core/dnsserver: support explicit registration for embedded hosts (#8525)

* core/dnsserver: test host-plugin embedding with forward

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

* core/dnsserver: add a directive setter for embedded hosts

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

* core/dnsserver: support explicit server registration for embedded hosts

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

---------

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

core/dnsserver/directives_test.go

@@ -0,0 +1,73 @@
+package dnsserver
+
+import (
+	"slices"
+	"strings"
+	"testing"
+
+	"github.com/coredns/caddy"
+)
+
+func TestSetDirectives(t *testing.T) {
+	original := Directives
+	t.Cleanup(func() { Directives = original })
+
+	for _, tc := range []struct {
+		name       string
+		directives []string
+		wantError  string
+	}{
+		{name: "ordered", directives: []string{"bind", "test_host", "forward"}},
+		{name: "replacement", directives: []string{"whoami", "bind"}},
+		{name: "nil", directives: nil},
+		{name: "empty", directives: []string{}},
+		{name: "empty name", directives: []string{"bind", ""}, wantError: "empty directive name"},
+		{name: "duplicate", directives: []string{"bind", "forward", "bind"}, wantError: `duplicate directive "bind"`},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			Directives = []string{"test_previous"}
+			previous := Directives
+			input := slices.Clone(tc.directives)
+			err := SetDirectives(input)
+			if tc.wantError != "" {
+				if err == nil || !strings.Contains(err.Error(), tc.wantError) {
+					t.Fatalf("SetDirectives(%v) = %v, want %q", input, err, tc.wantError)
+				}
+				if !slices.Equal(Directives, previous) || &Directives[0] != &previous[0] {
+					t.Fatalf("invalid list changed Directives: got %v, want %v", Directives, previous)
+				}
+			} else {
+				if err != nil {
+					t.Fatal(err)
+				}
+				if got := caddy.ValidDirectives("dns"); !slices.Equal(got, tc.directives) {
+					t.Fatalf("registered server directives = %v, want %v", got, tc.directives)
+				}
+				if len(input) > 0 {
+					input[0] = "test_mutated"
+					if !slices.Equal(Directives, tc.directives) {
+						t.Fatalf("caller mutation changed Directives: %v", Directives)
+					}
+					input[0] = tc.directives[0]
+				}
+			}
+			if !slices.Equal(input, tc.directives) {
+				t.Fatalf("SetDirectives changed the input: got %v, want %v", input, tc.directives)
+			}
+		})
+	}
+}
+
+func TestSetDirectivesCopiesCurrentList(t *testing.T) {
+	original := Directives
+	t.Cleanup(func() { Directives = original })
+	Directives = []string{"bind", "test_host", "forward"}
+	previous := Directives
+	if err := SetDirectives(Directives[1:]); err != nil {
+		t.Fatal(err)
+	}
+	previous[1] = "test_mutated"
+	if want := []string{"test_host", "forward"}; !slices.Equal(Directives, want) {
+		t.Fatalf("old list mutation changed Directives: got %v, want %v", Directives, want)
+	}
+}