testscript: add RequireUniqueNames parameter

Co-authored-by: Roger Peppe <rogpeppe@gmail.com>

testscript/testdata/testscript_duplicate_name.txt

@@ -0,0 +1,12 @@
+# 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
+stdout '.* would overwrite .* \(because RequireUniqueNames is enabled\)'
+
+-- 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.
@@ -372,6 +377,22 @@ type backgroundCmd struct {
 	neg  bool // if true, cmd should fail
 }
 
+func writeFile(name string, data []byte, perm fs.FileMode, excl bool) error {
+	oflags := os.O_WRONLY | os.O_CREATE | os.O_TRUNC
+	if excl {
+		oflags |= os.O_EXCL
+	}
+	f, err := os.OpenFile(name, oflags, perm)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	if _, err := f.Write(data); err != nil {
+		return fmt.Errorf("cannot write file contents: %v", err)
+	}
+	return nil
+}
+
 // setup sets up the test execution temporary directory and environment.
 // It returns the comment section of the txtar archive.
 func (ts *TestScript) setup() string {
@@ -433,7 +454,12 @@ func (ts *TestScript) setup() string {
 		name := ts.MkAbs(ts.expand(f.Name))
 		ts.scriptFiles[name] = f.Name
 		ts.Check(os.MkdirAll(filepath.Dir(name), 0o777))
-		ts.Check(ioutil.WriteFile(name, f.Data, 0o666))
+		switch err := writeFile(name, f.Data, 0o666, ts.params.RequireUniqueNames); {
+		case ts.params.RequireUniqueNames && errors.Is(err, fs.ErrExist):
+			ts.Check(fmt.Errorf("%s would overwrite %s (because RequireUniqueNames is enabled)", f.Name, name))
+		default:
+			ts.Check(err)
+		}
 	}
 	// Run any user-defined setup.
 	if ts.params.Setup != nil {

testscript/testscript_test.go

@@ -213,6 +213,7 @@ 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 {
@@ -229,6 +230,7 @@ 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) {
 							},