Go race detector 'WARNING: DATA RACE': how do I read the report and fix the race?

What it means: A data race is two goroutines accessing the same variable concurrently with at least one write. Build or test with -race (go test -race ./..., go run -race, go build -race). It needs cgo enabled and, on non-Darwin systems, a C compiler. It only finds races on code paths that actually run, so you need good tests or realistic load. Reading the report: It starts with WARNING: DATA RACE and shows two stack traces for the conflicting accesses (for example 'Read by goroutine 185' and 'Previous write by goroutine 184'), then where those goroutines were created. Go to the file:line of both accesses; the variable is shared between them. Typical causes and fixes from the Go docs: 1. Loop variable captured by a goroutine closure: pass a copy as an argument. (Since Go 1.22 each iteration has its own variable for loops declared with :=, but older go.mod versions keep the old behaviour.) 2. Accidentally shared variable such as err assigned inside a goroutine: use := to declare a new local. 3. Unprotected global map or slice: guard with a sync.Mutex or sync.RWMutex. 4. Plain counters or flags: use sync/atomic, a mutex or a channel. 5. Send and close on a channel with no ordering: receive before close, or coordinate shutdown with a WaitGroup. Tuning: GORACE accepts options like log_path, halt_on_error=1 and history_size. Expect roughly 5-10x memory and 2-20x slower execution, so use it in tests and staging, not usually production. Build tag race lets you exclude tests that are too slow under the detector. Verify: Re-run the same test several times with -race and see no WARNING: DATA RACE output; the exit status is not 66 (the default race exit code). Sources: https://go.dev/doc/articles/race_detector https://go.dev/doc/faq https://pkg.go.dev/sync