async-concurrency-reviewer

Description

Review async and concurrency code across any language. Finds real bugs — deadlocks, race conditions, missing cancellation, blocking calls in async contexts, and misused primitives. Returns a structured report with severity ratings and corrected code.

Use when

• "review my async code"
• "is this thread-safe"
• "could this deadlock"
• "check my concurrency"
• "await/promise/goroutine review"
• "is this race condition-safe"
• Any code with: async/await, promises, goroutines, threads, locks, channels, semaphores, actors

Supported languages

Python, JavaScript/TypeScript, C#, Go, Rust, Java, Kotlin — and any other language with async or concurrency primitives.

Input

Paste code. Optionally specify: language, runtime (Node, .NET, JVM, etc.), context (web server, CLI, background worker).

Output format

## Async/Concurrency Review

### Critical (fix before shipping)
- [Finding] — [why it causes bugs in production]
  ✗ Before: [problematic code]
  ✓ After:  [corrected code]

### Warnings (should fix)
- [Finding] — [explanation]

### Suggestions (nice to have)
- [Finding] — [explanation]

### What's correct
- [Specific patterns done right — always include at least one]

### Summary
[2–3 sentences: biggest risk, top fix, one pattern to adopt going forward]

Review checklist by language

Python

• asyncio.run() called inside an already-running event loop
• Blocking calls (time.sleep, requests.get) inside async def — use asyncio.sleep, httpx
• asyncio.create_task() result not stored — task gets garbage collected
• Missing await on coroutines (silent no-op bug)
• threading.Lock() inside async code — use asyncio.Lock()
• Shared mutable state across tasks without locks

JavaScript / TypeScript

• await inside forEach — use Promise.all with map
• Unhandled promise rejections
• async function called without await (fire and forget — often unintentional)
• Sequential await where parallel Promise.all would work
• Mixing callbacks and promises in the same flow
• setTimeout used as a poor-man's debounce in async context

C# / .NET

• .Result or .Wait() — deadlocks in sync-over-async
• async void — exceptions swallowed, no awaitable
• Missing CancellationToken on public async methods
• ConfigureAwait(false) missing in library code
• Task.Run wrapping sync CPU-bound work — correct; wrapping async — wrong
• lock keyword in async code — use SemaphoreSlim
• await inside lock — compiler error but watch for equivalent patterns

Go

• Goroutine leak — goroutine started, no way to stop it
• Channel send/receive without select+default or timeout — blocks forever
• WaitGroup.Add() called inside goroutine — race condition
• Shared map written from multiple goroutines without mutex
• Forgetting to close channels — range loop never terminates
• Context not passed through call chain — cancellation doesn't propagate

Rust

• block_on inside an async runtime — panic
• .unwrap() on JoinHandle — hides panics
• Holding a Mutex lock across an .await — deadlock
• spawn without storing the handle — fire and forget, errors lost
• Arc<Mutex<T>> contention hotspot — consider RwLock or message passing

Java / Kotlin

• CompletableFuture.get() blocking on the event thread
• Kotlin runBlocking inside a coroutine scope
• GlobalScope.launch — unstructured concurrency, leaks
• Missing Dispatchers.IO for blocking I/O in coroutines
• synchronized on wrong object — lock doesn't protect the shared state

Severity definitions

• Critical: Will deadlock, lose data, swallow exceptions, or cause race conditions in production
• Warning: Performance issue, resource leak, or hard-to-debug behaviour
• Suggestion: Style, clarity, or future-proofing improvement

Self-improvement instructions

After each review, note the most common finding. After 20 reviews, surface the top 3 patterns in your response as "Most common issues in [language]" to help users learn, not just fix.