Stop accessing user data in downstreamConn.authenticate

This becomes racy once user.Password is updated on-the-fly.

db.go

@@ -106,6 +106,21 @@ func (db *DB) ListUsers() ([]User, error) {
 	return users, nil
 }
 
+func (db *DB) GetUser(username string) (*User, error) {
+	db.lock.RLock()
+	defer db.lock.RUnlock()
+
+	user := &User{Username: username}
+
+	var password *string
+	row := db.db.QueryRow("SELECT password FROM User WHERE username = ?", username)
+	if err := row.Scan(&password); err != nil {
+		return nil, err
+	}
+	user.Password = fromStringPtr(password)
+	return user, nil
+}
+
 func (db *DB) CreateUser(user *User) error {
 	db.lock.Lock()
 	defer db.lock.Unlock()

downstream.go

@@ -586,19 +586,23 @@ func unmarshalUsername(rawUsername string) (username, network string) {
 func (dc *downstreamConn) authenticate(username, password string) error {
 	username, networkName := unmarshalUsername(username)
 
-	u := dc.srv.getUser(username)
+	u, err := dc.srv.db.GetUser(username)
-	if u == nil {
-		dc.logger.Printf("failed authentication for %q: unknown username", username)
-		return errAuthFailed
-	}
-
-	err := bcrypt.CompareHashAndPassword([]byte(u.Password), []byte(password))
 	if err != nil {
 		dc.logger.Printf("failed authentication for %q: %v", username, err)
 		return errAuthFailed
 	}
 
-	dc.user = u
+	err = bcrypt.CompareHashAndPassword([]byte(u.Password), []byte(password))
+	if err != nil {
+		dc.logger.Printf("failed authentication for %q: %v", username, err)
+		return errAuthFailed
+	}
+
+	dc.user = dc.srv.getUser(username)
+	if dc.user == nil {
+		dc.logger.Printf("failed authentication for %q: user not active", username)
+		return errAuthFailed
+	}
 	dc.networkName = networkName
 	return nil
 }