SIGSEGV in continuous batching when a streaming client disconnects mid-generation (block_manager.hpp:633 assertion, GPU, 2026.2.1)

Summary

A client that hard-closes its HTTP connection during a streaming
/v3/chat/completions generation reliably crashes the whole OVMS process within
seconds on Intel Arc GPUs. All other in-flight generations in the batch die with
it. Reproduced on demand; also observed 43× in 24 h under an agent fleet whose
callers time out and disconnect.

Environment

• Image: docker.io/openvino/model_server:2026.2.1-gpu (stock, unmodified)
• GPU: Intel Arc Pro B60 (xe driver) and Arc B580 — both affected, VM passthrough
• Model: Qwen3.6-27B INT4 OpenVINO IR (also seen with a 9B on the second card)
• Serving args:
  --task=text_generation --target_device=GPU --plugin_config='{"ENABLE_CPU_PINNING":false}' --tool_parser qwen3coder --enable_tool_guided_generation true --reasoning_parser qwen3 --kv_cache_precision u8 --cache_size 4 --enable_prefix_caching true --cache_interval_multiplier 64 --max_num_seqs 16 --max_num_batched_tokens 4096

Reproducer (deterministic)

1. POST a streaming chat completion ("stream": true, long generation).
2. Read a few SSE chunks.
3. Hard-close the client socket mid-stream (socket.close(), no graceful shutdown).
4. Within ~90 s the server logs the assertion below and the process exits.

Observed on the very first attempt; kernel gained exactly one segfault entry.

Logs

[llm_executor][error][llm_executor.hpp:132] Error occurred in LLM executor:
Check 'm_block_table.count(seq_id) > 0' failed at
../../../../../repos/openvino.genai/src/cpp/src/continuous_batching/cache/block_manager.hpp:633

Kernel (55+ occurrences over days, all error 4 read faults):

ovms[…]: segfault at … ip …6f3 … error 4 in ovms
libstdc++.so.6.0.33[…] from mediapipe/NNNNN threads

Symbolized against the shipped binary (BuildID a1b35868566ed20139449c9674ff41b1104ec73c):
the faulting IP lands in spdlog::logger::sink_it_+0x43 — i.e. the process dies
while logging, consistent with the executor thread's error path racing the
request-drop path that has already freed the sequence's block table entry.

Impact

• --max_num_seqs 16 means one disconnected client kills up to 15 innocent
  concurrent generations (All requests: 3; Scheduled requests: 3 logged at death).
• On a GPU deployment the recompile/reload takes 1–6 minutes per crash.
• Any latency spike becomes self-amplifying: slow generations → client timeouts →
  disconnects → crash → colder cache → slower generations.

Notes

• The xe "CAT error → engine reset" events sometimes seen after the crash FOLLOW
  the segfault by 300–460 ms and are absent for many crashes — downstream cleanup,
  not the cause.
• Not OOM (memory.events oom_kill 0), not concurrency-proportional (a second
  card at 2.3× the concurrent load crashes 51× less per stream-second — the
  discriminating variable is the mid-stream disconnect rate).
• 2026.1 release notes mention cancellation of requests from disconnected clients
  (NPU); this looks like the GPU continuous-batching cancellation path freeing
  seq_id from m_block_table while the executor still steps the sequence.

Start with openvino.genai/src/cpp/src/continuous_batching/cache/block_manager.hpp around line 633 and llm_executor.hpp around line 132, tracing cancellation after a streaming client disconnects. Reproduce with the supplied streaming request and hard close; done means the process remains alive and other in-flight generations continue when a client disconnects mid-generation.