Add guard against potentially blocking forever when NumGoroutines is 0

Closes #7462

Signed-off-by: Andrew Mason <[email protected]>

go/vt/concurrency/error_group.go

@@ -75,6 +75,10 @@ func (eg ErrorGroup) Wait(cancel context.CancelFunc, errors chan error) *AllErro
 	responseCounter := 0
 	rec := &AllErrorRecorder{}
 
+	if eg.NumGoroutines < 1 {
+		return rec
+	}
+
 	for err := range errors {
 		responseCounter++
 

go/vt/concurrency/error_group_test.go

@@ -0,0 +1,34 @@
+/*
+Copyright 2021 The Vitess Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+	http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package concurrency
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestErrorGroup(t *testing.T) {
+	t.Run("Wait() returns immediately when NumGoroutines = 0", func(t *testing.T) {
+		eg := ErrorGroup{
+			NumGoroutines: 0,
+		}
+
+		rec := eg.Wait(nil, nil)
+		assert.NoError(t, rec.Error())
+	})
+}