go/ast: fix bug handling the result of yield in Preorder

Once yield returns false, ast.Preorder must not call yield on any more
nodes. Even after the function passed to ast.Inspect returns false, it
may be invoked again with a non-nil node. Therefore, we must explicitly
truncate the inspection.

For #66339

Change-Id: I2b01e4e96a2d7aca785467c15ab59da13208c161
Reviewed-on: https://go-review.googlesource.com/c/go/+/585520
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/go/ast/walk.go

@@ -381,8 +381,9 @@ func Preorder(root Node) iter.Seq[Node] {
 	return func(yield func(Node) bool) {
 		ok := true
 		Inspect(root, func(n Node) bool {
-			if n != nil && !yield(n) {
-				ok = false
+			if n != nil {
+				// yield must not be called once ok is false.
+				ok = ok && yield(n)
 			}
 			return ok
 		})

src/go/ast/walk_test.go

@@ -0,0 +1,33 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast_test
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"testing"
+)
+
+func TestPreorderBreak(t *testing.T) {
+	// This test checks that Preorder correctly handles a break statement while
+	// in the middle of walking a node. Previously, incorrect handling of the
+	// boolean returned by the yield function resulted in the iterator calling
+	// yield for sibling nodes even after yield had returned false. With that
+	// bug, this test failed with a runtime panic.
+	src := "package p\ntype T struct {\n\tF int `json:\"f\"` // a field\n}\n"
+
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	for n := range ast.Preorder(f) {
+		if id, ok := n.(*ast.Ident); ok && id.Name == "F" {
+			break
+		}
+	}
+}