postmaster: async-signal-safe frame dump in the crash handler

[brandonros]

`backtrace_symbols_fd()` is not async-signal-safe: on macOS it calls `dladdr()` (dyld locks), and on glibc the first `backtrace()` call `dlopen()`s libgcc_s. A fault inside the allocator or the loader then hangs the handler instead of crashing, so the process never reaches `raise()` and leaves no core file.

This writes raw frame addresses with `write(2)` into a bounds-clamped stack buffer (Linux and macOS) and leaves symbolizing to `atos` / `addr2line`. The lint-determinism allow row grows by the two extra `write(2)` sites.

Split out of the objkv series (#92) as an independent fix; it does not depend on any of it.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved crash diagnostics on Linux and macOS by including raw hexadecimal backtrace addresses from fatal-signal handlers.
  • Prevented long backtraces from being truncated when diagnostic output spans multiple writes.
  • Crash output no longer includes symbolized function names directly; addresses can be symbolized separately.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Team

Run ID: fd23a136-a747-4025-b253-e33697ad7e17

📥 Commits

Reviewing files that changed from the base of the PR and between 4c2e49f and ad103f0.

📒 Files selected for processing (2)

• crates/_support/seams_init/tests/lint-determinism.allow
• crates/backend/postmaster/postmaster/src/crash_signals.rs

Included review availability: Your plan provides up to 8 included reviews per hour; 4 remain after this review.

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📝 Walkthrough

Walkthrough

The fatal-signal handler now emits raw hexadecimal frame addresses on Linux and macOS. It flushes buffered output across multiple writes. The lint configuration allows the additional write calls.

Changes

Crash signal backtrace output

Layer / File(s)	Summary
Raw backtrace emission crates/backend/postmaster/postmaster/src/crash_signals.rs, crates/_support/seams_init/tests/lint-determinism.allow	The handler replaces symbolized output with hexadecimal frame addresses, flushes the Writer when needed, enables macOS support, and increases the filesystem budget.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to ad103

Crash backtraces now emit raw hexadecimal frame addresses on Linux and macOS rather than symbolized frames, avoiding the unsafe symbolization call in the fatal-signal path. No merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (1 skipped: 1 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding an async-signal-safe frame dump to the postmaster crash handler.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (1 skipped: 1 unsupported.)

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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