node:fs: call callbacks from the native completion, not a promise reaction

src/js/internal/fs/binding.ts

@@ -123,4 +123,12 @@ interface FsBinding
   writev(fd: number, buffers: readonly NodeJS.ArrayBufferView[], position?: number | null): Promise<number>;
 }
 
-export default $rust("node_fs_binding.rs", "createBinding") as FsBinding;
+// `xCb(callback, ...args)`: the promise arm's arguments after the callback. `fs.cp` is a JS chain, so no `cpCb`.
+type FsCallbackArm<T> = {
+  [K in Exclude<keyof T & string, "cp"> as T[K] extends (...args: any[]) => Promise<any> ? `${K}Cb` : never]: (
+    callback: (err: any, value?: any) => void,
+    ...args: any[]
+  ) => void;
+};
+
+export default $rust("node_fs_binding.rs", "createBinding") as FsBinding & FsCallbackArm<FsBinding>;

src/js/internal/trace_events.ts

@@ -626,6 +626,8 @@ function installFsInstrumentation() {
   for (const method in asyncOps) {
     const original = binding[method];
     if (typeof original === "function") binding[method] = wrapFsAsyncMethod(original, asyncOps[method]);
+    const originalCb = binding[method + "Cb"];
+    if (typeof originalCb === "function") binding[method + "Cb"] = wrapFsCallbackMethod(originalCb, asyncOps[method]);
   }
   // fs.opendir never reaches the binding (Bun's Dir reads lazily), so wrap the
   // node:fs export for the node.fs_dir.async category.
@@ -690,6 +692,27 @@ function wrapFsAsyncMethod(original, names: string[]) {
   };
 }
 
+// The callback arm of the binding (`xCb(callback, ...args)`): the span ends when the operation calls back.
+function wrapFsCallbackMethod(original, names: string[]) {
+  return function (callback, ...args) {
+    if (suppressFsEvents || !isCategoryGroupEnabled(kFsAsyncCat)) return original.$apply(this, arguments);
+    for (let i = 0; i < names.length; i++) emitEvent("b", kFsAsyncCat, names[i]);
+    try {
+      return original.$call(
+        this,
+        function () {
+          emitFsAsyncEnd(names);
+          return callback.$apply(undefined, arguments);
+        },
+        ...args,
+      );
+    } catch (err) {
+      emitFsAsyncEnd(names);
+      throw err;
+    }
+  };
+}
+
 // The global console is native (not the JS Console class), so wrap its
 // counter/timer methods directly. Counts and timer labels are tracked in
 // parallel maps because the native implementations don't expose theirs;