Centralize atomic artifact writes

internal/fileutil/fileutil.go

@@ -0,0 +1,48 @@
+// Package fileutil provides narrow filesystem helpers for durable artifacts.
+package fileutil
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+func WriteFileAtomic(path string, data []byte) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return fmt.Errorf("create directory %q: %w", filepath.Dir(path), err)
+	}
+	tmp, err := os.CreateTemp(filepath.Dir(path), "."+filepath.Base(path)+".*.tmp")
+	if err != nil {
+		return fmt.Errorf("create temporary file for %q: %w", path, err)
+	}
+	tmpName := tmp.Name()
+	defer os.Remove(tmpName)
+	if _, err := tmp.Write(data); err != nil {
+		tmp.Close()
+		return fmt.Errorf("write temporary file for %q: %w", path, err)
+	}
+	if err := tmp.Close(); err != nil {
+		return fmt.Errorf("close temporary file for %q: %w", path, err)
+	}
+	if err := os.Rename(tmpName, path); err != nil {
+		return fmt.Errorf("save %q: %w", path, err)
+	}
+	return nil
+}
+
+func WriteJSONAtomic(path string, value any) error {
+	data, err := json.MarshalIndent(value, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshal %q: %w", path, err)
+	}
+	return WriteFileAtomic(path, data)
+}
+
+func CopyFileAtomic(source string, target string) error {
+	data, err := os.ReadFile(source)
+	if err != nil {
+		return fmt.Errorf("read %q: %w", source, err)
+	}
+	return WriteFileAtomic(target, data)
+}