RWMutex在条件变量中

Question

Why RWMutex in conditional variable doesn't have RLock and RUnLock methods?

package main

import (
    "sync"
)

func main() {
    cond := &sync.Cond{L: &sync.RWMutex{}}
    cond.L.RLock()
    cond.L.RUnlock()
}

I got an error:

cond.L.RLock undefined (type sync.Locker has no field or method RLock)
cond.L.RUnlock undefined (type sync.Locker has no field or method RUnlock)

https://play.golang.org/p/LdGYlrQQx86

Answer 1

A sync.Cond only makes use of the methods required by a sync.Locker: Lock and Unlock. The minimal interface needed by Cond is therefore just those two methods.

You can still use your RWMutex directly (as opposed to going through Cond), and Cond will still work.

func main() {
    myMutex := &sync.RWMutex{}
    cond := sync.NewCond(myMutex)
    // Use the RW mutex directly.
    myMutex.RLock()
    myMutex.RUnlock()
    // Use the mutex through cond. Lock and Unlock only.
    cond.L.Lock()
    cond.L.Unlock()
}

Or you can use it through Cond.L after making sure it is of the expected type:

func main() {
    cond := &sync.Cond{L: &sync.RWMutex{}}
    // Typecheck cond.L.
    myRWMutex, ok := cond.L.(*sync.RWMutex)
    if !ok {
        panic("AHHHH!!!")
    }
    myRWMutex.RLock()
    myRWMutex.RUnlock()
}

(you can skip the type check and just say cond.L.(*sync.RWMutex) but if it isn't a sync.RWMutex, your program will panic)