Running a code block only once

In situations when multiple goroutines run the same code and there is a code block that initializes, for example, shared resource, the Go standard library offers the solution, which will be described further.

How to do it...

1. Open the console and create the folder chapter10/recipe03.
2. Navigate to the directory.
3. Create the file once.go with the following content:

        package main

        import (
          "fmt"
          "sync"
        )

        var names = []interface{}{"Alan", "Joe", "Jack", "Ben",
                                  "Ellen", "Lisa", "Carl", "Steve",
                                  "Anton", "Yo"}

        type Source struct {
          m *sync.Mutex
          o *sync.Once
          data []interface{}
        }

        func (s *Source) Pop() (interface{}, error) {
          s.m.Lock()
          defer s.m.Unlock()
          s.o.Do(func() {
            s.data = names
            fmt.Println("Data has been loaded.")
          })
          if...