check whether test function will run in parallel

How can I check whether a *testing.T has been set to run in parallel?

Ideally the solution might be:

func isParellelT(t *testing.T) bool {
   // return some means to figure t has called t.Parallel().
}

testing.T does not make this information available. Your tests should not depend on it. If you want to do this, you almost certainly should do something else instead.

However, it is possible to access the private data using reflection, or derive it by checking whether testing.T.SetEnv panics 😈:

package para

import (
    "reflect"
    "strings"
    "testing"
)

func TestParallelTrue(t *testing.T) {
    t.Parallel()

    if !isParallel(t) {
        t.Errorf("isParallel: got = false; want = true")
    }
    if !isParallel2(t) {
        t.Errorf("isParallel2: got = false; want = true")
    }
}

func TestParallelFalse(t *testing.T) {
    if isParallel(t) {
        t.Errorf("isParallel: got = true; want = false")
    }
    if isParallel2(t) {
        t.Errorf("isParallel2: got = true; want = false")
    }
}

// WARNING: This is terrible and should never be used.
// It will likely break on old or future versions of Go.
func isParallel(t *testing.T) (b bool) {
    v := reflect.ValueOf(t).Elem()

    // Check testing.T.isParallel first.
    isPara := v.FieldByName("isParallel")
    if isPara.IsValid() {
        return isPara.Bool()
    }

    // Otherwise try testing.T.common.isParallel.
    common := v.FieldByName("common")
    if !common.IsValid() {
        t.Fatal("isParallel: unsupported testing.T implementation")
    }

    isPara = common.FieldByName("isParallel")
    if !isPara.IsValid() {
        t.Fatal("isParallel: unsupported testing.T implementation")
    }

    return isPara.Bool()
}

// WARNING: This is terrible and should never be used.
// Requires Go1.17+.
func isParallel2(t *testing.T) (b bool) {
    defer func() {
        v := recover()
        if v == nil {
            return
        }
        if s, ok := v.(string); ok && strings.Contains(s, "parallel tests") {
            b = true
            return
        }
        panic(v)
    }()
    t.Setenv("_dummy", "value")
    return false
}