From 4abb9e38b211439b1a21b46d065cea889ae11748 Mon Sep 17 00:00:00 2001
From: robobun <robobun@oven.sh>
Date: Thu, 16 Jul 2026 20:29:03 +0000
Subject: [PATCH] Pass the error instance to the stack string callback

ErrorInstance::finalizeUnconditionally computes and caches the stack
string through VM::onComputeErrorInfo, which does not receive the
ErrorInstance, so the embedder cannot include the error's name and
message in the cached string. A later .stack access serves that cached
string and the "Name: message" head line is lost.

Add an optional VM::onComputeErrorInfoWithInstance callback that also
receives the instance and is preferred over onComputeErrorInfo when
set. The existing callback keeps its signature and behavior, so current
embedders are unaffected until they opt in.

Needed for oven-sh/bun#34398.
---
 Source/JavaScriptCore/runtime/ErrorInstance.cpp | 11 ++++++++++-
 Source/JavaScriptCore/runtime/VM.h              |  6 ++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/Source/JavaScriptCore/runtime/ErrorInstance.cpp b/Source/JavaScriptCore/runtime/ErrorInstance.cpp
index 8cb916db573db..60d388797d614 100644
--- a/Source/JavaScriptCore/runtime/ErrorInstance.cpp
+++ b/Source/JavaScriptCore/runtime/ErrorInstance.cpp
@@ -372,9 +372,18 @@ void ErrorInstance::computeErrorInfo(VM& vm, bool allocationAllowed)
     UNUSED_PARAM(allocationAllowed);
 
     if (m_stackTrace && !m_stackTrace->isEmpty()) {
+        auto& fnWithInstance = vm.onComputeErrorInfoWithInstance();
         auto& fn = vm.onComputeErrorInfo();
         WTF::String stackString;
-        if (fn) {
+        if (fnWithInstance) {
+            // Preferred over onComputeErrorInfo: passing the instance lets the
+            // callback include the error's name and message in the stack string
+            // even when this runs from the GC finalizer (finalizeUnconditionally).
+            if (m_stackPropertyAlreadyMaterialized)
+                stackString = emptyString();
+            else
+                stackString = fnWithInstance(vm, *m_stackTrace.get(), m_lineColumn.line, m_lineColumn.column, m_sourceURL, this, this->bunErrorData());
+        } else if (fn) {
             if (m_stackPropertyAlreadyMaterialized)
                 stackString = emptyString();
             else
diff --git a/Source/JavaScriptCore/runtime/VM.h b/Source/JavaScriptCore/runtime/VM.h
index 6df90ecf4b5b8..b0c8fac58bfbc 100644
--- a/Source/JavaScriptCore/runtime/VM.h
+++ b/Source/JavaScriptCore/runtime/VM.h
@@ -170,6 +170,7 @@ constexpr bool validateDFGDoesGC = ENABLE_DFG_DOES_GC_VALIDATION;
 #if USE(BUN_JSC_ADDITIONS)
 using StackTraceAppenderFunction = WTF::Function<void(VM&, JSCell* owner, Vector<StackFrame>& stackTrace, size_t maxToAppend)>;
 using ErrorInfoFunction = WTF::Function<String(VM&, Vector<StackFrame>& stackTrace, unsigned& line, unsigned& column, String& sourceURL, void* bunErrorData)>;
+using ErrorInfoWithInstanceFunction = WTF::Function<String(VM&, Vector<StackFrame>& stackTrace, unsigned& line, unsigned& column, String& sourceURL, JSC::JSObject* errorInstance, void* bunErrorData)>;
 using ErrorInfoFunctionJSValue = WTF::Function<JSValue(VM&, Vector<StackFrame>& stackTrace, unsigned& line, unsigned& column, String& sourceURL, JSC::JSObject*, void* bunErrorData)>;
 #endif
 
@@ -1062,6 +1063,9 @@ class VM : public ThreadSafeRefCountedWithSuppressingSaferCPPChecking<VM> {
     const ErrorInfoFunction& onComputeErrorInfo() const { return m_onComputeErrorInfo; }
     ErrorInfoFunction& onComputeErrorInfo() { return m_onComputeErrorInfo; }
     
+    const ErrorInfoWithInstanceFunction& onComputeErrorInfoWithInstance() const { return m_onComputeErrorInfoWithInstance; }
+    ErrorInfoWithInstanceFunction& onComputeErrorInfoWithInstance() { return m_onComputeErrorInfoWithInstance; }
+    
     const ErrorInfoFunctionJSValue& onComputeErrorInfoJSValue() const { return m_onComputeErrorInfoJSValue; }
     ErrorInfoFunctionJSValue& onComputeErrorInfoJSValue() { return m_onComputeErrorInfoJSValue; }
     
@@ -1070,6 +1074,7 @@ class VM : public ThreadSafeRefCountedWithSuppressingSaferCPPChecking<VM> {
 
     void setOnAppendStackTrace(StackTraceAppenderFunction&& function) { m_onAppendStackTrace = WTF::move(function); }
     void setOnComputeErrorInfo(ErrorInfoFunction&& function) { m_onComputeErrorInfo = WTF::move(function); }
+    void setOnComputeErrorInfoWithInstance(ErrorInfoWithInstanceFunction&& function) { m_onComputeErrorInfoWithInstance = WTF::move(function); }
     void setOnComputeErrorInfoJSValue(ErrorInfoFunctionJSValue&& function) { m_onComputeErrorInfoJSValue = WTF::move(function); }
     void setComputeLineColumnWithSourcemap(WTF::Function<void(VM&, SourceProvider*, LineColumn&, String&)>&& function) { m_computeLineColumnWithSourcemap = WTF::move(function); }
 #endif
@@ -1287,6 +1292,7 @@ class VM : public ThreadSafeRefCountedWithSuppressingSaferCPPChecking<VM> {
     WTF::Function<void(VM&)> m_onEachMicrotaskTick;
 #if USE(BUN_JSC_ADDITIONS)
     ErrorInfoFunction m_onComputeErrorInfo;
+    ErrorInfoWithInstanceFunction m_onComputeErrorInfoWithInstance;
     ErrorInfoFunctionJSValue m_onComputeErrorInfoJSValue;
     StackTraceAppenderFunction m_onAppendStackTrace;
     WTF::Function<void(VM&, SourceProvider*, LineColumn&, String&)> m_computeLineColumnWithSourcemap;