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/less_test.go

@@ -3,7 +3,10 @@ package tree
 import (
 	"sort"
 	"strings"
+	"sync"
 	"testing"
+
+	"github.com/miekg/dns"
 )
 
 type set []string
@@ -78,3 +81,41 @@ Tests:
 		}
 	}
 }
+
+func TestLess_EmptyVsName(t *testing.T) {
+	if d := less("", "a."); d >= 0 {
+		t.Fatalf("expected < 0, got %d", d)
+	}
+	if d := less("a.", ""); d <= 0 {
+		t.Fatalf("expected > 0, got %d", d)
+	}
+}
+
+func TestLess_EmptyVsEmpty(t *testing.T) {
+	if d := less("", ""); d != 0 {
+		t.Fatalf("expected 0, got %d", d)
+	}
+}
+
+// Test that concurrent calls to Less (which calls Elem.Name) do not race or panic.
+// See issue #7561 for reference.
+func TestLess_ConcurrentNameAccess(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)
+	}
+	e := newElem(rr)
+
+	const n = 200
+	var wg sync.WaitGroup
+	wg.Add(n)
+	for range n {
+		go func() {
+			defer wg.Done()
+			// Compare the same name repeatedly; previously this could race due to lazy Name() writes.
+			_ = Less(e, "a.example.")
+			_ = e.Name()
+		}()
+	}
+	wg.Wait()
+}