How can we reproduce the crash?

This issue arose while consuming the Webview foreign library.

Webview opens a native OS browser window (webview), blocking the calling (Worker) thread until terminated. In order to interact with the Webview API after a browser window is opened, calls need to be made from the parent thread.

The bun minimal reproduction illustrates how Webview API commands using JSCallback work from the worker thread, but fail from the parent thread.

Observations:

• The Deno minimal reproduction works as expected for all Webview API commands from both threads.
• In Deno, the minimal reproduction works as expected using "node:worker_threads"
• The Bun minimal reproduction fails with native Worker and "node:worker_threads"
• Webview FFI commands not involving JSCallback work as expected from the parent thread.

I have thus formed an opinion that the issue does not lie with Bun Worker, but specifically with Bun FFI JSCallback.

In order to run the minimal reproduction (for linux x64) , some OS dependencies are required.
minimal.repro.zip contains all of the minimal reproduction code, as well as "libwebview.so" (the Webview library for linux x64)

Relevant log output

Overriding existing handler for signal 10. Set JSC_SIGNAL_FOR_GC if you want WebKit to use a different signal
============================================================
Bun v1.1.39 (1d485617) Linux x64
Linux Kernel v6.1.0 | glibc v2.36
CPU: sse42 popcnt avx avx2
Args: "bun" "./index.ts"
Features: jsc tsconfig workers_spawned 
Builtins: "bun:ffi" "bun:main" "node:path" 
Elapsed: 1229ms | User: 152ms | Sys: 114ms
RSS: 1.07GB | Peak: 0.16GB | Commit: 1.07GB | Faults: 0

panic(main thread): Segmentation fault at address 0x40
oh no: Bun has crashed. This indicates a bug in Bun, not your code.

To send a redacted crash report to Bun's team,
please file a GitHub issue using the link below:

 https://bun.report/1.1.39/la11d48561Ihgg8//DoqqomEk0/pvB__A2AgE

Illegal instruction

Stack Trace (bun.report)

Bun v1.1.39 (1d48561) on linux x86_64 [AutoCommand]

Segmentation fault at address 0x00000040

• ld-temp.o:0: icudt75_dat

Features: workers_spawned, jsc, tsconfig, tsconfig