Map LINE login errors and stop leaking raw errors into login copy

Wait() returned plain errors for verification and login failures, so the
provisioning API replaced them with a generic 500 M_UNKNOWN "Internal error in
login step". Map them onto declared RespErrors instead, reusing the existing
parseLoginErrorDetails so LINE's own short reason strings are quoted where it
gives one.

Also stop formatting the raw Go error into the login form instructions when no
reason could be parsed. That put internal detail directly in user-facing copy;
it now shows a fixed message.

CreateLogin ignored flowID entirely and returned the email login for any value,
including a typo. Validate it against the advertised flow ID and return
bridgev2.ErrInvalidLoginFlowID otherwise, and give that ID a named constant so
the flow list and the check cannot drift apart.

Note this does not change the larger contract question: a rejected password is
still reported as a fresh user_input step on HTTP 200 rather than an error, so
clients cannot distinguish it from a legitimate next step. That is worth
deciding deliberately before changing.

Author: Fizzadar

pkg/connector/client_lifecycle_test.go

@@ -78,11 +78,11 @@ func TestLineClientDisconnectBeforeConnectRejectsStartup(t *testing.T) {
 
 func TestCreateLoginSharesFinalizationLock(t *testing.T) {
 	connector := &LineConnector{}
-	firstProcess, err := connector.CreateLogin(context.Background(), nil, "")
+	firstProcess, err := connector.CreateLogin(context.Background(), nil, LoginFlowIDEmail)
 	if err != nil {
 		t.Fatalf("first CreateLogin returned error: %v", err)
 	}
-	secondProcess, err := connector.CreateLogin(context.Background(), nil, "")
+	secondProcess, err := connector.CreateLogin(context.Background(), nil, LoginFlowIDEmail)
 	if err != nil {
 		t.Fatalf("second CreateLogin returned error: %v", err)
 	}

pkg/connector/connector.go

@@ -145,15 +145,20 @@ func (lc *LineConnector) LoadUserLogin(ctx context.Context, login *bridgev2.User
 	return nil
 }
 
+const LoginFlowIDEmail = "dev.highest.matrix.line.email_login"
+
 func (lc *LineConnector) GetLoginFlows() []bridgev2.LoginFlow {
 	return []bridgev2.LoginFlow{{
 		Name:        "Login",
 		Description: "Login with your LINE Email and Password",
-		ID:          "dev.highest.matrix.line.email_login",
+		ID:          LoginFlowIDEmail,
 	}}
 }
 
 func (lc *LineConnector) CreateLogin(ctx context.Context, user *bridgev2.User, flowID string) (bridgev2.LoginProcess, error) {
+	if flowID != LoginFlowIDEmail {
+		return nil, bridgev2.ErrInvalidLoginFlowID
+	}
 	return &LineEmailLogin{User: user, finalizeMu: &lc.loginFinalizeMu}, nil
 }
 
@@ -233,7 +238,7 @@ func (ll *LineEmailLogin) StartWithOverride(ctx context.Context, override *bridg
 		ll.logLoginFailure(err, "reconnect")
 		reason := loginErrorReason(err)
 		if reason == "" {
-			reason = fmt.Sprintf("Login failed: %v", err)
+			reason = genericLoginFailureReason
 		}
 		return ll.loginErrorStep(reason), nil
 	}
@@ -260,7 +265,7 @@ func (ll *LineEmailLogin) SubmitUserInput(ctx context.Context, input map[string]
 		ll.logLoginFailure(err, "credentials")
 		reason := loginErrorReason(err)
 		if reason == "" {
-			reason = fmt.Sprintf("Login failed: %v", err)
+			reason = genericLoginFailureReason
 		}
 		return ll.loginErrorStep(reason), nil
 	}
@@ -456,10 +461,10 @@ func (ll *LineEmailLogin) Wait(ctx context.Context) (*bridgev2.LoginStep, error)
 			if res.AuthToken != "" {
 				return ll.finishLogin(ctx, res)
 			}
-			return nil, fmt.Errorf("verification failed: no auth token received")
+			return nil, ErrLoginVerificationFailed
 		case err := <-ll.pollErr:
 			ll.logLoginFailure(err, "verification_poll")
-			return nil, fmt.Errorf("verification failed: %w", err)
+			return nil, wrapLineLoginError(err)
 		case <-ctx.Done():
 			return nil, ctx.Err()
 		}
@@ -469,7 +474,7 @@ func (ll *LineEmailLogin) Wait(ctx context.Context) (*bridgev2.LoginStep, error)
 		res, err := loginWithCredentials(ll.Email, ll.Password, ll.Certificate)
 		if err != nil {
 			ll.logLoginFailure(err, "pin_continuation")
-			return nil, fmt.Errorf("login failed: %w", err)
+			return nil, wrapLineLoginError(err)
 		}
 		return ll.handleLoginResponse(ctx, res)
 	}
@@ -581,7 +586,7 @@ func (ll *LineEmailLogin) finishLogin(ctx context.Context, res *line.LoginResult
 		ll.User.Bridge.Log.Info().Int("keys", len(meta.ExportedKeyMap)).Msg("Preserved existing E2EE keys after re-login")
 	}
 	if !res.NoE2EE && len(meta.ExportedKeyMap) == 0 {
-		return nil, fmt.Errorf("LINE login completed without E2EE keychain; please reconnect again and complete the LINE verification prompt")
+		return nil, ErrLoginNoKeychain
 	}
 
 	detectedLineID := networkid.UserLoginID(mid)

pkg/connector/loginerrors.go

@@ -0,0 +1,67 @@
+package connector
+
+import (
+	"fmt"
+	"net/http"
+
+	"maunium.net/go/mautrix/bridgev2"
+)
+
+// genericLoginFailureReason is shown in the login form when LINE rejects the sign-in but
+// gives no reason we can quote. It replaces formatting the raw Go error into the
+// instructions, which leaked internal detail into user-facing copy.
+const genericLoginFailureReason = "LINE rejected the sign-in. Please check your email and password and try again."
+
+var (
+	ErrLoginVerificationFailed = bridgev2.RespError{
+		ErrCode:    "DEV.HIGHEST.LINE.VERIFICATION_FAILED",
+		Err:        "LINE didn't confirm the verification. Please start the login again.",
+		StatusCode: http.StatusBadRequest,
+	}
+	ErrLoginNoKeychain = bridgev2.RespError{
+		ErrCode:    "DEV.HIGHEST.LINE.NO_KEYCHAIN",
+		Err:        "LINE finished signing in without sending the encryption keychain. Please reconnect and complete the verification prompt in the LINE app.",
+		StatusCode: http.StatusBadRequest,
+	}
+	ErrLoginTooManyAttempts = bridgev2.RespError{
+		ErrCode:    "DEV.HIGHEST.LINE.TOO_MANY_ATTEMPTS",
+		Err:        loginTooManyAttemptsReason,
+		StatusCode: http.StatusTooManyRequests,
+	}
+	ErrLoginRejected = bridgev2.RespError{
+		ErrCode:    "DEV.HIGHEST.LINE.LOGIN_REJECTED",
+		Err:        genericLoginFailureReason,
+		StatusCode: http.StatusUnauthorized,
+	}
+	ErrLoginUnknown = bridgev2.RespError{
+		ErrCode:    "M_UNKNOWN",
+		Err:        "Internal error logging in to LINE",
+		StatusCode: http.StatusInternalServerError,
+	}
+)
+
+// wrapLineLoginError translates a LINE error into one the client can act on, keeping the
+// original in the chain with %w so logs are unaffected.
+func wrapLineLoginError(err error) error {
+	if err == nil {
+		return nil
+	}
+	mapped := ErrLoginUnknown
+	details := parseLoginErrorDetails(err)
+	reason := details.ErrorReason
+	if reason == "" {
+		reason = details.ErrorMessage
+	}
+	switch {
+	case isBlockedUserLoginError(reason):
+		mapped = ErrLoginTooManyAttempts
+	case details.HTTPStatus == http.StatusTooManyRequests:
+		mapped = ErrLoginTooManyAttempts
+	case details.HTTPStatus == http.StatusUnauthorized, details.HTTPStatus == http.StatusForbidden:
+		mapped = ErrLoginRejected
+	case reason != "":
+		// LINE's own reason strings are short and user-facing, so quote them.
+		mapped = ErrLoginRejected.WithMessage("%s", reason)
+	}
+	return fmt.Errorf("%w: %w", mapped, err)
+}