fix stalling on PagedCollection, when value is empty but nextLink is …

…valid (#999)

* fix stalling when values is empty but nextLink is valid

* comments

Tests/Fluent.Tests/PagedCollectionTests.cs

@@ -0,0 +1,62 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License. See License.txt in the project root for license information.
+
+using Microsoft.Azure.KeyVault.Models;
+using Microsoft.Azure.Management.ResourceManager.Fluent.Core;
+using Microsoft.Rest.Azure;
+using Newtonsoft.Json;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Fluent.Tests
+{
+    // For serialzation to Page<T>
+    [JsonObject]
+    public class MockPage<T>
+    {
+        [JsonProperty("nextLink")]
+        public string NextPageLink { get; set; }
+
+        [JsonProperty("value")]
+        public IList<T> Items { get; set; }
+    }
+
+    public class PagedCollection
+    {
+        [Fact]
+        public void CanLoadEmptyPageWithNextLink()
+        {
+            var taskLoadPage = PagedCollection<string, string>.LoadPage(
+                // first page, valid nextLink
+                (cancellation) => Task.FromResult(ConvertToPage(new MockPage<string> { Items = new List<string> { "1", "2" }, NextPageLink = "2" })),
+                (nextLink, cancellation) => 
+                {
+                    if (nextLink == "2")
+                    {
+                        // empty values, valid nextLink
+                        return Task.FromResult(ConvertToPage(new MockPage<string> { Items = new List<string>(), NextPageLink = "3" }));
+                    }
+                    else if (nextLink == "3")
+                    {
+                        // non-empty values, null nextLink
+                        return Task.FromResult(ConvertToPage(new MockPage<string> { Items = new List<string> { "3", "4", "5" }, NextPageLink = null }));
+                    }
+                    else
+                    {
+                        Assert.False(true);
+                        return Task.FromResult(ConvertToPage(new MockPage<string> { Items = new List<string> (), NextPageLink = null }));
+                    }
+                },
+                (str) => str, true, CancellationToken.None);
+
+            Assert.Equal(new List<string> { "1", "2", "3", "4", "5" }, taskLoadPage.Result);
+        }
+
+        private static IPage<T> ConvertToPage<T>(MockPage<T> mockPage)
+        {
+            return Microsoft.Rest.Serialization.SafeJsonConvert.DeserializeObject<Page<T>>(Microsoft.Rest.Serialization.SafeJsonConvert.SerializeObject(mockPage));
+        }
+    }
+}

src/ResourceManagement/ResourceManager/Core/PagedCollection.cs

@@ -118,11 +118,14 @@ IEnumerator IEnumerable.GetEnumerator()
         private async Task AddCollection(IPage<InnerResourceT> currentPage, PagedCollection<IFluentResourceT, InnerResourceT> pagedCollection,
             CancellationToken cancellationToken)
         {
-            if (currentPage != null && currentPage.Any())
+            if (currentPage != null)
             {
                 pagedCollection.NextPageLink = currentPage.NextPageLink;
-                var resources = await Task.WhenAll(currentPage.Select(async (inner) => await this.WrapModelAsyncDelegate(inner, cancellationToken)));
-                ((List<IFluentResourceT>)pagedCollection.innerCollection).AddRange(resources);
+                if (currentPage.Any())
+                {
+                    var resources = await Task.WhenAll(currentPage.Select(async (inner) => await this.WrapModelAsyncDelegate(inner, cancellationToken)));
+                    ((List<IFluentResourceT>)pagedCollection.innerCollection).AddRange(resources);
+                }
             }
         }