Aias00 opened a new pull request, #3523:
URL: https://github.com/apache/dubbo-go/pull/3523
## What
`newP2CLoadBalance` and `newAdaptiveServiceCluster` read the package-level
`instance` **outside** `sync.Once.Do` (an `if instance == nil` fast path +
`return instance`), racing first-time concurrent initialization.
## Why
```go
var (
once sync.Once
instance loadbalance.LoadBalance
)
func newP2CLoadBalance(r randomPicker) loadbalance.LoadBalance {
if r == nil { r = defaultRnd }
if instance == nil { // unsynchronized read
once.Do(func() { instance = &p2cLoadBalance{...} })
}
return instance // unsynchronized read when check was
false
}
```
`sync.Once.Do` only establishes happens-before for callers that **call
`Do`**. A goroutine that observes `instance != nil` (check false) skips `Do`
and reads `instance` with no happens-before to the goroutine that wrote it
inside `Do`. Two concurrent first-time callers race on `instance`. The
identical pattern is in
`cluster/cluster/adaptivesvc/cluster.go:newAdaptiveServiceCluster`. Both
factories are registered in `init()`, so concurrent RPCs at process start hit
this path. `go test -race` flags it.
## Fix
Drop the `if instance == nil` fast path; always `once.Do(...)` then `return
instance`. `sync.Once.Do` guarantees all callers see the writes done inside
`Do`, and there is no read of `instance` outside `Do`. The p2c `r == nil`
defaulting moves inside the `once.Do` closure (only the first call's `r` is
used, identical to before).
## Tests
Added `TestNewP2CLoadBalanceConcurrent`: 100 goroutines racing the factory,
passing under `-race`. `p2c` package passes under `-race`.
Fixes #3522
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