check whether slices are empty before popping

Signed-off-by: Andres Taylor <andres@planetscale.com>

go/tools/asthelpergen/integration/ast_path_test.go

@@ -43,7 +43,7 @@ func TestWalkAllPartsOfAST(t *testing.T) {
 	}
 
 	var leafPaths []ASTPath
-	RewriteWithPaths(ast, func(c *Cursor) bool {
+	Rewrite(ast, func(c *Cursor) bool {
 		node := c.Node()
 		if !reflect.TypeOf(node).Comparable() {
 			return true

go/tools/asthelpergen/integration/test_helpers.go

@@ -83,25 +83,9 @@ func (c *Cursor) ReplaceAndRevisit(newNode AST) {
 
 type replacerFunc func(newNode, parent AST)
 
-// Rewrite is the api.
 func Rewrite(node AST, pre, post ApplyFunc) AST {
 	outer := &struct{ AST }{node}
 
-	a := &application{
-		pre:  pre,
-		post: post,
-	}
-
-	a.rewriteAST(outer, node, func(newNode, parent AST) {
-		outer.AST = newNode
-	})
-
-	return outer.AST
-}
-
-func RewriteWithPaths(node AST, pre, post ApplyFunc) AST {
-	outer := &struct{ AST }{node}
-
 	a := &application{
 		pre:          pre,
 		post:         post,

go/tools/asthelpergen/rewrite_gen.go

@@ -100,12 +100,17 @@ func (r *rewriteGen) interfaceMethod(t types.Type, iface *types.Interface, spi g
 }
 
 func (r *rewriteGen) visitStructFields(t types.Type, strct *types.Struct, spi generatorSPI, fail bool) (stmts []jen.Code) {
-	fields := r.rewriteAllStructFields(t, strct, spi, fail)
+	fields, prevSliceField := r.rewriteAllStructFields(t, strct, spi, fail)
 	stmts = append(stmts, r.executePre())
 	stmts = append(stmts, fields...)
 
 	if len(fields) > 0 {
-		stmts = append(stmts, jen.If(jen.Id("a.collectPaths")).Block(jen.Id("a.cur.current.Pop").Params()))
+		ifCondition := jen.Id("a.collectPaths")
+		if prevSliceField != "" {
+			ifCondition = ifCondition.Op("&&").Len(jen.Id("node." + prevSliceField)).Op(">").Lit(0)
+		}
+
+		stmts = append(stmts, jen.If(ifCondition).Block(jen.Id("a.cur.current.Pop").Params()))
 	}
 	stmts = append(stmts, executePost(len(fields) > 0))
 	stmts = append(stmts, returnTrue())
@@ -284,7 +289,7 @@ func (r *rewriteGen) rewriteFunc(t types.Type, stmts []jen.Code, source string)
 	r.file.Add(code)
 }
 
-func (r *rewriteGen) rewriteAllStructFields(t types.Type, strct *types.Struct, spi generatorSPI, fail bool) []jen.Code {
+func (r *rewriteGen) rewriteAllStructFields(t types.Type, strct *types.Struct, spi generatorSPI, fail bool) ([]jen.Code, string) {
 	/*
 		if errF := rewriteAST(node, node.ASTType, func(newNode, parent AST) {
 			err = vterrors.New(vtrpcpb.Code_INTERNAL, "[BUG] tried to replace '%s' on '%s'")
@@ -326,7 +331,7 @@ func (r *rewriteGen) rewriteAllStructFields(t types.Type, strct *types.Struct, s
 			fieldNumber++
 		}
 	}
-	return output
+	return output, prevSliceField
 }
 
 func failReplacer(t types.Type, f string) *jen.Statement {

go/vt/sqlparser/parse_test.go

@@ -3917,6 +3917,10 @@ func TestValid(t *testing.T) {
 			_ = Walk(func(node SQLNode) (bool, error) {
 				return true, nil
 			}, tree)
+
+			_ = RewriteWithPath(tree, func(cursor *Cursor) bool {
+				return true
+			}, nil)
 		})
 	}
 }