feat(user): implement registration for the first user with concurrency handling

database/user.go

@@ -4,6 +4,7 @@ import (
 	"crypto/sha256"
 	"database/sql"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -81,12 +82,36 @@ func HashPassword(password string) string {
 	return hex.EncodeToString(hash[:])
 }
 
-// CreateUser inserts a new user into the database.
-func CreateUser(username, password, level string) (int64, error) {
+// ErrUsersExist is returned by RegisterFirstUser when the users table is not
+// empty. Registration is only ever allowed for the very first user.
+var ErrUsersExist = errors.New("registration rejected: only the first user can be registered via this endpoint")
+
+// RegisterFirstUser atomically creates the first user, but only while the users
+// table is empty. The emptiness check and the insert run inside a single
+// transaction. Because InitUserDB caps the pool at one connection, a concurrent
+// registration blocks at Begin until the in-flight transaction commits, so it
+// cannot observe the table as empty between the check and the insert. This
+// closes the check-then-insert race two simultaneous first-run registrations
+// would otherwise hit.
+func RegisterFirstUser(username, password, level string) (int64, error) {
 	hashedPassword := HashPassword(password)
 	registerDate := time.Now().Format("2006-01-02 15:04:05")
 
-	result, err := UserDB.Exec(
+	tx, err := UserDB.Begin()
+	if err != nil {
+		return 0, fmt.Errorf("failed to begin transaction: %w", err)
+	}
+	defer tx.Rollback()
+
+	var count int
+	if err := tx.QueryRow("SELECT COUNT(*) FROM users").Scan(&count); err != nil {
+		return 0, fmt.Errorf("failed to check existing users: %w", err)
+	}
+	if count > 0 {
+		return 0, errors.New("registration closed: users already exist")
+	}
+
+	result, err := tx.Exec(
 		"INSERT INTO users (username, password, level, register_date) VALUES (?, ?, ?, ?)",
 		username, hashedPassword, level, registerDate,
 	)
@@ -94,6 +119,9 @@ func CreateUser(username, password, level string) (int64, error) {
 		return 0, fmt.Errorf("failed to create user: %w", err)
 	}
 
+	if err := tx.Commit(); err != nil {
+		return 0, fmt.Errorf("failed to commit user creation: %w", err)
+	}
 	return result.LastInsertId()
 }
 

database/user_test.go

@@ -0,0 +1,80 @@
+package database
+
+import (
+	"errors"
+	"path/filepath"
+	"sync"
+	"testing"
+)
+
+// initTempDB opens a fresh user database in a temp directory and registers a
+// cleanup that closes it when the test finishes.
+func initTempDB(t *testing.T) {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "user.db")
+	if err := InitUserDB(path); err != nil {
+		t.Fatalf("InitUserDB: %v", err)
+	}
+	t.Cleanup(CloseUserDB)
+}
+
+func TestRegisterFirstUserClosedAfterFirst(t *testing.T) {
+	initTempDB(t)
+
+	if _, err := RegisterFirstUser("alice", "password1", "admin"); err != nil {
+		t.Fatalf("first registration should succeed: %v", err)
+	}
+
+	if _, err := RegisterFirstUser("bob", "password2", "admin"); !errors.Is(err, ErrUsersExist) {
+		t.Fatalf("second registration error = %v, want ErrUsersExist", err)
+	}
+
+	count, err := GetUserCount()
+	if err != nil {
+		t.Fatalf("GetUserCount: %v", err)
+	}
+	if count != 1 {
+		t.Fatalf("user count = %d, want 1", count)
+	}
+}
+
+func TestRegisterFirstUserConcurrentOnlyOneWins(t *testing.T) {
+	initTempDB(t)
+
+	const n = 8
+	var wg sync.WaitGroup
+	results := make(chan error, n)
+	for i := range n {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			_, err := RegisterFirstUser("user"+string(rune('a'+i)), "password", "admin")
+			results <- err
+		}(i)
+	}
+	wg.Wait()
+	close(results)
+
+	successes := 0
+	for err := range results {
+		switch {
+		case err == nil:
+			successes++
+		case errors.Is(err, ErrUsersExist):
+			// Expected for every registration that lost the race.
+		default:
+			t.Fatalf("unexpected error: %v", err)
+		}
+	}
+	if successes != 1 {
+		t.Fatalf("concurrent registrations: %d succeeded, want exactly 1", successes)
+	}
+
+	count, err := GetUserCount()
+	if err != nil {
+		t.Fatalf("GetUserCount: %v", err)
+	}
+	if count != 1 {
+		t.Fatalf("user count = %d, want 1", count)
+	}
+}