fix: guard yaml/flock/TOCTOU/atomic writes across small surfaces (#28018)#28593
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1. trajectory_compressor.py: yaml.safe_load() returns None on empty
files, crashing with TypeError on `if 'tokenizer' in data`. Fix by
adding `or {}` fallback. (HIGH — blocks startup with empty config)
2. 6 files with fcntl.flock(LOCK_UN) in finally blocks without
try/except: cron/scheduler.py, hermes_cli/auth.py,
agent/shell_hooks.py, tools/skill_usage.py,
tools/environments/file_sync.py, tools/memory_tool.py. If unlock
raises OSError, fd.close() is skipped and the lock is held forever.
The msvcrt branches already had try/except; the fcntl branches did
not. Fix by wrapping in try/except (OSError, IOError): pass.
3. agent/copilot_acp_client.py line 639: TOCTOU race — path.exists()
followed by path.read_text() with no try/except. If file is deleted
between the check and the read, FileNotFoundError propagates. Fix
by using try/except FileNotFoundError.
4. gateway/sticker_cache.py: non-atomic write via Path.write_text()
can leave truncated JSON on crash, causing JSONDecodeError on next
load. Fix by writing to tempfile + fsync + os.replace (atomic).
3 tasks
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Salvage of #28018 by @vanthinh6886.
What: Several small defensive gaps across multiple modules that all share the same pattern (race condition between check + use, or no exception handler around a syscall that can fail under load/edge cases):
agent/copilot_acp_client.py:path.exists()→path.read_text()is a TOCTOU race; replace withtry/except FileNotFoundError.agent/shell_hooks.py,cron/scheduler.py,hermes_cli/auth.py:fcntl.flock(.., LOCK_UN)in finally blocks could raise OSError if the lock fd was already closed; wrap in try/except.gateway/sticker_cache.py: cache save wasCACHE_PATH.write_text(...)(non-atomic, partial-write corruption risk); switch totempfile.mkstemp+os.fsync+os.replace.Why: None of these are bugs hit in the happy path — they're hardening against partial reads/writes, lock fd reuse, and races on heavily-loaded systems.
Original PR: #28018