Keep a hop's parent after rewriting its header

traceOf read the caller's ids as slices of a view over the header
buffer, then rewrote that buffer in place: every shipped span named
itself as its parent, and once the connection served its next request
the span carried that request's trace id.

Claude-Session: https://claude.ai/code/session_01HuMDfgFssPUptYUzgZ6Cp8

telemetry.go

@@ -653,7 +653,11 @@ type trace struct {
 // request is the one that has to begin it.
 func traceOf(c *Ctx) trace {
 	t := trace{}
-	if id, parent, ok := parseTrace(c.fc.Get(HeaderTrace)); ok {
+	// CLONED, because the ids are slices of it and it is a view over the header
+	// buffer: the Set below rewrites that buffer in place, which would turn the
+	// parent into this hop's own span, and the connection's next request rewrites
+	// it again before the span ships.
+	if id, parent, ok := parseTrace(strings.Clone(c.fc.Get(HeaderTrace))); ok {
 		t.trace, t.parent = id, parent
 	} else {
 		t.trace = mint(16)

traceparent_test.go

@@ -54,3 +54,20 @@ func TestTraceparentHook(t *testing.T) {
 		t.Fatalf("hook answer: sent %q, want %q over the request's", overridden, own)
 	}
 }
+
+// The span a hop ships names its caller as parent and its own id as itself, even
+// after the hop has rewritten the header the caller's ids were read from, and
+// after the connection's next request has overwritten it.
+func TestTraceOfKeepsTheCallersIDs(t *testing.T) {
+	const (
+		caller = "4bf92f3577b34da6a3ce929d0e0e4736"
+		parent = "00f067aa0ba902b7"
+	)
+	c := New(Config{AppName: "front", DisableStartupMessage: true}).TestCtx("GET", "/ask")
+	c.fc.Request().Header.Set(HeaderTrace, "00-"+caller+"-"+parent+"-01")
+	tr := traceOf(c)
+	c.fc.Request().Header.Set(HeaderTrace, "00-ffffffffffffffffffffffffffffffff-ffffffffffffffff-01")
+	if tr.trace != caller || tr.parent != parent || tr.span == parent {
+		t.Fatalf("trace = %+v, want trace %s parent %s and a span of its own", tr, caller, parent)
+	}
+}