Copying a Mutex Value

Why This Happens

sync.Mutex must not be copied after first use because the copy gets a snapshot of the lock state. If the original is locked, the copy starts in a locked state, leading to deadlocks. Always use a pointer to a mutex.

The Problem

type SafeMap struct {
    mu sync.Mutex
    m  map[string]int
}

func process(sm SafeMap) { // copies the mutex!
    sm.mu.Lock()
    defer sm.mu.Unlock()
    sm.m["key"] = 1
}

The Fix

func process(sm *SafeMap) { // pointer, no copy
    sm.mu.Lock()
    defer sm.mu.Unlock()
    sm.m["key"] = 1
}

Step-by-Step Fix

1. 1

   Identify the copy

   Find where a struct containing a sync.Mutex is passed by value or assigned to another variable.

2. 2

   Use a pointer

   Change the function parameter or assignment to use a pointer instead of a value.

3. 3

   Check all call sites

   Verify all places that use the struct pass it by pointer.