interp: record runner state at the first Reset call

This is perhaps a bit confusing, but it's a good middle ground. This
lets us support Reset while not undoing changes that the user has done
outside of New, which is common.

The downside is that making any changes after the first call to Run or
Reset are undone by further calls to Reset. However, that's fine; Reset
is meant to return a runner to its initial state, not any other previous
or arbitrary state.

interp/interp.go

@@ -57,12 +57,6 @@ func New(opts ...func(*Runner) error) (*Runner, error) {
 	if r.Stdout == nil || r.Stderr == nil {
 		StdIO(r.Stdin, r.Stdout, r.Stderr)(r)
 	}
-	r.origDir = r.Dir
-	r.origParams = r.Params
-	r.origOpts = r.opts
-	r.origStdin = r.Stdin
-	r.origStdout = r.Stdout
-	r.origStderr = r.Stderr
 	return r, nil
 }
 
@@ -494,8 +488,8 @@ const (
 	optGlobStar
 )
 
-// Reset returns a runner to its initial state, before any programs were
-// interpreted via the Run method.
+// Reset returns a runner to its initial state, right before the first call to
+// Run or Reset.
 //
 // Typically, this function only needs to be called if a runner is reused to run
 // multiple programs non-incrementally. Not calling Reset between each run will
@@ -505,6 +499,14 @@ func (r *Runner) Reset() {
 	if !r.usedNew {
 		panic("use interp.New to construct a Runner")
 	}
+	if !r.didReset {
+		r.origDir = r.Dir
+		r.origParams = r.Params
+		r.origOpts = r.opts
+		r.origStdin = r.Stdin
+		r.origStdout = r.Stdout
+		r.origStderr = r.Stderr
+	}
 	// reset the internal state
 	*r = Runner{
 		Env:         r.Env,

interp/interp_test.go

@@ -2958,8 +2958,11 @@ func TestRunnerResetFields(t *testing.T) {
 	r, _ := New(
 		Params("-f", "--", "first", dir, logFile.Name()),
 		Dir(dir),
-		StdIO(nil, logFile, os.Stderr),
 	)
+	// Check that using option funcs and Runner fields directly is still
+	// kept by Reset.
+	StdIO(nil, logFile, os.Stderr)(r)
+	r.Env = expand.ListEnviron(append(os.Environ(), "GLOBAL=foo")...)
 
 	file := parse(t, nil, `
 # Params set 3 arguments
@@ -2977,10 +2980,15 @@ echo line1
 echo line2
 [[ "$(wc -l <$3)" == "2" ]] || exit 14
 
+# $GLOBAL was set directly via the Env field
+[[ "$GLOBAL" == "foo" ]] || exit 15
+
 # Change all of the above within the script. Reset should undo this.
 set +f -- newargs
 cd
 exec >/dev/null 2>/dev/null
+GLOBAL=
+export GLOBAL=
 `)
 	ctx := context.Background()
 	for i := 0; i < 3; i++ {