Fix panic due to nil map (Azure#19396)

* Fix panic due to nil map

* rename fake endpoint

Author: jhendrixMSFT

sdk/data/azappconfig/policy_sync_token.go

@@ -26,7 +26,7 @@ type syncTokenPolicy struct {
 }
 
 func newSyncTokenPolicy() *syncTokenPolicy {
-	return &syncTokenPolicy{}
+	return &syncTokenPolicy{syncTokens: map[string]syncToken{}}
 }
 
 func parseToken(tok string) (syncToken, error) {

sdk/data/azappconfig/policy_sync_token_test.go

@@ -0,0 +1,80 @@
+//go:build go1.18
+// +build go1.18
+
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License. See License.txt in the project root for license information.
+
+package azappconfig
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"testing"
+
+	"github.com/Azure/azure-sdk-for-go/sdk/azcore/policy"
+	"github.com/Azure/azure-sdk-for-go/sdk/azcore/runtime"
+	"github.com/Azure/azure-sdk-for-go/sdk/data/azappconfig/internal/generated"
+	"github.com/stretchr/testify/require"
+)
+
+// TransportFunc is a helper to use a first-class func to satisfy the Transporter interface.
+type TransportFunc func(*http.Request) (*http.Response, error)
+
+// Do implements the Transporter interface for the TransportFunc type.
+func (pf TransportFunc) Do(req *http.Request) (*http.Response, error) {
+	return pf(req)
+}
+
+type nonRetriableError struct {
+	error
+}
+
+func (nonRetriableError) NonRetriable() {}
+
+func TestSyncTokenPolicy(t *testing.T) {
+	stp := newSyncTokenPolicy()
+	require.NotNil(t, stp)
+
+	pl := runtime.NewPipeline("TestSyncTokenPolicy", generated.ModuleVersion, runtime.PipelineOptions{PerRetry: []policy.Policy{stp}}, &policy.ClientOptions{
+		Transport: TransportFunc(func(req *http.Request) (*http.Response, error) {
+			return &http.Response{
+				StatusCode: http.StatusOK,
+				Body:       http.NoBody,
+				Header: http.Header{
+					"Sync-Token": []string{"jtqGc1I4=MDoyOA==;sn=28"},
+				},
+			}, nil
+		}),
+	})
+
+	req, err := runtime.NewRequest(context.Background(), http.MethodGet, "http://test.contoso.com")
+	require.NoError(t, err)
+
+	resp, err := pl.Do(req)
+	require.NoError(t, err)
+	require.NotNil(t, resp)
+	st := stp.syncTokens["jtqGc1I4"]
+	require.NotZero(t, st)
+	require.Equal(t, "jtqGc1I4", st.id)
+	require.Equal(t, int64(28), st.seqNo)
+	require.Equal(t, "MDoyOA==", st.value)
+}
+
+func TestSyncTokenPolicyError(t *testing.T) {
+	stp := newSyncTokenPolicy()
+	require.NotNil(t, stp)
+
+	pl := runtime.NewPipeline("TestSyncTokenPolicy", generated.ModuleVersion, runtime.PipelineOptions{PerRetry: []policy.Policy{stp}}, &policy.ClientOptions{
+		Transport: TransportFunc(func(req *http.Request) (*http.Response, error) {
+			return nil, nonRetriableError{errors.New("failed")}
+		}),
+	})
+
+	req, err := runtime.NewRequest(context.Background(), http.MethodGet, "http://test.contoso.com")
+	require.NoError(t, err)
+
+	resp, err := pl.Do(req)
+	require.Error(t, err)
+	require.Nil(t, resp)
+}