Fixed bug when downloading zero length StorageResources in a StorageResourceContainer will throw an exception. (Azure#36368)

Author: amnguye

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

@@ -13,6 +13,7 @@
 - Fix to prevent empty strings or null to be passed as paths for `LocalFileStorageResource` and `LocalDirectoryStorageResourceContainer`.
 - Fixed `ErrorHandlingOptions.ContinueOnFailure` not be respected.
 - Fixed bug where resuming a transfer where the source and destination is a `StorageResourceContainer` would throw a null reference exception. 
+- Fixed bug when downloading zero length `StorageResource`s in a `StorageResourceContainer` will throw an exception.
 
 ### Other Changes
 

sdk/storage/Azure.Storage.DataMovement/src/UriToStreamJobPart.cs

@@ -193,23 +193,7 @@ internal async Task UnknownDownloadInternal()
                 // length is 0 bytes.
                 if (initialResult == default || initialLength == 0)
                 {
-                    // We just need to at minimum create the file
-                    bool succesfulCreation = await CopyToStreamInternal(
-                        offset: 0,
-                        sourceLength: 0,
-                        source: default,
-                        expectedLength: 0).ConfigureAwait(false);
-                    if (succesfulCreation)
-                    {
-                        // Queue the work to end the download
-                        await QueueChunkToChannelAsync(
-                            async () =>
-                            await CompleteFileDownload().ConfigureAwait(false)).ConfigureAwait(false);
-                    }
-                    else
-                    {
-                        await CheckAndUpdateCancellationStatusAsync().ConfigureAwait(false);
-                    }
+                    await CreateZeroLengthDownload().ConfigureAwait(false);
                     return;
                 }
 
@@ -272,7 +256,11 @@ await DownloadStreamingInternal(range: httpRange).ConfigureAwait(false))
         internal async Task LengthKnownDownloadInternal()
         {
             long totalLength = _sourceResource.Length.Value;
-            if (_initialTransferSize <= totalLength)
+            if (totalLength == 0)
+            {
+                await CreateZeroLengthDownload().ConfigureAwait(false);
+            }
+            else if (_initialTransferSize <= totalLength)
             {
                 // To prevent requesting a range that is invalid when
                 // we already know the length we can just make one get blob request.
@@ -309,6 +297,7 @@ await CompleteFileDownload().ConfigureAwait(false))
 
                 // Get list of ranges of the blob
                 IList<HttpRange> ranges = GetRangesList(0, totalLength, rangeSize);
+
                 // Create Download Chunk event handler to manage when the ranges finish downloading
                 _downloadChunkHandler = GetDownloadChunkHandler(
                     currentTranferred: 0,
@@ -503,5 +492,26 @@ internal async Task DisposeHandlers()
                 await _downloadChunkHandler.DisposeAsync().ConfigureAwait(false);
             }
         }
+
+        private async Task CreateZeroLengthDownload()
+        {
+            // We just need to at minimum create the file
+            bool succesfulCreation = await CopyToStreamInternal(
+                offset: 0,
+                sourceLength: 0,
+                source: default,
+                expectedLength: 0).ConfigureAwait(false);
+            if (succesfulCreation)
+            {
+                // Queue the work to end the download
+                await QueueChunkToChannelAsync(
+                    async () =>
+                    await CompleteFileDownload().ConfigureAwait(false)).ConfigureAwait(false);
+            }
+            else
+            {
+                await CheckAndUpdateCancellationStatusAsync().ConfigureAwait(false);
+            }
+        }
     }
 }