From 5b4f0c2c1bfbefebc2617958166fcb784b9cb18d Mon Sep 17 00:00:00 2001
From: Nigro Simone <nigro.simone@gmail.com>
Date: Sat, 19 Sep 2026 07:03:46 +0200
Subject: [PATCH] Keep onData, onDataV2 and onWritable callbacks in
 MoveOnlyFunction inline storage

---
 src/HttpResponseWrapper.h | 18 +++++++++---------
 src/Utilities.h           | 11 +++++++++++
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/src/HttpResponseWrapper.h b/src/HttpResponseWrapper.h
index ec326ce48..516fd95b2 100644
--- a/src/HttpResponseWrapper.h
+++ b/src/HttpResponseWrapper.h
@@ -94,8 +94,8 @@ struct HttpResponseWrapper {
         Isolate *isolate = args.GetIsolate();
         auto *res = getHttpResponse<SSL>(args);
         if (res) {
-            /* This thing perfectly fits in with unique_function, and will Reset on destructor */
-            UniquePersistent<Function> p(isolate, Local<Function>::Cast(args[0]));
+            /* Noexcept move keeps the 16 byte capture in MoveOnlyFunction's inline storage */
+            NoexceptPersistent<Function> p(isolate, Local<Function>::Cast(args[0]));
 
             res->onData([p = std::move(p), isolate](std::string_view data, bool last) {
                 HandleScope hs(isolate);
@@ -103,7 +103,7 @@ struct HttpResponseWrapper {
                 Local<ArrayBuffer> dataArrayBuffer = ArrayBuffer_New(isolate, (void *) data.data(), data.length());
 
                 Local<Value> argv[] = {dataArrayBuffer, Boolean::New(isolate, last)};
-                CallJS(isolate, Local<Function>::New(isolate, p), 2, argv);
+                CallJS(isolate, Local<Function>::New(isolate, p.p), 2, argv);
 
                 dataArrayBuffer->Detach();
             });
@@ -197,8 +197,8 @@ struct HttpResponseWrapper {
         Isolate *isolate = args.GetIsolate();
         auto *res = getHttpResponse<SSL>(args);
         if (res) {
-            /* This thing perfectly fits in with unique_function, and will Reset on destructor */
-            UniquePersistent<Function> p(isolate, Local<Function>::Cast(args[0]));
+            /* Noexcept move keeps the 16 byte capture in MoveOnlyFunction's inline storage */
+            NoexceptPersistent<Function> p(isolate, Local<Function>::Cast(args[0]));
 
             res->onDataV2([p = std::move(p), isolate](std::string_view data, uint64_t maxRemainingBodyLength) {
                 HandleScope hs(isolate);
@@ -207,7 +207,7 @@ struct HttpResponseWrapper {
 
                 /* Pass maxRemainingBodyLength so user can preallocate; 0 signals the last chunk */
                 Local<Value> argv[] = {dataArrayBuffer, BigInt::NewFromUnsigned(isolate, maxRemainingBodyLength)};
-                CallJS(isolate, Local<Function>::New(isolate, p), 2, argv);
+                CallJS(isolate, Local<Function>::New(isolate, p.p), 2, argv);
 
                 dataArrayBuffer->Detach();
             });
@@ -341,8 +341,8 @@ struct HttpResponseWrapper {
         Isolate *isolate = args.GetIsolate();
         auto *res = getHttpResponse<SSL>(args);
         if (res) {
-            /* This thing perfectly fits in with unique_function, and will Reset on destructor */
-            UniquePersistent<Function> p(isolate, Local<Function>::Cast(args[0]));
+            /* Noexcept move keeps the 16 byte capture in MoveOnlyFunction's inline storage */
+            NoexceptPersistent<Function> p(isolate, Local<Function>::Cast(args[0]));
 
             res->onWritable([p = std::move(p), isolate](size_t offset) -> bool {
                 HandleScope hs(isolate);
@@ -350,7 +350,7 @@ struct HttpResponseWrapper {
                 Local<Value> argv[] = {Number::New(isolate, offset)};
 
                 /* We should check if this is really here! */
-                MaybeLocal<Value> maybeBoolean = CallJS(isolate, Local<Function>::New(isolate, p), 1, argv);
+                MaybeLocal<Value> maybeBoolean = CallJS(isolate, Local<Function>::New(isolate, p.p), 1, argv);
                 if (maybeBoolean.IsEmpty()) {
                     std::cerr << "Warning: uWS.HttpResponse.onWritable callback should return Boolean. See documentation for uWS.HttpResponse.onWritable and consult the user manual." << std::endl;
                     /* The default should be true, as it only adds a potential extra send, rather than erroneously avoid it */
diff --git a/src/Utilities.h b/src/Utilities.h
index 78ce93845..297a9e2f0 100644
--- a/src/Utilities.h
+++ b/src/Utilities.h
@@ -112,6 +112,17 @@ static inline bool missingArguments(int length, const FunctionCallbackInfo<Value
     return false;
 }
 
+/* v8::Global's move constructor is not noexcept, so a lambda capturing one fails
+ * MoveOnlyFunction's small-object test and heap-allocates. The underlying move is
+ * a pointer swap and cannot throw, this wrapper only restores the noexcept */
+template <class T>
+struct NoexceptPersistent {
+    UniquePersistent<T> p;
+    NoexceptPersistent(Isolate *isolate, const Local<T> &v) : p(isolate, v) {}
+    NoexceptPersistent(NoexceptPersistent &&other) noexcept : p(std::move(other.p)) {}
+};
+static_assert(std::is_nothrow_move_constructible<NoexceptPersistent<Function>>::value, "NoexceptPersistent must be nothrow movable");
+
 struct Callback {
     bool invalid = false;
     UniquePersistent<Function> f;