fix(file): fix data race in tree Elem.Name (#7574)

Eagerly set name in newElem and make Name() read-only to avoid
racy lazy writes under concurrent lookups. Add tests for empty-name
comparisons and concurrent access to Less/Name(). In addition,
regression tests to CloudDNS plugin.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/file/tree/elem_test.go

@@ -0,0 +1,41 @@
+package tree
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+// Test that Name() falls back to reading from the stored RRs when the cached name is empty.
+func TestElemName_FallbackWhenCachedEmpty(t *testing.T) {
+	rr, err := dns.NewRR("a.example. 3600 IN A 1.2.3.4")
+	if err != nil {
+		t.Fatalf("failed to create RR: %v", err)
+	}
+
+	// Build via newElem to ensure m is populated
+	e := newElem(rr)
+	got := e.Name()
+	want := "a.example."
+	if got != want {
+		t.Fatalf("unexpected name; want %q, got %q", want, got)
+	}
+
+	// clear the cached name
+	e.name = ""
+
+	got = e.Name()
+	want = "a.example."
+	if got != want {
+		t.Fatalf("unexpected name; want %q, got %q", want, got)
+	}
+
+	// clear the map
+	e.m = make(map[uint16][]dns.RR, 0)
+
+	got = e.Name()
+	want = ""
+	if got != want {
+		t.Fatalf("unexpected name after clearing RR map; want %q, got %q", want, got)
+	}
+}