environment: Error when t.Parallel was called before Protect

Protecting the environment relies on the shared state (containers,
images, etc) which might already be mutated by other tests if the test
opted in into the Parallel execution before Protect was called.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

testutil/environment/protect.go

@@ -10,6 +10,7 @@ import (
 	"github.com/docker/docker/api/types/image"
 	"github.com/docker/docker/api/types/volume"
 	"github.com/docker/docker/errdefs"
+	"github.com/docker/docker/testutil"
 	"go.opentelemetry.io/otel"
 	"gotest.tools/v3/assert"
 )
@@ -38,6 +39,8 @@ func newProtectedElements() protectedElements {
 // volumes, and, on Linux, plugins) from being cleaned up at the end of test
 // runs
 func ProtectAll(ctx context.Context, t testing.TB, testEnv *Execution) {
+	testutil.CheckNotParallel(t)
+
 	t.Helper()
 	ctx, span := otel.Tracer("").Start(ctx, "ProtectAll")
 	defer span.End()

testutil/helpers.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"io"
 	"os"
+	"reflect"
 	"strings"
 	"sync"
 	"testing"
@@ -156,3 +157,21 @@ func SetContext(t TestingT, ctx context.Context) {
 func CleanupContext(t *testing.T) {
 	testContexts.Delete(t)
 }
+
+// CheckNotParallel checks if t.Parallel() was not called on the current test.
+// There's no public method to check this, so we use reflection to check the
+// internal field set by t.Parallel()
+// https://github.com/golang/go/blob/8e658eee9c7a67a8a79a8308695920ac9917566c/src/testing/testing.go#L1449
+//
+// Since this is not a public API, it might change at any time.
+func CheckNotParallel(t testing.TB) {
+	t.Helper()
+	field := reflect.ValueOf(t).Elem().FieldByName("isParallel")
+	if field.IsValid() {
+		if field.Bool() {
+			t.Fatal("t.Parallel() was called before")
+		}
+	} else {
+		t.Logf("FIXME: CheckParallel could not determine if test %s is parallel - did the t.Parallel() implementation change?", t.Name())
+	}
+}