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

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)
+	}
+}