Golang documentation for the condition variables states the following:

In the terminology of the Go memory model, Cond arranges that a call
to Broadcast or Signal “synchronizes before” any Wait call that it
unblocks.

However, the following code (run via go run -race .) gives a DATA RACE flag:

package main

import (
    "fmt"
    "sync"
    "time"
)

func main() {
    m := sync.Mutex{}
    c := sync.NewCond(&m)
    var a = 10
    go func() {
        time.Sleep(time.Second)
        a = 20
        c.Signal()
    }()

    go func() {
        defer m.Unlock()
        m.Lock()
        c.Wait()
        fmt.Println(a)
    }()

    select {}
}

Why?

Second goroutine gets into Wait(). After some time (one second) first goroutine makes a signal by invoking Signal(). I use time.Sleep here to ensure second goroutine first gets into Wait before Signal happens. Since call to Signal() is synchronized before any Wait() call that it unblock, I don’t understand how can it be a data race on a = 20 and fmt.Println(a).

Since a = 20 is sequenced before c.Signal() which is synchronized before unblocking c.Wait() which is sequenced before fmt.Println(a), it results a = 20 happens before fmt.Println(a) thus it shouldn’t have a data race.