < result.length; ++i) {
     bytes[i] = static_cast<uint8_t>((i + 1) * 10);
   }
-  return buffer;
+  return result;
 }
 
-RCT_EXPORT_METHOD(testMethodWhichStoresArrayBuffer : (NSMutableData *)payload)
+RCT_EXPORT_METHOD(testMethodWhichStoresArrayBuffer : (NSData *)payload)
 {
   self.lastReceivedPayload = [payload copy];
 }
@@ -169,8 +170,8 @@ - (void)testSyncArrayBufferRoundTrip
   auto result = module.invokeObjCMethod(
       *rt,
       ArrayBufferKind,
-      "testMethodWhichMutatesArrayBuffer",
-      @selector(testMethodWhichMutatesArrayBuffer:),
+      "testMethodWhichTransformsArrayBuffer",
+      @selector(testMethodWhichTransformsArrayBuffer:),
       args,
       1);