refactor(server): generate runtime configs and use .build/config for development

go/internal/server/configfile/default.go

@@ -0,0 +1,51 @@
+// Package configfile publishes default configuration without replacing user files.
+package configfile
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"log/slog"
+	"os"
+	"path/filepath"
+)
+
+// Ensure publishes a complete JSON file only when path does not exist.
+// Linking a same-directory temporary file makes publication atomic and refuses
+// to replace a configuration created by another process in the meantime.
+func Ensure(path string, defaults any) error {
+	if _, err := os.Stat(path); err == nil {
+		return nil
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return err
+	}
+	data, err := json.MarshalIndent(defaults, "", "  ")
+	if err != nil {
+		return err
+	}
+	if err = os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return fmt.Errorf("create configuration directory: %w", err)
+	}
+	file, err := os.CreateTemp(filepath.Dir(path), ".config-*.tmp")
+	if err != nil {
+		return err
+	}
+	defer os.Remove(file.Name())
+	if _, err = file.Write(append(data, '\n')); err == nil {
+		err = file.Sync()
+	}
+	closeErr := file.Close()
+	if err != nil {
+		return err
+	}
+	if closeErr != nil {
+		return closeErr
+	}
+	if err = os.Link(file.Name(), path); errors.Is(err, os.ErrExist) {
+		return nil
+	} else if err != nil {
+		return fmt.Errorf("create default configuration %s: %w", path, err)
+	}
+	slog.Info("default configuration created", "path", path)
+	return nil
+}