fix: check a password even when the username does not exist

internal/app/identity/account.go

@@ -117,6 +117,13 @@ func (a *Accounts) Login(ctx context.Context, current member.Member, currentToke
 func (a *Accounts) verify(ctx context.Context, username, password string) (port.Account, error) {
 	account, err := a.members.AccountByUsername(ctx, username)
 	if errors.Is(err, port.ErrMemberNotFound) {
+		dummy, err := dummyPasswordHash()
+		if err != nil {
+			return port.Account{}, err
+		}
+		if _, err := checkPassword(ctx, dummy, password); err != nil {
+			return port.Account{}, err
+		}
 		return port.Account{}, ErrInvalidCredentials
 	}
 	if err != nil {

internal/app/identity/account_test.go

@@ -450,3 +450,21 @@ func TestLoginRejectedWhenGuestSessionOrPasswordChangesMeanwhile(t *testing.T) {
 		t.Error("登录失败时游客的 token 应仍然有效")
 	}
 }
+
+// 名额占满、请求已经取消时，要做密码计算的请求以 context.Canceled 结束，不做的直接返回结果。
+func TestLoginDoesPasswordWorkForUnknownUsername(t *testing.T) {
+	a, s, _, _, _ := newAccounts(t)
+	registerAccount(t, a, s, "alice", "correct horse")
+	guest := createMember(t, s)
+	if _, err := dummyPasswordHash(); err != nil {
+		t.Fatal(err)
+	}
+	occupyArgonSlots(t)
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+	for _, username := range []string{"alice", "nobody"} {
+		if _, _, err := a.Login(ctx, guest.Member, guest.MemberToken, clientIP, username, "wrong horse"); !errors.Is(err, context.Canceled) {
+			t.Errorf("用户名 %s：Login = %v，期望同样等待密码计算的名额", username, err)
+		}
+	}
+}

internal/app/identity/password.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"runtime"
 	"strings"
+	"sync"
 
 	"golang.org/x/crypto/argon2"
 )
@@ -52,6 +53,12 @@ func HashPassword(ctx context.Context, password string) (string, error) {
 	return passwordPrefix + base64.RawStdEncoding.EncodeToString(salt) + "$" + base64.RawStdEncoding.EncodeToString(key), nil
 }
 
+// 用户名不存在时拿它做一次同样的计算，响应时间不暴露用户名是否存在。密码是随机值，不会与任何人输入的密码匹配。
+var dummyPasswordHash = sync.OnceValues(func() (string, error) {
+	password, _ := newSecret()
+	return HashPassword(context.Background(), password)
+})
+
 // 只接受按当前参数生成的哈希：存储中的密码哈希都由 HashPassword 生成。
 func checkPassword(ctx context.Context, hash, password string) (bool, error) {
 	encoded, ok := strings.CutPrefix(hash, passwordPrefix)

internal/app/identity/password_test.go

@@ -87,3 +87,18 @@ func TestPasswordWorkReturnsItsSlot(t *testing.T) {
 		}
 	}
 }
+
+func TestDummyPasswordHashMatchesNoPassword(t *testing.T) {
+	hash, err := dummyPasswordHash()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if again, _ := dummyPasswordHash(); again != hash {
+		t.Error("假哈希应只生成一次")
+	}
+	for _, password := range []string{"", "correct horse", "12345678"} {
+		if ok, err := checkPassword(t.Context(), hash, password); err != nil || ok {
+			t.Errorf("checkPassword(假哈希, %q) = %v, %v，期望能解析且不匹配", password, ok, err)
+		}
+	}
+}