296261 2025-07-21 00:29:54 -0700 [JSC] Add async stack traces behind the flag 2025-09-04 17:44:30 -0700 1 1 1 Unclassified WebKit JavaScriptCore WebKit Nightly Build Unspecified Unspecified RESOLVED FIXED InRadar P2 Normal --- 1 sosuke sosuke webkit-bug-importer oldest_to_newest 2131525 0 sosuke 2025-07-21 00:29:54 -0700 JSC cannot obtain sufficient async stack traces. For example, this code: async function one(x) { await two(x); } async function two(x) { await x; throw new Error("error from two"); } one(1).catch((err) => { print(err.stack); }); prints this stack trace: two@./test.js:6:20 When an error occurs within an async function, the stack trace is cut off at the most recent await point, so higher-level functions in the async call chain are not displayed. This happens because when an async function is suspended at an await statement and later resumed from the microtask queue, the original call stack is lost. While this behavior is correct from the semantics of call stacks and microtask queues, it is inconvenient for users. This patch builds async stack traces through the following approach (only when the --useAsyncStackTrace flag is enabled): - Detect calls to `@asyncFunctionResume` during stack trace construction and retrieve the JSGenerator object hidden behind async-await from its arguments - Obtain references to awaited Promises held in each generator's Context field - Trace AsyncContext (parent generators recorded as @context fields) from the Promise's reaction chain - Walk up the chain of parent generators and construct stack frames from function information stored in each async function's Next field After this patch, the previous code outputs this stack trace: two@./WebKitBuild/Debug/test.js:6:20 one@./WebKitBuild/Debug/test.js:2:14 V8 constructs async stack traces using a similar approach [1][2]. [1]: https://docs.google.com/document/d/13Sy_kBIJGP0XT34V1CV3nkWya4TwYx9L3Yv45LdGB6Q/edit?tab=t.0#heading=h.9 ss45aibqpw2 [2]: https://issues.chromium.org/issues/42210758 2131526 1 sosuke 2025-07-21 00:31:06 -0700 Pull request: https://github.com/WebKit/WebKit/pull/48314 2132799 2 webkit-bug-importer 2025-07-28 00:30:36 -0700 <rdar://problem/156890795> 2140502 3 ews-feeder 2025-09-02 21:07:41 -0700 Committed 299482@main (f8a073dfbecf): <https://commits.webkit.org/299482@main> Reviewed commits have been landed. Closing PR #48314 and removing active labels. 2140857 4 sosuke 2025-09-04 00:22:13 -0700 Re-opening for pull request https://github.com/WebKit/WebKit/pull/50290 2141095 5 ews-feeder 2025-09-04 17:43:38 -0700 Committed 299595@main (74c38785fa38): <https://commits.webkit.org/299595@main> Reviewed commits have been landed. Closing PR #50290 and removing active labels.