JSON continuation tests (#42393)

* Repro #42070

* formatting

* namespace

* Fix

* never forget the header

* More tests
- Test continuation at every position inside the tested object
- Many member with primitive and nullable types
- One more level of nested object
- All combinations of class/struct for tested and nested object
- tested and nested object with parametrized ctor for some properties

* Addressed feedback

* refactoring

* Test with original repro data from #42070

* custom converter to ensure the padding is written in front of the tested object

* rename

* test data moved to Strings.resx

* Using test data from SR

* Generalize continuation tests for payloads of any length
Tweak the payload and expect `JsonException`

* merge

* Test deserialize with Utf8JsonReader and ReadOnlySequence

* Again with value typed nested object

* Add tests for splitted whitespaces

* Addressed feedback
Added dictionary test

* Validate line and position of failure in tweaked payloads
more tweaks

* Fixed comment

Co-authored-by: Layomi Akinrinade <[email protected]>

src/libraries/System.Text.Json/tests/Serialization/ContinuationTests.NullToken.cs

@@ -0,0 +1,146 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace System.Text.Json.Serialization.Tests
+{
+    public static partial class ContinuationTests
+    {
+        // From https://github.com/dotnet/runtime/issues/42070
+        [Theory]
+        [MemberData(nameof(ContinuationAtNullTokenTestData))]
+        public static async Task ContinuationAtNullToken(string payload)
+        {
+            using (Stream stream = new MemoryStream(Encoding.UTF8.GetBytes(payload)))
+            {
+                CustomerCollectionResponse response = await JsonSerializer.DeserializeAsync<CustomerCollectionResponse>(stream, new JsonSerializerOptions { IgnoreNullValues = true });
+                Assert.Equal(50, response.Customers.Count);
+            }
+        }
+
+        public static IEnumerable<object[]> ContinuationAtNullTokenTestData
+            => new[]
+            {
+                new[] { SR.CustomerSearchApi108KB },
+                new[] { SR.CustomerSearchApi107KB },
+            };
+
+        private class CustomerCollectionResponse
+        {
+            [JsonPropertyName("customers")]
+            public List<Customer> Customers { get; set; }
+        }
+
+        private class CustomerAddress
+        {
+            [JsonPropertyName("first_name")]
+            public string FirstName { get; set; }
+
+            [JsonPropertyName("address1")]
+            public string Address1 { get; set; }
+
+            [JsonPropertyName("phone")]
+            public string Phone { get; set; }
+
+            [JsonPropertyName("city")]
+            public string City { get; set; }
+
+            [JsonPropertyName("zip")]
+            public string Zip { get; set; }
+
+            [JsonPropertyName("province")]
+            public string Province { get; set; }
+
+            [JsonPropertyName("country")]
+            public string Country { get; set; }
+
+            [JsonPropertyName("last_name")]
+            public string LastName { get; set; }
+
+            [JsonPropertyName("address2")]
+            public string Address2 { get; set; }
+
+            [JsonPropertyName("company")]
+            public string Company { get; set; }
+
+            [JsonPropertyName("latitude")]
+            public float? Latitude { get; set; }
+
+            [JsonPropertyName("longitude")]
+            public float? Longitude { get; set; }
+
+            [JsonPropertyName("name")]
+            public string Name { get; set; }
+
+            [JsonPropertyName("country_code")]
+            public string CountryCode { get; set; }
+
+            [JsonPropertyName("province_code")]
+            public string ProvinceCode { get; set; }
+        }
+
+        private class Customer
+        {
+            [JsonPropertyName("id")]
+            public long Id { get; set; }
+
+            [JsonPropertyName("email")]
+            public string Email { get; set; }
+
+            [JsonPropertyName("accepts_marketing")]
+            public bool AcceptsMarketing { get; set; }
+
+            [JsonPropertyName("created_at")]
+            public DateTimeOffset? CreatedAt { get; set; }
+
+            [JsonPropertyName("updated_at")]
+            public DateTimeOffset? UpdatedAt { get; set; }
+
+            [JsonPropertyName("first_name")]
+            public string FirstName { get; set; }
+
+            [JsonPropertyName("last_name")]
+            public string LastName { get; set; }
+
+            [JsonPropertyName("orders_count")]
+            public int OrdersCount { get; set; }
+
+            [JsonPropertyName("state")]
+            public string State { get; set; }
+
+            [JsonPropertyName("total_spent")]
+            public string TotalSpent { get; set; }
+
+            [JsonPropertyName("last_order_id")]
+            public long? LastOrderId { get; set; }
+
+            [JsonPropertyName("note")]
+            public string Note { get; set; }
+
+            [JsonPropertyName("verified_email")]
+            public bool VerifiedEmail { get; set; }
+
+            [JsonPropertyName("multipass_identifier")]
+            public string MultipassIdentifier { get; set; }
+
+            [JsonPropertyName("tax_exempt")]
+            public bool TaxExempt { get; set; }
+
+            [JsonPropertyName("tags")]
+            public string Tags { get; set; }
+
+            [JsonPropertyName("last_order_name")]
+            public string LastOrderName { get; set; }
+
+            [JsonPropertyName("default_address")]
+            public CustomerAddress DefaultAddress { get; set; }
+
+            [JsonPropertyName("addresses")]
+            public IList<CustomerAddress> Addresses { get; set; }
+        }
+    }
+}