### What does this PR do?
Fixes the crash handler failing to capture and display stack traces on
Linux ARM64 systems.
**Before:**
```
============================================================
panic(main thread): cast causes pointer to be null
```
No stack trace shown.
**After:**
```
============================================================
panic(main thread): cast causes pointer to be null
bun.js.api.FFIObject.Reader.u8
/workspace/bun/src/bun.js/api/FFIObject.zig:67:41
bun.js.jsc.host_fn.toJSHostCall__anon_2545765
/workspace/bun/src/bun.js/jsc/host_fn.zig:93:5
```
Full stack trace with source locations.
#### Root Cause
- Zig's `std.debug.captureStackTrace` uses `StackIterator.init()` which
falls back to frame pointer-based unwinding when no context is provided
- Frame pointer-based unwinding doesn't work reliably on ARM64, even
with `-fno-omit-frame-pointer` enabled
- This resulted in 0 frames being captured (`trace.index == 0`)
#### Changes
1. **Use glibc's backtrace() on Linux**: On Linux with glibc (not musl),
always use glibc's `backtrace()` function instead of Zig's
StackIterator. glibc's implementation properly uses DWARF unwinding
information from `.eh_frame` sections.
2. **Skip crash handler frames**: After capturing with `backtrace()`,
find the desired `begin_addr` in the trace (within 128 byte tolerance)
and filter out crash handler internal frames for cleaner output. If
`begin_addr` is not found, use the complete backtrace.
3. **Preserve existing behavior**:
- Non-debug builds: Use WTF printer (fast, no external deps)
- Debug builds: Fall through to llvm-symbolizer (detailed source info)
### How did you verify your code works?
Reproduced the crash:
```bash
bun-debug --print 'Bun.FFI.read.u8(0)'
```
Verified that:
- ✅ Stack traces now appear on Linux ARM64 with proper source locations
- ✅ Crash handler frames are properly filtered out
- ✅ llvm-symbolizer integration works for debug builds
- ✅ WTF printer is used for release builds
- ✅ When begin_addr is not found, complete backtrace is used
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
f00e1816ef
