testscript: add RequireUniqueNames parameter

testscript/testdata/testscript_duplicate_name.txt

@@ -0,0 +1,11 @@
+# Check that RequireUniqueNames works;
+# it should reject txtar archives with duplicate names as defined by the host system.
+
+unquote scripts-normalized/testscript.txt
+
+testscript scripts-normalized
+! testscript -unique-names scripts-normalized
+
+-- scripts-normalized/testscript.txt --
+>-- file --
+>-- dir/../file --

testscript/testscript.go

@@ -14,6 +14,7 @@ import (
 	"flag"
 	"fmt"
 	"go/build"
+	"io/fs"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -174,6 +175,10 @@ type Params struct {
 	// executions explicit.
 	RequireExplicitExec bool
 
+	// RequireUniqueNames requires that names in the txtar archive are unique.
+	// By default, later entries silently overwrite earlier ones.
+	RequireUniqueNames bool
+
 	// ContinueOnError causes a testscript to try to continue in
 	// the face of errors. Once an error has occurred, the script
 	// will continue as if in verbose mode.
@@ -426,6 +431,16 @@ func (ts *TestScript) setup() string {
 	ts.archive = a
 	for _, f := range a.Files {
 		name := ts.MkAbs(ts.expand(f.Name))
+		if ts.params.RequireUniqueNames {
+			switch _, err := os.Lstat(name); {
+			case err == nil:
+				ts.Fatalf("'%s' would overwrite '%s' (because RequireUniqueNames is enabled)", f.Name, name)
+			case errors.Is(err, fs.ErrNotExist):
+				// OK.
+			default:
+				ts.Fatalf("%s: %v", f.Name, err)
+			}
+		}
 		ts.scriptFiles[name] = f.Name
 		ts.Check(os.MkdirAll(filepath.Dir(name), 0o777))
 		ts.Check(ioutil.WriteFile(name, f.Data, 0o666))

testscript/testscript_test.go

@@ -188,6 +188,7 @@ func TestScripts(t *testing.T) {
 				fset := flag.NewFlagSet("testscript", flag.ContinueOnError)
 				fUpdate := fset.Bool("update", false, "update scripts when cmp fails")
 				fExplicitExec := fset.Bool("explicit-exec", false, "require explicit use of exec for commands")
+				fUniqueNames := fset.Bool("unique-names", false, "require unique names in txtar archive")
 				fVerbose := fset.Bool("v", false, "be verbose with output")
 				fContinue := fset.Bool("continue", false, "continue on error")
 				if err := fset.Parse(args); err != nil {
@@ -210,6 +211,7 @@ func TestScripts(t *testing.T) {
 						Dir:                 ts.MkAbs(dir),
 						UpdateScripts:       *fUpdate,
 						RequireExplicitExec: *fExplicitExec,
+						RequireUniqueNames:  *fUniqueNames,
 						Cmds: map[string]func(ts *TestScript, neg bool, args []string){
 							"some-param-cmd": func(ts *TestScript, neg bool, args []string) {
 							},