Update tracer to guard for a nil ctx

sdk/trace/trace_test.go

@@ -363,6 +363,21 @@ func TestStartSpanWithParent(t *testing.T) {
 	}
 }
 
+// Test we get a successful span as a new root if a nil context is sent in, as opposed to a panic.
+// See https://github.com/open-telemetry/opentelemetry-go/issues/3109
+func TestStartSpanWithNilContext(t *testing.T) {
+	alwaysSampleTp := NewTracerProvider()
+	sampledTr := alwaysSampleTp.Tracer("AlwaysSampled")
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Error("unexpected panic creating span with nil context")
+		}
+	}()
+	//nolint // no nil context, but that's the point of the test
+	sampledTr.Start(nil, "should-not-panic")
+}
+
 func TestStartSpanNewRootNotSampled(t *testing.T) {
 	alwaysSampleTp := NewTracerProvider()
 	sampledTr := alwaysSampleTp.Tracer("AlwaysSampled")

sdk/trace/tracer.go

@@ -37,6 +37,11 @@ var _ trace.Tracer = &tracer{}
 func (tr *tracer) Start(ctx context.Context, name string, options ...trace.SpanStartOption) (context.Context, trace.Span) {
 	config := trace.NewSpanStartConfig(options...)
 
+	// If ctx is nil, set to context.Background() as context.WithValue will panic on a nil value.
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
 	// For local spans created by this SDK, track child span count.
 	if p := trace.SpanFromContext(ctx); p != nil {
 		if sdkSpan, ok := p.(*recordingSpan); ok {