[BugFix] Add lock to avoid generating nan when using storage cache

[juncaipeng]

Motivation

Add lock to avoid generating nan when using storage cache

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Modifications

fastdeploy/cache_manager/prefix_cache_manager.py

Usage or Command

none

Accuracy Tests

none

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
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[paddle-bot]

Thanks for your contribution!

[Copilot]

Pull request overview

该 PR 旨在通过对 KVCache storage cache 的读/写回任务加锁，避免在特定并发访问场景下出现 NaN（通过 FD_USE_KVCACHE_LOCK 控制启用）。

Changes:

• 在 issue_write_back_storage_task 中对写回 storage 的流程加锁，并限制当前仅支持同步模式。
• 在 issue_prefetch_storage_task 中对从 storage 预取到 GPU 的流程加锁，并限制当前仅支持同步模式。

Comments suppressed due to low confidence (1)

fastdeploy/cache_manager/prefix_cache_manager.py:1260

• 这里在 acquire lock 之后如果触发后续异常（例如 keys/gpu_block_ids 长度不一致时 raise ValueError，或 put_transfer_task 抛异常），会导致锁永远不释放，后续 worker 可能被永久阻塞。建议把参数校验放到加锁之前，并用 try/finally（或 contextmanager）保证 _release_kvcache_lock() 一定执行。

        assert is_sync, "Only support is_sync=True for now."
        self._acquire_kvcache_lock()

        if len(task.keys) != len(task.gpu_block_ids):
            err_msg = (
                f"write_back_storage error: hash_keys({len(task.keys)}) != gpu_block_ids({len(task.gpu_block_ids)})"
            )
            logger.error(err_msg)
            raise ValueError(err_msg)

[Copilot]

这里用 assert 来限制 is_sync=True：1) Python -O 会跳过 assert，约束不生效；2) 现有单测 tests/cache_manager/test_prefix_cache_manager.py 覆盖了 is_sync=False 的分支并期望抛 ValueError（key/gpu_block_ids 不匹配），这次改动会直接变成 AssertionError 导致用例失败。建议改为显式的运行时检查并抛出 NotImplementedError/ValueError，或保留对 is_sync=False 的兼容实现并同步更新单测。

[codecov-commenter]

Codecov Report

❌ Patch coverage is 68.18182% with 7 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@25d64ef). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/cache_manager/prefix_cache_manager.py	71.42%	3 Missing and 3 partials ⚠️
.../transfer_factory/mooncake_store/mooncake_store.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #7046   +/-   ##
==========================================
  Coverage           ?   72.99%           
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  Files              ?      399           
  Lines              ?    56419           
  Branches           ?     8919           
==========================================
  Hits               ?    41183           
  Misses             ?    12309           
  Partials           ?     2927           

Flag	Coverage Δ
GPU	72.99% <68.18%> (?)

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