How does GOMAXPROCS=1 change behaviour and when is it actually useful?

runtime.GOMAXPROCS(1)

// With GOMAXPROCS=1: only one goroutine runs at a time
// There is no true parallelism
// Some races become less likely to manifest — but they still exist!

// WRONG: GOMAXPROCS=1 does NOT eliminate races
// Cooperative preemption at function calls still allows interleaving
var x int
go func() {
    x++           // goroutine may yield here
    fmt.Println(x) // x may have changed between ++ and Println
}()

// ALWAYS use -race flag — not GOMAXPROCS=1 — to detect races

// Legitimate uses of GOMAXPROCS=1:
// 1. Reproduce bugs that require specific goroutine ordering
// 2. Test that code produces correct results without parallelism
//    (if code only works with multiple Ps, that's a design flaw)
// 3. Performance comparison: scheduling overhead vs pure compute

// Testing sequential correctness:
func TestSequential(t *testing.T) {
    prev := runtime.GOMAXPROCS(1)
    defer runtime.GOMAXPROCS(prev) // restore after test

    result := concurrentAlgorithm([]int{1, 2, 3, 4, 5})
    if result != 15 {
        t.Errorf("expected 15, got %d", result)
    }
}