在Go中使用映射时忽略goroutine /线程安全的危险是什么？

Question

Go's map is said not to be goroutine-safe (see here and here). I'm interested at finding out what could happen in a case where I neglect to protect access to the map using mutex/etc.

Specifically, can any of the following happen?

1. Assuming I have a map with keys k1, k2, ..., kn, can a concurrency issue lead to getting map[ki] when I asked for map[kj] (i != j)?
2. Can it lead to a panic in the application?

Answer 1

As the comments have already stated, races are bad. Go has very weak guarantees, unlike Java, and hence a program that has any race is allowed to have undefined behavior even when the race-containing code is not executed. In C, this is called "catch-fire semantics". The presence of a race means any outcome is possible, to include your computer catching on fire.

However, in Go it is easy to make a map thread-safe. Consider the following:

// Global variable defining a map
var safemap = struct {
    sync.RWMutex
    m map[string]string
}{m: make(map[string]string)}

You can do safe reads from the map like this:

// Get a read lock, then read from the map
safemap.RLock()
defer safemap.RUnlock()
return safemap.m[mykey] == myval

And you can do safe modifications like this:

// Delete from the map
safemap.Lock()
delete(safemap.m, mykey)
safemap.Unlock()

or this:

// Insert into the map
safemap.Lock()
safemap.m[mykey] = myval
safemap.Unlock()