NCBC-3758: PersistentQueue does not dequeue items in original FIFO order

Motivation
----------
This bug was introduced in SDK 3.4.15 following this change NCBC-3578.

A bug in the PersistentQueue datastructure dequeues items in LIFO order (removes item at index -1), and returns the first item (at index 0) on each Dequeue().

Changes
-------
Dequeue now targets the first element of the queue rather than the last.

Change-Id: I2ca7a7eb7e7b2626513c82b7ade80df602321479
Reviewed-on: https://review.couchbase.org/c/couchbase-net-client/+/208536
Reviewed-by: Richard Ponton <richard.ponton@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>

Author: emilienbev

src/Couchbase/DataStructures/PersistentQueue.cs

@@ -35,11 +35,11 @@ internal PersistentQueue(ICouchbaseCollection collection, string key, QueueOptio
             {
                 try
                 {
-                    var result = await Collection.LookupInAsync(Key, builder => builder.Get("[-1]"))
+                    var result = await Collection.LookupInAsync(Key, builder => builder.Get("[0]"))
                         .ConfigureAwait(false);
                     var item = result.ContentAs<TValue>(0);
 
-                    await Collection.MutateInAsync(Key, builder => builder.Remove("[-1]"),
+                    await Collection.MutateInAsync(Key, builder => builder.Remove("[0]"),
                         options => options.Cas(result.Cas)).ConfigureAwait(false);
 
                     return item;

tests/Couchbase.CombinationTests/Tests/DataStructures/PersistentQueueTests.cs

@@ -0,0 +1,61 @@
+using System;
+using System.Threading.Tasks;
+using Couchbase.CombinationTests.Fixtures;
+using Couchbase.Core.Exceptions.KeyValue;
+using Couchbase.KeyValue;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Couchbase.CombinationTests.Tests.DataStructures;
+
+
+[Collection(CombinationTestingCollection.Name)]
+public class PersistentQueueTests
+{
+    private readonly CouchbaseFixture _fixture;
+    private readonly ITestOutputHelper _outputHelper;
+    private TestHelper _testHelper;
+
+    public PersistentQueueTests(CouchbaseFixture fixture, ITestOutputHelper outputHelper)
+    {
+        _fixture = fixture;
+        _outputHelper = outputHelper;
+        _testHelper = new TestHelper(fixture);
+    }
+
+
+    [Fact]
+    async Task Test_PersistentQueue_Enqueues_And_Dequeues_In_Correct_FIFO_Order()
+    {
+        var collection = await _fixture.GetDefaultCollection().ConfigureAwait(false);
+
+        var documentId = "QueueTest" + Guid.NewGuid();
+        var queue = collection.Queue<int>(documentId);
+
+        for (var queueItem = 1; queueItem <= 5; queueItem++)
+        {
+            _outputHelper.WriteLine($"Enqueing {queueItem}");
+            await queue.EnqueueAsync(queueItem).ConfigureAwait(false);
+        }
+
+        var count = 1;
+        while (true)
+        {
+            int currentPeek;
+            try
+            {
+                currentPeek = await queue.PeekAsync().ConfigureAwait(false);
+            }
+            catch (PathNotFoundException)
+            {
+                break;
+            }
+
+            Assert.Equal(count, currentPeek);
+            _outputHelper.WriteLine($"Peeking at {currentPeek} before dequeing");
+            await queue.DequeueAsync().ConfigureAwait(false);
+            count++;
+
+        }
+    }
+}