Default implementation for empty BatchProcessor (#5239)

Ensure an empty BatchProcessor does not panic when any method is called.
Default an empty BatchProcessor as being shut down.

sdk/log/batch.go

@@ -29,7 +29,9 @@ const (
 var _ Processor = (*BatchProcessor)(nil)
 
 // BatchProcessor is a processor that exports batches of log records.
-// A BatchProcessor must be created with [NewBatchProcessor].
+//
+// Use [NewBatchProcessor] to create a BatchProcessor. An empty BatchProcessor
+// is shut down by default, no records will be batched or exported.
 type BatchProcessor struct {
 	// The BatchProcessor is designed to provide the highest throughput of
 	// log records possible while being compatible with OpenTelemetry. The
@@ -170,7 +172,7 @@ func (b *BatchProcessor) poll(interval time.Duration) (done chan struct{}) {
 
 // OnEmit batches provided log record.
 func (b *BatchProcessor) OnEmit(_ context.Context, r Record) error {
-	if b.stopped.Load() {
+	if b.stopped.Load() || b.q == nil {
 		return nil
 	}
 	if n := b.q.Enqueue(r); n >= b.batchSize {
@@ -187,12 +189,12 @@ func (b *BatchProcessor) OnEmit(_ context.Context, r Record) error {
 
 // Enabled returns if b is enabled.
 func (b *BatchProcessor) Enabled(context.Context, Record) bool {
-	return !b.stopped.Load()
+	return !b.stopped.Load() && b.q != nil
 }
 
 // Shutdown flushes queued log records and shuts down the decorated exporter.
 func (b *BatchProcessor) Shutdown(ctx context.Context) error {
-	if b.stopped.Swap(true) {
+	if b.stopped.Swap(true) || b.q == nil {
 		return nil
 	}
 
@@ -219,7 +221,7 @@ var ctxErr = func(ctx context.Context) error {
 
 // ForceFlush flushes queued log records and flushes the decorated exporter.
 func (b *BatchProcessor) ForceFlush(ctx context.Context) error {
-	if b.stopped.Load() {
+	if b.stopped.Load() || b.q == nil {
 		return nil
 	}
 

sdk/log/batch_test.go

@@ -18,6 +18,18 @@ import (
 	"go.opentelemetry.io/otel/log"
 )
 
+func TestEmptyBatchConfig(t *testing.T) {
+	assert.NotPanics(t, func() {
+		var bp BatchProcessor
+		ctx := context.Background()
+		var record Record
+		assert.NoError(t, bp.OnEmit(ctx, record), "OnEmit")
+		assert.False(t, bp.Enabled(ctx, record), "Enabled")
+		assert.NoError(t, bp.ForceFlush(ctx), "ForceFlush")
+		assert.NoError(t, bp.Shutdown(ctx), "Shutdown")
+	})
+}
+
 func TestNewBatchConfig(t *testing.T) {
 	otel.SetErrorHandler(otel.ErrorHandlerFunc(func(err error) {
 		t.Log(err)