[berelels]

What

After VK_ERROR_DEVICE_LOST the replay wrote the checkpoint report and kept
running. Nothing it submitted completed again, so captures and staging were
never retired: in #1275 captures kept went from 25 to 6527 and staging from
256 MB to 1408 MB, until system RAM ran out with a black window.

The four VK_ERROR_DEVICE_LOST checks in gpu/vk/vk_replay.cpp now call a new
device_lost(), which writes the report once through std::call_once and then
ends the process through the existing replay_fatal (exit code 3). With
call_once, a second thread that sees the loss at the same time (the #1275 log
shows several) waits for the report instead of ending the process before it is
written.

This does not fix the GPU hang itself in #1275, only what happens after it.

Tests

• tests/test_device_lost_exit.py (new, game-free): every
  VK_ERROR_DEVICE_LOST check must call device_lost(); device_lost() is cut
  into a fixture and called from five threads with a slow report, which must be
  written once before the exit. It fails on the old code path and on a
  check-once flag without call_once.
• tests/test_parallel_init_order.py: its fixture gained a device_lost stub
  (the segment-submit block it cuts now calls it); passes.
• Not tested: a real device lost on hardware, and a full build (only the
  changed function was compiled, in the fixture). Linux, g++.

Question

Do you want the process to stay alive after a device lost in some debugging
setup (e.g. with RX_VK_FAULT_INFO)? If so, I can put the exit behind an
opt-out variable.

Refs #1275

AI assistance

Assisted-by: Claude Opus 5.5
Assisted-by: gpt-oss-120b (LLM council)
Assisted-by: Qwen3.8-27B (LLM council)
Assisted-by: Gemini 3.5 Flash-Lite (LLM council)
Assisted-by: dots-3-note-preview (LLM council)
Assisted-by: Nemotron 3.5 Lightning (LLM council)
Assisted-by: Nemotron 3 Ultra 550B (LLM council)
Assisted-by: Nemotron 3.5 Super VL (LLM council)
Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com

The LLM council (several models answer the same question independently, then
rank each other's answers anonymously) was used to define the approach and the
solution: which problem to take on, how to fix it and what to avoid. It saw a
summary of the issues, not the code. The change and its test were written with
Claude Code, then reviewed and tested by me.

[berelels]

Thanks for the careful read. Fixed in fb879db: MkwVkHost gets log_flush (hos_log_flush, API version 10), and replay_fatal calls it before _Exit; test_device_lost_exit.py checks both. On the report hanging with RX_VK_FAULT_INFO: agreed, I left it, since it only happens with that debug variable. Keeping exit-by-default as you suggest.