From 0b5fdcc34625e4aa2b499e982fd49ecb089af585 Mon Sep 17 00:00:00 2001
From: Pieter Noordhuis <pieter.noordhuis@databricks.com>
Date: Wed, 7 Feb 2024 10:25:53 +0100
Subject: [PATCH] Zero destination struct in `convert.ToTyped` (#1178)

## Changes

Not doing this means that the output struct is not a true representation
of the `dyn.Value` and unrepresentable state (e.g. unexported fields)
can be carried over across `convert.ToTyped` calls.

## Tests

Unit tests.
---
 libs/dyn/convert/to_typed.go      |  4 ++++
 libs/dyn/convert/to_typed_test.go | 21 +++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/libs/dyn/convert/to_typed.go b/libs/dyn/convert/to_typed.go
index 209de12c..715d3f67 100644
--- a/libs/dyn/convert/to_typed.go
+++ b/libs/dyn/convert/to_typed.go
@@ -53,6 +53,10 @@ func ToTyped(dst any, src dyn.Value) error {
 func toTypedStruct(dst reflect.Value, src dyn.Value) error {
 	switch src.Kind() {
 	case dyn.KindMap:
+		// Zero the destination struct such that fields
+		// that aren't present in [src] are cleared.
+		dst.SetZero()
+
 		info := getStructInfo(dst.Type())
 		for k, v := range src.MustMap() {
 			index, ok := info.Fields[k]
diff --git a/libs/dyn/convert/to_typed_test.go b/libs/dyn/convert/to_typed_test.go
index 3adc94c7..fd399b93 100644
--- a/libs/dyn/convert/to_typed_test.go
+++ b/libs/dyn/convert/to_typed_test.go
@@ -59,6 +59,27 @@ func TestToTypedStructOverwrite(t *testing.T) {
 	assert.Equal(t, "baz", out.Bar)
 }
 
+func TestToTypedStructClearFields(t *testing.T) {
+	type Tmp struct {
+		Foo string `json:"foo"`
+		Bar string `json:"bar,omitempty"`
+	}
+
+	// Struct value with non-empty fields.
+	var out = Tmp{
+		Foo: "baz",
+		Bar: "qux",
+	}
+
+	// Value is an empty map.
+	v := dyn.V(map[string]dyn.Value{})
+
+	// The previously set fields should be cleared.
+	err := ToTyped(&out, v)
+	require.NoError(t, err)
+	assert.Equal(t, Tmp{}, out)
+}
+
 func TestToTypedStructAnonymousByValue(t *testing.T) {
 	type Bar struct {
 		Bar string `json:"bar"`