From 4f89424851dd11003c1abf647e5ac56538fb150d Mon Sep 17 00:00:00 2001
From: James M Snell <jasnell@gmail.com>
Date: Fri, 20 Sep 2024 12:06:08 -0700
Subject: [PATCH] Attempt to log the stack where cross-request promises are
 being resolved

---
 src/workerd/io/worker.c++ | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/src/workerd/io/worker.c++ b/src/workerd/io/worker.c++
index 98c2ae019c4..6214482dfc1 100644
--- a/src/workerd/io/worker.c++
+++ b/src/workerd/io/worker.c++
@@ -1194,6 +1194,35 @@ Worker::Isolate::Isolate(kj::Own<Api> apiParam,
                   v8::Local<v8::Value> argument)> callback) -> v8::Maybe<void> {
         try {
           auto& js = jsg::Lock::from(isolate);
+
+          {
+            // TODO(soon): Hopefully this logging is temporary. We want to get an idea
+            // of where the cross request promises are being resolved just in general.
+            // To do so we need to try to capture a stack. We do so by creating an error
+            // object and logging it.
+            v8::HandleScope handleScope(isolate);
+            auto err =
+                v8::Exception::Error(js.str("Cross Request Promise Resolve"_kj)).As<v8::Object>();
+            jsg::check(err->Set(js.v8Context(), js.str("name"_kj), js.str("Warning"_kj)));
+            auto stack = jsg::check(err->Get(js.v8Context(), js.str("stack"_kj)));
+            // Let's only log if there's actually a useful stack in the log.
+            auto msg = kj::str("NOSENTRY ", stack);
+            // This check is meant to prevent spamming the logs with messages that do
+            // not actually provide any useful information.
+            if (msg != "NOSENTRY Warning: Cross Request Promise Resolve") {
+              KJ_LOG(WARNING, kj::str("NOSENTRY ", stack));
+            } else {
+              // If we get here, it means we don't have a useful JS stack trace.
+              // Either the stack trace limit was set to zero for some reason or
+              // this resolve/reject originated from inside the runtime where a
+              // JS stack trace is not available. Let's emit the warning with a
+              // C++ stack instead.
+              // TODO(review): Is this worthwhile? Does it give us enough useful signal?
+              LOG_PERIODICALLY(WARNING, "NOSENTRY Warning: Cross Request Promise Resolve\n",
+                  kj::getStackTrace());
+            }
+          }
+
           // The promise tag is generally opaque except for right here. The tag
           // wraps an instanceof kj::Own<IoCrossContextExecutor>, which wraps an atomically
           // refcounted pointer to the DeleteQueue for the correct isolate.