Add user exception metadata to internal exception

danlapid · danlapid · commit 08b4a3d517db · 2025-11-04T18:11:15.000Z
diff --git a/src/workerd/jsg/BUILD.bazel b/src/workerd/jsg/BUILD.bazel
@@ -118,6 +118,7 @@ wd_cc_library(
     tags = ["no-clang-tidy"],
     deps = [
         ":exception",
+        ":exception_metadata_capnp",
         ":macro-meta",
         ":memory-tracker",
         ":meta",
@@ -222,6 +223,12 @@ wd_cc_capnp_library(
     visibility = ["//visibility:public"],
 )
 
+wd_cc_capnp_library(
+    name = "exception_metadata_capnp",
+    srcs = ["exception-metadata.capnp"],
+    visibility = ["//visibility:public"],
+)
+
 js_capnp_library(
     name = "rtti_capnp_js",
     srcs = ["rtti.capnp"],
diff --git a/src/workerd/jsg/exception-metadata.capnp b/src/workerd/jsg/exception-metadata.capnp
@@ -0,0 +1,15 @@
+@0xa9ae63464030fcef;
+# Schema for JavaScript exception metadata passed through KJ exception details
+
+using Cxx = import "/capnp/c++.capnp";
+$Cxx.namespace("workerd::jsg");
+$Cxx.allowCancellation;
+
+struct JsExceptionMetadata {
+  # JavaScript error type (e.g., "Error", "TypeError", "RangeError")
+  errorType @0 :Text;
+
+  # Full stack trace string as produced by V8
+  # Example: "Error: User-thrown exception\n    at Object.fetch (foo:4:11)"
+  stackTrace @1 :Text;
+}
diff --git a/src/workerd/jsg/util.c++ b/src/workerd/jsg/util.c++
@@ -7,8 +7,12 @@
 #include "ser.h"
 #include "setup.h"
 
+#include <workerd/jsg/exception-metadata.capnp.h>
+
 #include <openssl/rand.h>
 
+#include <capnp/message.h>
+#include <capnp/serialize.h>
 #include <kj/debug.h>
 
 #include <cstdlib>
@@ -454,6 +458,48 @@ void addExceptionDetail(Lock& js, kj::Exception& exception, v8::Local<v8::Value>
   }
 }
 
+void addJsExceptionMetadata(Lock& js, kj::Exception& exception, v8::Local<v8::Value> handle) {
+  // Extract JavaScript error type and stack trace
+  if (!handle->IsObject()) {
+    return;  // Not an error object, nothing to extract
+  }
+
+  auto errorObj = jsg::JsObject(handle.As<v8::Object>());
+
+  // Build Cap'n Proto message
+  capnp::MallocMessageBuilder message;
+  auto metadata = message.initRoot<JsExceptionMetadata>();
+
+  // Limit for user-controlled fields (4KB)
+  constexpr size_t MAX_FIELD_SIZE = 4096;
+
+  // Extract error name (e.g., "Error", "TypeError", "RangeError")
+  auto nameProp = errorObj.get(js, "name"_kj);
+  if (nameProp.isString()) {
+    auto errorType = nameProp.toString(js);
+    // Truncate to 4KB if needed
+    if (errorType.size() > MAX_FIELD_SIZE) {
+      errorType = kj::str(errorType.slice(0, MAX_FIELD_SIZE));
+    }
+    metadata.setErrorType(errorType);
+  }
+
+  // Extract stack trace string
+  auto stackProp = errorObj.get(js, "stack"_kj);
+  if (stackProp.isString()) {
+    auto stackTrace = stackProp.toString(js);
+    // Truncate to 4KB if needed
+    if (stackTrace.size() > MAX_FIELD_SIZE) {
+      stackTrace = kj::str(stackTrace.slice(0, MAX_FIELD_SIZE));
+    }
+    metadata.setStackTrace(stackTrace);
+  }
+
+  // Serialize to bytes using Cap'n Proto
+  auto words = capnp::messageToFlatArray(message);
+  exception.setDetail(JS_EXCEPTION_METADATA_DETAIL_ID, kj::heapArray(words.asBytes()));
+}
+
 static kj::String typeErrorMessage(TypeErrorContext c, const char* expectedType) {
   kj::String type;
 
diff --git a/src/workerd/jsg/util.h b/src/workerd/jsg/util.h
@@ -89,9 +89,16 @@ void throwInternalError(
 
 constexpr kj::Exception::DetailTypeId TUNNELED_EXCEPTION_DETAIL_ID = 0xe8027292171b1646ull;
 
+// Detail type for JavaScript exception metadata (error type and stack trace)
+constexpr kj::Exception::DetailTypeId JS_EXCEPTION_METADATA_DETAIL_ID = 0xa9ae63464030fcefull;
+
 // Add a serialized copy of the exception value to the KJ exception, as a "detail".
 void addExceptionDetail(Lock& js, kj::Exception& exception, v8::Local<v8::Value> handle);
 
+// Extract and add JavaScript exception metadata (error type and stack trace) to the KJ exception.
+// Serializes using Cap'n Proto schema defined in exception-metadata.capnp.
+void addJsExceptionMetadata(Lock& js, kj::Exception& exception, v8::Local<v8::Value> handle);
+
 struct TypeErrorContext {
   enum Kind : uint8_t {
     METHOD_ARGUMENT,       // has type name, member (method) name, and argument index
diff --git a/src/workerd/jsg/value.h b/src/workerd/jsg/value.h
@@ -1503,6 +1503,7 @@ class ExceptionWrapper {
     }();
 
     addExceptionDetail(js, result, handle);
+    addJsExceptionMetadata(js, result, handle);
     return result;
   }
 };

Original file line number	Diff line number	Diff line change
`@@ -1503,6 +1503,7 @@ class ExceptionWrapper {`
`1503`	`1503`	`}();`
`1504`	`1504`
`1505`	`1505`	`addExceptionDetail(js, result, handle);`
	`1506`	`+ addJsExceptionMetadata(js, result, handle);`
`1506`	`1507`	`return result;`
`1507`	`1508`	`}`
`1508`	`1509`	`};`