Go Once

Internal

• The sync Package
• Goroutines | Synchronization

Overview

Once is a Go synchronization construct that is used when a piece of code must be executed only once. This is a useful idiom in initialization. Note that the code is executed synchronously, when it is executed: the invocation to Once#Do() executes in the same goroutine that makes the Once#Do() invocation.

This pattern is thread-safe when the initialization is attempted by more than one goroutine.

It is recommended that any usage of sync.Once is wrapped in a small lexical block, either a small function or a type.

var someOnce sync.Once

// someFunc has no parameters
func someFunc() {
  ...
}

...
someOnce.Do(someFunc) // no parameters are passed

Initialization Pattern

var log Log
var initLogOnce sync.Once

...
func GetLog() {
  initLogOnce.Do(func() {
     log = ... // thread-safely init the log
  })
  return log // because the function passed to Do() executes synchronously, log will be initialized here.
}

Methods

Do()

The only argument of Do() is a function value, not a function invocation. Do() will use the function value to invoke the function, internally. All invokers will block, except one, which executes synchronously in the same goroutine that performed the Do() invocation. The blocked invokers will be released after the one that does initialization finishes executing, and none of them will execute.

Note that Do() will execute only once, ever, not only once per unique function it is invoked with. Do() counts the number of times it is called, not how many times unique functions were passed to it.

How to invoke a function that has arguments? The only solution I found so far was to wrap into the definition of an anonymous function:

someOnce.Do(func() {
  someArg := ...
  theFunctionThatMustBeRunOnlyOnce(someArg)
})