Feature/storage/open write page blob bug

sdk/storage/Azure.Storage.Blobs/CHANGELOG.md

@@ -3,6 +3,7 @@
 ## 12.6.0-preview.1 (Unreleased)
 - Fixed bug where BlobClient.Upload(), BlockBlobClient.Upload(), AppendBlobClient.AppendBlock(), and PageBlobClient.UploadPages() would deadlock if the content stream's position was not 0.
 - Fixed bug in BlobBaseClient.OpenRead() causing us to do more download called than necessary.
+- Fixed bug where PageBlobWriteStream would advance Position 2x the number of written bytes.
 
 ## 12.5.1 (2020-08-18)
 - Fixed bug in TaskExtensions.EnsureCompleted method that causes it to unconditionally throw an exception in the environments with synchronization context

sdk/storage/Azure.Storage.Blobs/src/PageBlobWriteStream.cs

@@ -86,7 +86,6 @@ await WriteToBufferInternal(
                     }
                 }
             }
-            _position += count;
         }
 
         protected override async Task AppendInternal(bool async, CancellationToken cancellationToken)

sdk/storage/Azure.Storage.Blobs/tests/PageBlobClientTests.cs

@@ -3384,6 +3384,47 @@ await TestHelper.CatchAsync<Exception>(
             }
         }
 
+        [Test]
+        public async Task OpenWriteAsync_Position()
+        {
+            // Arrange
+            await using DisposingContainer test = await GetTestContainerAsync();
+            PageBlobClient blob = await CreatePageBlobClientAsync(test.Container, Constants.KB);
+
+            byte[] data0 = GetRandomBuffer(512);
+            byte[] data1 = GetRandomBuffer(512);
+            using Stream dataStream0 = new MemoryStream(data0);
+            using Stream dataStream1 = new MemoryStream(data1);
+            byte[] expectedData = new byte[Constants.KB];
+            Array.Copy(data0, expectedData, 512);
+            Array.Copy(data1, 0, expectedData, 512, 512);
+
+            // Act
+            Stream openWriteStream = await blob.OpenWriteAsync(
+                overwrite: false,
+                position: 0);
+
+            Assert.AreEqual(0, openWriteStream.Position);
+
+            await dataStream0.CopyToAsync(openWriteStream);
+
+            Assert.AreEqual(512, openWriteStream.Position);
+
+            await dataStream1.CopyToAsync(openWriteStream);
+
+            Assert.AreEqual(1024, openWriteStream.Position);
+
+            await openWriteStream.FlushAsync();
+
+            Assert.AreEqual(1024, openWriteStream.Position);
+
+            Response<BlobDownloadInfo> result = await blob.DownloadAsync();
+            MemoryStream dataResult = new MemoryStream();
+            await result.Value.Content.CopyToAsync(dataResult);
+            Assert.AreEqual(expectedData.Length, dataResult.Length);
+            TestHelper.AssertSequenceEqual(expectedData, dataResult.ToArray());
+        }
+
         private PageBlobRequestConditions BuildAccessConditions(
             AccessConditionParameters parameters,
             bool lease = false,