Fixed race condition in OnEnd and added a unit test (#3951)

* Fixed race condition in OnEnd and added a test

* fixed code review comments

* fixed lint

* Update CHANGELOG.md

Co-authored-by: Robert Pająk <pellared@hotmail.com>

* Update sdk/trace/simple_span_processor_test.go

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

* Update sdk/trace/simple_span_processor_test.go

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

* Update sdk/trace/simple_span_processor_test.go

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

* fixed panic check

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>
Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

sdk/trace/simple_span_processor_test.go

@@ -17,9 +17,12 @@ package trace_test
 import (
 	"context"
 	"errors"
+	"sync"
 	"testing"
 	"time"
 
+	"github.com/stretchr/testify/assert"
+
 	sdktrace "go.opentelemetry.io/otel/sdk/trace"
 	"go.opentelemetry.io/otel/trace"
 )
@@ -150,6 +153,32 @@ func TestSimpleSpanProcessorShutdownOnEndConcurrency(t *testing.T) {
 	<-done
 }
 
+func TestSimpleSpanProcessorShutdownOnEndRace(t *testing.T) {
+	exporter := &testExporter{}
+	ssp := sdktrace.NewSimpleSpanProcessor(exporter)
+	tp := basicTracerProvider(t)
+	tp.RegisterSpanProcessor(ssp)
+
+	var wg sync.WaitGroup
+	wg.Add(2)
+
+	span := func(spanName string) {
+		assert.NotPanics(t, func() {
+			defer wg.Done()
+			_, span := tp.Tracer("test").Start(context.Background(), spanName)
+			span.End()
+		})
+	}
+
+	go span("test-span-1")
+	go span("test-span-2")
+
+	wg.Wait()
+
+	assert.NoError(t, ssp.Shutdown(context.Background()))
+	assert.True(t, exporter.shutdown, "exporter shutdown")
+}
+
 func TestSimpleSpanProcessorShutdownHonorsContextDeadline(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), time.Nanosecond)
 	defer cancel()