From 65bdb4e3fb63454b30ee33096ded2c1a9c963350 Mon Sep 17 00:00:00 2001
From: Luc Talatinian <lucix@amazon.com>
Date: Thu, 29 Jun 2023 13:23:51 -0400
Subject: [PATCH] fix header serialization of empty enum lists in restxml

---
 CHANGELOG_PENDING.md                |  2 ++
 private/protocol/rest/build.go      |  4 ++++
 private/protocol/rest/build_test.go | 22 ++++++++++++++++++++++
 3 files changed, 28 insertions(+)

diff --git a/CHANGELOG_PENDING.md b/CHANGELOG_PENDING.md
index 8a1927a39ca..44916363a31 100644
--- a/CHANGELOG_PENDING.md
+++ b/CHANGELOG_PENDING.md
@@ -3,3 +3,5 @@
 ### SDK Enhancements
 
 ### SDK Bugs
+* `private/protocol`: Fix header serialization of empty enum lists in restxml.
+  * Header was serialized as the empty string if list was nil/empty.
diff --git a/private/protocol/rest/build.go b/private/protocol/rest/build.go
index 1d273ff0ec6..ecc521f88f1 100644
--- a/private/protocol/rest/build.go
+++ b/private/protocol/rest/build.go
@@ -287,6 +287,10 @@ func convertType(v reflect.Value, tag reflect.StructTag) (str string, err error)
 		if tag.Get("location") != "header" || tag.Get("enum") == "" {
 			return "", fmt.Errorf("%T is only supported with location header and enum shapes", value)
 		}
+		if len(value) == 0 {
+			return "", errValueNotSet
+		}
+
 		buff := &bytes.Buffer{}
 		for i, sv := range value {
 			if sv == nil || len(*sv) == 0 {
diff --git a/private/protocol/rest/build_test.go b/private/protocol/rest/build_test.go
index 5b48482bc7d..4760e683721 100644
--- a/private/protocol/rest/build_test.go
+++ b/private/protocol/rest/build_test.go
@@ -147,6 +147,28 @@ func TestListOfEnums(t *testing.T) {
 				"X-Amz-Test-Header": {`"f,o,o","\"bar\""`},
 			},
 		},
+		{
+			Input: func() interface{} {
+				type v struct {
+					List []*string `type:"list" location:"header" locationName:"x-amz-test-header" enum:"FooBar"`
+				}
+				return &v{
+					List: nil,
+				}
+			}(),
+			Expected: http.Header{},
+		},
+		{
+			Input: func() interface{} {
+				type v struct {
+					List []*string `type:"list" location:"header" locationName:"x-amz-test-header" enum:"FooBar"`
+				}
+				return &v{
+					List: []*string{},
+				}
+			}(),
+			Expected: http.Header{},
+		},
 	}
 
 	for i, tt := range cases {