Go race condition trivia

How private is private

Trivia time!

I recently ran into a -race error message that stumped me for a while, so I’ll retell it here in phases to see if you can catch it.

The code

The intent of the code was a service abstraction that ran a goroutine in the background to send runtime stats to Stastd. A small example of the code follows:

You can be 100% sure that I never run Loop() for the same Stats struct. And 
you’re 100% sure I only make a single Stats{} struct in my program. Before reading forward, inspect the above code and see if you can find the race condition I hit while using Stats.

A hint

The race condition occurred on line 19 that looks like.

p.prev = ms

Does that help find the race condition?

A second, bigger hint

The code that created my service structure looked somewhat like the following.

Private doesn’t mean undiscoverable

When you try to print a structure in Go with the Sprint or Println family of operations it will check to see if it’s a Formatter or Stringer and usually use those methods, otherwise it will use reflection to invent a way to print the structure. You can inspect the conditions for when is uses Formatter or Stringer inside print.go.

This particular code stumped me because I kept looking at Stats{} and thinking there’s no way that is a race. I keep prev private and never reference it. However, with Go, you can use reflection to inspect private variables. This inspection created the read operation that caused a race condition to trigger. This is very reasonable, especially since you can trigger a read of private variables even without reflection by taking the value of a structure

s := Stats{}
q := s // Will read stats to make a copy inside q

Thankfully, we can’t use reflection to set a private variable. See Value.CanSet for a description of when you can and cannot.

The moral of this little trivia is to remember that private variables can cause read -race errors, via copy and reflection, even if looking at the struct itself it appears safe.

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