Preflight report output filenames

internal/fileutil/fileutil.go

@@ -8,11 +8,28 @@ import (
 	"path/filepath"
 )
 
+const (
+	maxFileNameBytes     = 255
+	temporaryFilePattern = ".weatherreporter-*.tmp"
+)
+
+// ValidateAtomicPath verifies that the final path can be safely used with this
+// package's same-directory atomic-write implementation.
+func ValidateAtomicPath(path string) error {
+	if len(filepath.Base(path)) > maxFileNameBytes {
+		return fmt.Errorf("final file name exceeds the %d-byte limit", maxFileNameBytes)
+	}
+	return nil
+}
+
 func WriteFileAtomic(path string, data []byte) error {
+	if err := ValidateAtomicPath(path); err != nil {
+		return err
+	}
 	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")
+	tmp, err := os.CreateTemp(filepath.Dir(path), temporaryFilePattern)
 	if err != nil {
 		return fmt.Errorf("create temporary file for %q: %w", path, err)
 	}