otelhttptrace: handle missing getconn hook without panic (#5965)

Fork of
#5187
updated with main branch and tests, this PR adds nil dereference check
for clientTracer.root in `end()` when span events are used instead of
sub spans

---------

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
Co-authored-by: Tonis Tiigi <tonistiigi@gmail.com>
Co-authored-by: Damien Mathieu <42@dmathieu.com>

CHANGELOG.md

@@ -8,6 +8,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Fixed
+
+- Possible nil dereference panic in `go.opentelemetry.io/contrib/instrumentation/net/http/httptrace/otelhttptrace`. (#5965)
+
 <!-- Released section -->
 <!-- Don't change this section unless doing release -->
 

instrumentation/net/http/httptrace/otelhttptrace/clienttrace.go

@@ -215,6 +215,10 @@ func (ct *clientTracer) start(hook, spanName string, attrs ...attribute.KeyValue
 
 func (ct *clientTracer) end(hook string, err error, attrs ...attribute.KeyValue) {
 	if !ct.useSpans {
+		// sometimes end may be called without previous start
+		if ct.root == nil {
+			ct.root = trace.SpanFromContext(ct.Context)
+		}
 		if err != nil {
 			attrs = append(attrs, attribute.String(hook+".error", err.Error()))
 		}

instrumentation/net/http/httptrace/otelhttptrace/test/clienttrace_test.go

@@ -250,6 +250,22 @@ func TestEndBeforeStartCreatesSpan(t *testing.T) {
 	require.Len(t, spans, 1)
 }
 
+func TestEndBeforeStartWithoutSubSpansDoesNotPanic(t *testing.T) {
+	sr := tracetest.NewSpanRecorder()
+	tp := trace.NewTracerProvider(trace.WithSpanProcessor(sr))
+	otel.SetTracerProvider(tp)
+
+	ct := otelhttptrace.NewClientTrace(context.Background(), otelhttptrace.WithoutSubSpans())
+
+	require.NotPanics(t, func() {
+		ct.DNSDone(httptrace.DNSDoneInfo{})
+	})
+
+	// no spans created because we were just using background context without span
+	// and Start wasn't called which would have started a span
+	require.Len(t, sr.Ended(), 0)
+}
+
 type clientTraceTestFixture struct {
 	Address      string
 	URL          string