Snippet

Safe Concurrent Table-Driven Subtests via Closure Variable Re-binding

When combining table-driven tests with t.Parallel(), subtests run concurrently in separate goroutines. Capturing the loop iteration variable directly inside the closure leads to race conditions where subtests read mutated loop variables. Re-binding 'tt := tt' inside the loop scope ensures each subtest receives a distinct, isolated memory copy of its test case data.

snippet.go

package validator_test
 
import (
	"testing"
)
 
func TestValidateInput(t *testing.T) {
	tests := []struct {
		name    string
		input   string
		isValid bool
	}{
		{"valid email", "[email protected]", true},
		{"empty string", "", false},
		{"no domain", "user@", false},
	}
 
	for _, tt := range tests {
		tt := tt
		t.Run(tt.name, func(t *testing.T) {
			t.Parallel()
			if result := (len(tt.input) > 3); result != tt.isValid {
				t.Errorf("got %v, want %v", result, tt.isValid)
			}
		})
	}
}

Breakdown

tt := tt

Re-binds the loop iteration variable to a block-scoped variable to isolate it per goroutine.

t.Run(tt.name, func(t *testing.T) {

Spawns a named subtest closure that safely references the local copy of tt.

t.Parallel()

Signals to the Go test runner that this subtest can be executed concurrently alongside other parallel tests.

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Safe Concurrent Table-Driven Subtests via Closure Variable Re-binding

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