Agent Skills: Concurrency Auditor (L3 Worker)

Paths: File paths (shared/, references/, ../ln-*) are relative to skills repo root. If not found at CWD, locate this SKILL.md directory and go up one level for repo root. If shared/ is missing, fetch files via WebFetch from https://raw.githubusercontent.com/levnikolaevich/claude-code-skills/master/skills/{path}.

Concurrency Auditor (L3 Worker)

Type: L3 Worker

Specialized worker auditing concurrency, async patterns, and cross-process resource access.

Purpose & Scope

• Audit concurrency (Category 11: High Priority)
• 7 checks: async races, thread safety, TOCTOU, deadlocks, blocking I/O, resource contention, cross-process races
• Two-layer detection: grep finds candidates, agent reasons about context
• Calculate compliance score (X/10)

Inputs

MANDATORY READ: Load shared/references/audit_worker_core_contract.md.

Receives contextStore with: tech_stack, best_practices, codebase_root, output_dir.

Workflow

MANDATORY READ: Load shared/references/two_layer_detection.md for detection methodology.

1. Parse context -- extract tech_stack, language, output_dir from contextStore
2. Per check (1-7):
   • Layer 1: Grep/Glob scan to find candidates
   • Layer 2: Read 20-50 lines around each candidate. Apply check-specific critical questions. Classify: confirmed / false positive / needs-context
3. Collect confirmed findings with severity, location, effort, recommendation
4. Calculate score per shared/references/audit_scoring.md
5. Write Report -- build in memory, write to {output_dir}/ln-628--global.md (atomic single Write)
6. Return Summary to coordinator

Audit Rules

Unified severity escalation: For ALL checks -- if finding affects payment/auth/financial code -> escalate to CRITICAL regardless of other factors.

1. Async/Event-Loop Races (CWE-362)

What: Shared state corrupted across await/yield boundaries in single-threaded async code.

Layer 1 -- Grep patterns:

| Language | Pattern | Grep | |----------|---------|------| | JS/TS | Read-modify-write across await | \w+\s*[+\-*/]?=\s*.*await (e.g., result += await something) | | JS/TS | Check-then-initialize race | if\s*\(!?\w+\) followed by \w+\s*=\s*await in same block | | Python | Read-modify-write across await | \w+\s*[+\-*/]?=\s*await inside async def | | Python | Shared module-level state in async | Module-level \w+\s*= + modified inside async def | | All | Shared cache without lock | \.set\(|\.put\(|\[\w+\]\s*= in async function without lock/mutex nearby |

Layer 2 -- Critical questions:

• Is the variable shared (module/global scope) or local?
• Can two async tasks interleave at this await point?
• Is there a lock/mutex/semaphore guarding the access?

Severity: CRITICAL (payment/auth) | HIGH (user-facing) | MEDIUM (background)

Safe pattern exclusions: Local variables, const declarations, single-use await (no interleaving possible).

Effort: M

2. Thread/Goroutine Safety (CWE-366)

What: Shared mutable state accessed from multiple threads/goroutines without synchronization.

Layer 1 -- Grep patterns:

| Language | Pattern | Grep | |----------|---------|------| | Go | Map access without mutex | map\[.*\].*= in struct without sync.Mutex or sync.RWMutex | | Go | Variable captured by goroutine | go func + variable from outer scope modified | | Python | Global modified in threads | global\s+\w+ in function + threading.Thread in same file | | Java | HashMap shared between threads | HashMap + Thread\|Executor\|Runnable in same class without synchronized\|ConcurrentHashMap | | Rust | Rc in multi-thread context | Rc<RefCell + thread::spawn\|tokio::spawn in same file | | Node.js | Worker Threads shared state | workerData\|SharedArrayBuffer\|parentPort + mutable access without Atomics |

Layer 2 -- Critical questions:

• Is this struct/object actually shared between threads? (single-threaded code -> FP)
• Is mutex/lock in embedded struct or imported module? (grep may miss it)
• Is go func capturing by value (safe) or by reference (unsafe)?

Severity: CRITICAL (payment/auth) | HIGH (data corruption possible) | MEDIUM (internal)

Safe pattern exclusions: Go map in init() or main() before goroutines start. Rust Arc<Mutex<T>> (already safe). Java Collections.synchronizedMap().

Effort: M

3. TOCTOU -- Time-of-Check Time-of-Use (CWE-367)

What: Resource state checked, then used, but state can change between check and use.

Layer 1 -- Grep patterns:

| Language | Check | Use | Grep | |----------|-------|-----|------| | Python | os.path.exists() | open() | os\.path\.exists\( near open\( on same variable | | Python | os.access() | os.open() | os\.access\( near os\.open\(\|open\( | | Node.js | fs.existsSync() | fs.readFileSync() | existsSync\( near readFileSync\(\|readFile\( | | Node.js | fs.accessSync() | fs.openSync() | accessSync\( near openSync\( | | Go | os.Stat() | os.Open() | os\.Stat\( near os\.Open\(\|os\.Create\( | | Java | .exists() | new FileInputStream | \.exists\(\) near new File\|FileInputStream\|FileOutputStream |

Layer 2 -- Critical questions:

• Is the check used for control flow (vulnerable) or just logging (safe)?
• Is there a lock/retry around the check-then-use sequence?
• Is the file in a temp directory controlled by the application (lower risk)?
• Could an attacker substitute the file (symlink attack)?

Severity: CRITICAL (security-sensitive: permissions, auth tokens, configs) | HIGH (user-facing file ops) | MEDIUM (internal/background)

Safe pattern exclusions: Check inside try/catch with retry. Check for logging/metrics only. Check + use wrapped in file lock.

Effort: S-M (replace check-then-use with direct use + error handling)

4. Deadlock Potential (CWE-833)

What: Lock acquisition in inconsistent order, or lock held during blocking operation.

Layer 1 -- Grep patterns:

| Language | Pattern | Grep | |----------|---------|------| | Python | Nested locks | with\s+\w+_lock: (multiline: two different locks nested) | | Python | Lock in loop | for.*: with \.acquire\(\) inside loop body | | Python | Lock + external call | \.acquire\(\) followed by await\|requests\.\|urllib before release | | Go | Missing defer unlock | \.Lock\(\) without defer.*\.Unlock\(\) on next line | | Go | Nested locks | Two \.Lock\(\) calls in same function without intervening \.Unlock\(\) | | Java | Nested synchronized | synchronized\s*\( (multiline: nested blocks with different monitors) | | JS | Async mutex nesting | await\s+\w+\.acquire\(\) (two different mutexes in same function) |

Layer 2 -- Critical questions:

• Are these the same lock (reentrant = OK) or different locks (deadlock risk)?
• Is the lock ordering consistent across all call sites?
• Does the external call inside lock have a timeout?

Severity: CRITICAL (payment/auth) | HIGH (app freeze risk)

Safe pattern exclusions: Reentrant locks (same lock acquired twice). Locks with explicit timeout (asyncio.wait_for, tryLock).

Effort: L (lock ordering redesign)

5. Blocking I/O in Async Context (CWE-400)

What: Synchronous blocking calls inside async functions or event loop handlers.

Layer 1 -- Grep patterns:

| Language | Blocking Call | Grep | Replacement | |----------|--------------|------|-------------| | Python | time.sleep in async def | time\.sleep inside async def | await asyncio.sleep | | Python | requests.* in async def | requests\.(get\|post\|put\|delete) inside async def | httpx or aiohttp | | Python | open() in async def | open\( inside async def | aiofiles.open | | Node.js | fs.readFileSync in async | fs\.readFileSync\|fs\.writeFileSync\|fs\.mkdirSync | fs.promises.* | | Node.js | execSync in async | execSync\|spawnSync in async handler | exec with promises | | Node.js | Sync crypto in async | crypto\.pbkdf2Sync\|crypto\.scryptSync | crypto.pbkdf2 (callback) |

Layer 2 -- Critical questions:

• Is this in a hot path (API handler) or cold path (startup script)?
• Is the blocking duration significant (>100ms)?
• Is there a legitimate reason (e.g., sync read of small config at startup)?

Severity: HIGH (blocks event loop/async context) | MEDIUM (minor blocking <100ms)

Safe pattern exclusions: Blocking call in if __name__ == "__main__" (startup). readFileSync in config loading at init time. Sync crypto for small inputs.

Effort: S-M (replace with async alternative)

6. Resource Contention (CWE-362)

What: Multiple concurrent accessors compete for same resource without coordination.

Layer 1 -- Grep patterns:

| Pattern | Risk | Grep | |---------|------|------| | Shared memory without sync | Data corruption | SharedArrayBuffer\|SharedMemory\|shm_open\|mmap without Atomics\|Mutex\|Lock nearby | | IPC without coordination | Message ordering | process\.send\|parentPort\.postMessage in concurrent loops | | Concurrent file append | Interleaved writes | Multiple appendFile\|fs\.write to same path from parallel tasks |

Layer 2 -- Critical questions:

• Are multiple writers actually concurrent? (Sequential = safe)
• Is there OS-level atomicity guarantee? (e.g., O_APPEND for small writes)
• Is ordering important for correctness?

Severity: HIGH (data corruption) | MEDIUM (ordering issues)

Safe pattern exclusions: Single writer pattern. OS-guaranteed atomic operations (small pipe writes, O_APPEND). Message queues with ordering guarantees.

Effort: M

7. Cross-Process & Invisible Side Effects (CWE-362, CWE-421)

What: Multiple processes or process+OS accessing same exclusive resource, including operations with non-obvious side effects on shared OS resources.

Layer 1 -- Grep entry points:

| Pattern | Risk | Grep | |---------|------|------| | Clipboard dual access | OSC 52 + native clipboard in same flow | osc52\|\\x1b\\]52 AND clipboard\|SetClipboardData\|pbcopy\|xclip in same file | | Subprocess + shared file | Parent and child write same file | spawn\|exec\|Popen + writeFile\|open.*"w" on same path | | OS exclusive resource | Win32 clipboard, serial port, named pipe | OpenClipboard\|serial\.Serial\|CreateNamedPipe\|mkfifo | | Terminal escape sequences | stdout triggers terminal OS access | \\x1b\\]\|\\033\\]\|writeOsc\|xterm | | External clipboard tools | Clipboard via spawned process | pbcopy\|xclip\|xsel\|clip\.exe |

Layer 2 -- This check relies on reasoning more than any other:

1. Build Resource Inventory:

   | Resource | Exclusive? | Accessor 1 | Accessor 2 | Sync present? | |----------|-----------|------------|------------|---------------|

2. Trace Timeline:

   t=0ms  operation_A() -> resource_X accessed
   t=?ms  side_effect   -> resource_X accessed by external process
   t=?ms  operation_B() -> resource_X accessed again -> CONFLICT?
   

3. Critical Questions:

   • Can another process (terminal, OS, child) access this resource simultaneously?
   • Does this operation have invisible side effects on shared OS resources?
   • What happens if the external process is slower/faster than expected?
   • What happens if user triggers this action twice rapidly?

Severity: CRITICAL (two accessors to exclusive OS resource without sync) | HIGH (subprocess + shared file without lock) | HIGH (invisible side effect detected via reasoning)

Safe pattern exclusions: Single accessor. Retry/backoff pattern present. Operations sequenced with explicit delay/await.

Effort: M-L (may require removing redundant access path)

Scoring Algorithm

MANDATORY READ: Load shared/references/audit_worker_core_contract.md and shared/references/audit_scoring.md.

Output Format

MANDATORY READ: Load shared/references/audit_worker_core_contract.md and shared/templates/audit_worker_report_template.md.

Write JSON summary per shared/references/audit_summary_contract.md. In managed mode the caller passes both runId and summaryArtifactPath; in standalone mode the worker generates its own run-scoped artifact path per shared contract.

Write report to {output_dir}/ln-628--global.md with category: "Concurrency" and checks: async_races, thread_safety, toctou, deadlock_potential, blocking_io, resource_contention, cross_process_races.

Return summary per shared/references/audit_summary_contract.md.

When summaryArtifactPath is absent, write the standalone runtime summary under .hex-skills/runtime-artifacts/runs/{run_id}/evaluation-worker/{worker}--{identifier}.json and optionally echo the same summary in structured output.

Report written: .hex-skills/runtime-artifacts/runs/{run_id}/audit-report/ln-628--global.md
Score: X.X/10 | Issues: N (C:N H:N M:N L:N)

Critical Rules

MANDATORY READ: Load shared/references/audit_worker_core_contract.md.

• Do not auto-fix: Report only -- concurrency fixes require careful human review
• Two-layer detection: Always apply Layer 2 reasoning after Layer 1 grep. Never report raw grep matches without context analysis
• Language-aware detection: Use language-specific patterns per check
• Unified CRITICAL escalation: Any finding in payment/auth/financial code = CRITICAL
• Effort realism: S = <1h, M = 1-4h, L = >4h
• Exclusions: Skip test files, skip single-threaded CLI tools, skip generated code

Definition of Done

MANDATORY READ: Load shared/references/audit_worker_core_contract.md.

• [ ] contextStore parsed (language, concurrency model, output_dir)
• [ ] All 7 checks completed with two-layer detection:
  • async races, thread safety, TOCTOU, deadlock potential, blocking I/O, resource contention, cross-process races
• [ ] Layer 2 reasoning applied to each candidate (confirmed / FP / needs-context)
• [ ] Findings collected with severity, location, effort, recommendation
• [ ] Score calculated per shared/references/audit_scoring.md
• [ ] Report written to {output_dir}/ln-628--global.md (atomic single Write call)
• [ ] Summary written per contract

Reference Files

• Two-layer detection methodology: shared/references/two_layer_detection.md
• Audit output schema: shared/references/audit_output_schema.md

Version: 4.0.0 Last Updated: 2026-03-04