Test exclusive tone-test output creation

tonetest_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"os"
 	"path/filepath"
 	"testing"
 )
@@ -25,6 +26,23 @@ func TestNewAudioFileSaverReportsUnavailableOutputPath(t *testing.T) {
 	}
 }
 
+func TestNewAudioFileSaverDoesNotOverwriteExistingOutput(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "tone.pcm")
+	if err := os.WriteFile(path, []byte("existing"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	if saver, err := NewAudioFileSaver(path); err == nil || saver != nil {
+		t.Fatalf("got saver=%v err=%v", saver, err)
+	}
+	contents, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(contents) != "existing" {
+		t.Fatalf("output was overwritten: %q", contents)
+	}
+}
+
 type testDetacher struct{ detached bool }
 
 func (d *testDetacher) Detach() { d.detached = true }