Update tracer to guard for a nil ctx

CHANGELOG.md

@@ -20,6 +20,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - The exponential histogram mapping functions have been updated with
   exact upper-inclusive boundary support following the [corresponding
   specification change](https://github.com/open-telemetry/opentelemetry-specification/pull/2633). (#2982)
+- Attempting to start a span with a nil `context` will no longer cause a panic. (#3110)
 
 ## [1.9.0/0.0.3] - 2022-08-01
 

sdk/trace/trace_test.go

@@ -363,6 +363,16 @@ 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) {
+	tp := NewTracerProvider()
+	tr := tp.Tracer("NoPanic")
+
+	// nolint:staticcheck // no nil context, but that's the point of the test.
+	assert.NotPanics(t, func() { tr.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 == nil {
+		// Prevent trace.ContextWithSpan from panicking.
+		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 {