otel-thread-ctx: publish V8 + ObjectWrap layout constants for readers

bindings/otel-thread-ctx.cc

@@ -679,6 +679,28 @@ constexpr int WRAPPED_OBJECT_OFFSET = 0;
 #endif
 constexpr int TAGGED_SIZE = v8::internal::kApiTaggedSize;
 
+// sizeof(node::ObjectWrap). Given a pointer to a CtxWrap — or any other
+// ObjectWrap-derived C++ object attached to a JSObject via the V8
+// wrapped-object slot — add this offset to reach the derived class's own
+// fields. For CtxWrap, that's `record_` (see the static_assert on its
+// offset above).
+constexpr int NATIVE_WRAP_FIELDS_OFFSET =
+    static_cast<int>(sizeof(node::ObjectWrap));
+
+// V8 JSMap layout: kTableOffset within the JSMap object holds a tagged
+// pointer to the backing OrderedHashMap table. Not exposed in V8's
+// public headers; kept in sync with
+// deps/v8/src/objects/js-collection.h (JSCollection::kTableOffset)
+// and the torque-generated JSCollection layout.
+constexpr int JS_MAP_TABLE_OFFSET = 0x18;
+
+// V8 OrderedHashMap layout: the on-heap table starts with a 16-byte
+// header before the element_count / deleted_element_count /
+// number_of_buckets fields. Not exposed in V8's public headers; kept in
+// sync with deps/v8/src/objects/ordered-hash-table.h
+// (OrderedHashTable base layout).
+constexpr int ORDERED_HASH_MAP_HEADER_SIZE = 0x10;
+
 }  // namespace
 
 void OtelThreadCtx::Init(Local<Object> exports) {
@@ -688,17 +710,19 @@ void OtelThreadCtx::Init(Local<Object> exports) {
 
   Isolate* isolate = Isolate::GetCurrent();
   Local<Context> ctx = isolate->GetCurrentContext();
-  exports
-      ->Set(ctx,
-            String::NewFromUtf8Literal(isolate,
-                                       "otelThreadCtxWrappedObjectOffset"),
-            Integer::New(isolate, WRAPPED_OBJECT_OFFSET))
-      .FromJust();
-  exports
-      ->Set(ctx,
-            String::NewFromUtf8Literal(isolate, "otelThreadCtxTaggedSize"),
-            Integer::New(isolate, TAGGED_SIZE))
-      .FromJust();
+  auto publish_int = [&](const char* name, int value) {
+    exports
+        ->Set(ctx,
+              String::NewFromUtf8(isolate, name).ToLocalChecked(),
+              Integer::New(isolate, value))
+        .FromJust();
+  };
+  publish_int("otelThreadCtxJsMapTableOffset", JS_MAP_TABLE_OFFSET);
+  publish_int("otelThreadCtxNativeWrapFieldsOffset", NATIVE_WRAP_FIELDS_OFFSET);
+  publish_int("otelThreadCtxOrderedHashMapHeaderSize",
+              ORDERED_HASH_MAP_HEADER_SIZE);
+  publish_int("otelThreadCtxTaggedSize", TAGGED_SIZE);
+  publish_int("otelThreadCtxWrappedObjectOffset", WRAPPED_OBJECT_OFFSET);
 }
 
 }  // namespace dd

ts/src/otel-thread-ctx.ts

@@ -40,6 +40,9 @@ export interface ProcessContextAttributes {
   readonly 'threadlocal.attribute_key_map': readonly string[];
   readonly 'threadlocal.wrapped_object_offset': number;
   readonly 'threadlocal.tagged_size': number;
+  readonly 'threadlocal.native_wrap_fields_offset': number;
+  readonly 'threadlocal.js_map_table_offset': number;
+  readonly 'threadlocal.ordered_hash_map_header_size': number;
 }
 
 /**
@@ -103,6 +106,9 @@ interface Addon {
   otelThreadCtxGetStoredAlsHash(): number;
   otelThreadCtxWrappedObjectOffset: number;
   otelThreadCtxTaggedSize: number;
+  otelThreadCtxNativeWrapFieldsOffset: number;
+  otelThreadCtxJsMapTableOffset: number;
+  otelThreadCtxOrderedHashMapHeaderSize: number;
 }
 
 const SCHEMA_VERSION = 'nodejs_v1_dev';
@@ -114,6 +120,9 @@ const SCHEMA_VERSION = 'nodejs_v1_dev';
 // consistent in shape.
 let WRAPPED_OBJECT_OFFSET = 24;
 let TAGGED_SIZE = 8;
+let NATIVE_WRAP_FIELDS_OFFSET = 24;
+let JS_MAP_TABLE_OFFSET = 0x18;
+let ORDERED_HASH_MAP_HEADER_SIZE = 0x10;
 
 /** {@inheritDoc ThreadContextCtor} */
 export let ThreadContext: ThreadContextCtor;
@@ -140,6 +149,9 @@ if (process.platform === 'linux') {
   const addon: Addon = findBinding(join(__dirname, '..', '..'));
   WRAPPED_OBJECT_OFFSET = addon.otelThreadCtxWrappedObjectOffset;
   TAGGED_SIZE = addon.otelThreadCtxTaggedSize;
+  NATIVE_WRAP_FIELDS_OFFSET = addon.otelThreadCtxNativeWrapFieldsOffset;
+  JS_MAP_TABLE_OFFSET = addon.otelThreadCtxJsMapTableOffset;
+  ORDERED_HASH_MAP_HEADER_SIZE = addon.otelThreadCtxOrderedHashMapHeaderSize;
 
   ThreadContext = addon.threadContext;
 
@@ -266,5 +278,8 @@ export function getProcessContextAttributes(
     'threadlocal.attribute_key_map': Object.freeze(keys.slice()),
     'threadlocal.wrapped_object_offset': WRAPPED_OBJECT_OFFSET,
     'threadlocal.tagged_size': TAGGED_SIZE,
+    'threadlocal.native_wrap_fields_offset': NATIVE_WRAP_FIELDS_OFFSET,
+    'threadlocal.js_map_table_offset': JS_MAP_TABLE_OFFSET,
+    'threadlocal.ordered_hash_map_header_size': ORDERED_HASH_MAP_HEADER_SIZE,
   }) as ProcessContextAttributes;
 }

ts/test/test-otel-thread-ctx.ts

@@ -733,8 +733,17 @@ function captureBytes(opts: {
         strictAssert.deepEqual(pca['threadlocal.attribute_key_map'], keys);
         strictAssert.equal(pca['threadlocal.wrapped_object_offset'], 24);
         strictAssert.equal(pca['threadlocal.tagged_size'], 8);
+        strictAssert.equal(pca['threadlocal.native_wrap_fields_offset'], 24);
+        strictAssert.equal(pca['threadlocal.js_map_table_offset'], 0x18);
+        strictAssert.equal(
+          pca['threadlocal.ordered_hash_map_header_size'],
+          0x10,
+        );
         strictAssert.deepEqual(Object.keys(pca).sort(), [
           'threadlocal.attribute_key_map',
+          'threadlocal.js_map_table_offset',
+          'threadlocal.native_wrap_fields_offset',
+          'threadlocal.ordered_hash_map_header_size',
           'threadlocal.schema_version',
           'threadlocal.tagged_size',
           'threadlocal.wrapped_object_offset',