Why Do My Goroutines All Print the Same Value Despite Using a Slice in Go?

Understanding the Behavior of Goroutines in a Data Race

The code snippet provided demonstrates a classic data race issue in Go. While the aim is to print different values ("one", "two", "three") from a list of fields, the actual result observed is that "three" is printed three times.

To resolve this issue, we need to understand why and where the data race occurs.

Identifying the Data Race

The issue lies in the argument evaluation of the goroutine function. When the loop iterates over the data slice, a range variable v is assigned to the loop body and is used in the call to v.print(). However, the v variable is a copy of the elements, and since the slice is changed during the iteration, the goroutines eventually all point to the same last element of the slice.

This means that when the goroutines execute, they all print the last value ("three") instead of the unique values ("one", "two", "three") that we intended.

Possible Solutions

There are several ways to address this issue and eliminate the data race:

1. Create a New Variable Inside the Loop:

   • By declaring a new variable v within the loop using the v := v syntax, we ensure that each goroutine operates on its own copy of the data.

2. Use a Slice of Pointers:

   • Instead of passing the original slice, we can use a slice of pointers (e.g., []*field). When passed to the goroutines, each pointer points to a unique element in the slice, avoiding the data race.

3. Pass the Address of the Slice Element:

   • Alternatively, we can explicitly pass the address of each slice element to the goroutine using the syntax &data[i]. This ensures that each goroutine has a unique pointer to the intended element.

4. Use an Anonymous Function with Arguments:

   • If the goroutine function is within an anonymous function, we can pass the range variable as an argument to the anonymous function, isolating the goroutine's access to the data.

By applying one of these solutions, we can eliminate the data race and ensure that the goroutines print the intended unique values ("one", "two", "three") in any order.

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