Fix export overwrite confirmation on EOF (#1342)

Sources/tart/Commands/Export.swift

@@ -37,7 +37,7 @@ struct Export: AsyncParsableCommand {
   func userWantsOverwrite(_ filename: String) -> Bool {
     print("file \(filename) already exists, are you sure you want to overwrite it? (yes, [no])? ", terminator: "")
 
-    let answer = readLine()!
+    let answer = readLine()
 
     return answer == "yes"
   }

integration-tests/export_darwin_test.go

@@ -0,0 +1,60 @@
+package integration_test
+
+import (
+	"bytes"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestExportOverwriteConfirmation(t *testing.T) {
+	for _, tt := range []struct {
+		name      string
+		input     string
+		overwrite bool
+	}{
+		{name: "EOF"},
+		{name: "empty line", input: "\n"},
+		{name: "no", input: "no\n"},
+		{name: "yes", input: "yes\n", overwrite: true},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			home := t.TempDir()
+			t.Setenv("TART_HOME", home)
+			t.Setenv("TART_NO_AUTO_PRUNE", "1")
+			createSyntheticVM(t, home, "source", "92:81:b5:ab:39:37")
+
+			directory := t.TempDir()
+			destination := filepath.Join(directory, "source.tvm")
+			original := []byte("existing archive")
+			if err := os.WriteFile(destination, original, 0600); err != nil {
+				t.Fatal(err)
+			}
+
+			cmd := exec.CommandContext(t.Context(), "tart", "export", "source")
+			cmd.Dir = directory
+			cmd.Stdin = strings.NewReader(tt.input)
+			output, err := cmd.CombinedOutput()
+			if err != nil {
+				t.Fatalf("export: %v: %s", err, output)
+			}
+			if !strings.Contains(string(output), "are you sure you want to overwrite it?") {
+				t.Fatalf("expected overwrite confirmation: %s", output)
+			}
+			if strings.Contains(string(output), "exporting...") != tt.overwrite {
+				t.Fatalf("unexpected export behavior: %s", output)
+			}
+
+			current, err := os.ReadFile(destination)
+			if err != nil {
+				t.Fatal(err)
+			}
+			changed := !bytes.Equal(original, current)
+			if changed != tt.overwrite {
+				t.Fatalf("destination changed = %t, want %t", changed, tt.overwrite)
+			}
+		})
+	}
+}