Counter that rotates that is safe for concurrent use - go

Question

I want to write something like this:
type RoundRobinList struct {
lst []string
idx uint32
}
// rr_list is a RoundRobinList
func loop(rr_list) {
start = rr_list.idx
rr_list.idx = (rr_list.idx + 1)%len(rr_list.lst)
print(rr_list.lst[idx])
}
If rr_list.lst = ["a", "b", "c"], and loop is called over and over, I would expect the following printed:
"a"
"b"
"c"
"a"
"b"
"c" ...
Is this safe? rr_list.idx = (rr_list.idx + 1)%len(rr_list.lst)

Answer 1

Whenever you have a read and write of a value that's not protected in some way (using a mutex, or some of the other things in the sync package), you have a race, and that means you can't use it safely in a concurrent setting.
Here, you're reading and writing the idx field of your RoundRobinList structure without any protection, so you'll have a race if you use it concurrently.
As a first line of defense you should understand the rules for memory safety and follow them carefully, and not write code unless you're pretty sure it's safe. As a second line of defense, you can use the race detector to find a lot of problems with lack of safety of concurrent access.
Here's a simple test case, in file a_test.go. I had to also fix some bugs in the code to get it to compile. It starts two goroutines which call loop 1000 times each on a shared RoundRobinList.
package main
import (
"sync"
"testing"
)
type RoundRobinList struct {
lst []string
idx uint32
}
func loop(rr *RoundRobinList) {
rr.idx = (rr.idx + 1) % uint32(len(rr.lst))
}
func TestLoop(t *testing.T) {
rr := RoundRobinList{[]string{"a", "b", "c"}, 0}
var wg sync.WaitGroup
wg.Add(2)
for n := 0; n < 2; n++ {
go func() {
for i := 0; i < 1000; i++ {
loop(&rr)
}
wg.Done()
}()
}
wg.Wait()
}
Running with go test -race ./a_test.go results in:
==================
WARNING: DATA RACE
Read at 0x00c0000b0078 by goroutine 9:
command-line-arguments.loop()
/mnt/c/Users/paul/Desktop/a_test.go:14 +0xa1
command-line-arguments.TestLoop.func1()
/mnt/c/Users/paul/Desktop/a_test.go:24 +0x78
Previous write at 0x00c0000b0078 by goroutine 8:
command-line-arguments.loop()
/mnt/c/Users/paul/Desktop/a_test.go:14 +0x4e
command-line-arguments.TestLoop.func1()
/mnt/c/Users/paul/Desktop/a_test.go:24 +0x78
Goroutine 9 (running) created at:
command-line-arguments.TestLoop()
/mnt/c/Users/paul/Desktop/a_test.go:22 +0x1cc
testing.tRunner()
/usr/local/go/src/testing/testing.go:992 +0x1eb
Goroutine 8 (finished) created at:
command-line-arguments.TestLoop()
/mnt/c/Users/paul/Desktop/a_test.go:22 +0x1cc
testing.tRunner()
/usr/local/go/src/testing/testing.go:992 +0x1eb
==================
--- FAIL: TestLoop (0.00s)
testing.go:906: race detected during execution of test
FAIL
FAIL command-line-arguments 0.006s
FAIL