Native crash (SIGTRAP) in fff-scan thread during watcher resubscribe after scan — FileItem::write_absolute_path overflows fixed 1024-byte buffer (macOS arm64)

Summary

opencode crashes with a native Rust panic in the bundled fff file-indexer during a scan, killing the whole process with no error in opencode.log.

• opencode: 1.18.18 (latest at time of filing)
• Platform: macOS 26.5.2, Apple Silicon (arm64)
• Crash reports: ~/Library/Logs/DiagnosticReports/opencode-2026-08-18-210957.ips and opencode-2026-08-18-222412.ips — two crashes ~75 min apart, byte-identical faulting stack.

Crash signature

EXC_BREAKPOINT / SIGTRAP
faultingThread: 33, thread name: "fff-scan"

fff_search::types::FileItem::write_absolute_path::h6be4aaca40dafd85
fff_search::scan::rescubscribe_watcher_post_scan::hd9b19e5fc8d2229e
fff_search::scan::ScanJob::run::hb82750f8c22d9326
std::sys::backtrace::__rust_begin_short_backtrace
core::ops::function::FnOnce::call_once{{vtable.shim}}
std::sys::thread::Thread::new::thread_start

What was happening

Both crashes occurred during long agentic sessions in a git repo with multiple worktrees (jot-brain, jot-brain-wt119, jot-brain-wtsec). The app log shows the session running a rescan loop and stopping abruptly mid-step with no error line:

...loop session.id=ses_... step=243
[trace/breakpoint trap]  ← process dies here, nothing logged

Root cause hypothesis (from fff source)

The panic is in FileItem::write_absolute_path (fff-core/src/types.rs), which writes base_path + '/' + relative_path into a fixed-size stack buffer:

#[cfg(not(target_os = "windows"))]
pub const PATH_BUF_SIZE: usize = libc::PATH_MAX as usize;   // 1024 on macOS

pub(crate) fn write_absolute_path<'a>(&self, arena, base_path, buf: &'a mut [u8; PATH_BUF_SIZE]) {
    let base_len = base.len();
    buf[..base_len].copy_from_slice(base);          // panics if base_len > 1024
    ...
    buf[base_len] = std::path::MAIN_SEPARATOR as u8; // panics if base_len == 1024
    ...
}

There is no bounds check. During ScanJob::run → rescubscribe_watcher_post_scan (fff-core/src/scan.rs), the watcher is re-subscribed to every indexed directory; any directory path ≥ PATH_BUF_SIZE (1024 on macOS) causes a slice index out of range Rust panic.

Rust panics in the native lib abort the process via an int3 trap → recorded by macOS as EXC_BREAKPOINT/SIGTRAP, and bypass the opencode JS logger, so the app log just ends mid-loop with no error. The two identical .ips files confirm a deterministic trigger rather than two unrelated incidents.

Related

• #42557 — fff watcher / libgit2 busy-loop (same fff code area)
• #32511, #35791, #37740 — fff over-indexing / watch explosion
• #31682, #31747, #31701, #31850 — earlier (closed) fff scan crashes in 1.17.x

Ask

• Bounds-check the path write in write_absolute_path (fall back to heap PathBuf when PATH_BUF_SIZE is exceeded).
• Consider raising PATH_BUF_SIZE / not truncating silently.

Crash reports available on request.