Race condition in Go (see `letter` variable)

race_conditions.go

package main

import (
	"fmt"
	"sync"
)

const numThreads = 5

var ch chan string
var wg sync.WaitGroup

var letter string

func main() {
	ch = make(chan string, numThreads)

	// Establish connection to source
	// and destination database

	// Data generator
	wg.Add(1)
	go generate()

	// Worker threads
	workers()

	wg.Wait()
	close(ch)

	fmt.Println(letter)

}

func generate() {
	/* For as long as we have data in database,
	   - Fetch the data in batch
	   - Delegate the task to workers via channel
	*/

	// Simulating database access delay
	// time.Sleep(300 * time.Millisecond)

	data := []string{"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l"}
	wg.Add(len(data))
	for i := range data {
		// Data process delay
		// time.Sleep(100 * time.Millisecond)
		ch <- data[i]
	}

	// No more data to work on, notify main routine
	wg.Done()

}

func workers() {
	// Spin N number of worker threads
	for i := 0; i < numThreads; i++ {
		go func(w int) {
			for p := range ch {
				fmt.Printf("Worker %d: %s\n", w, p)
				// Transform the data, wait for 1000 records
				// and save it to postgres
				// time.Sleep(20 * time.Millisecond)
				wg.Done()
				letter = p
			}
			fmt.Printf("No more work to do, shutting down worker %d\n", w)
		}(i)
	}
}