Fix: Azure Device Behavior for Non-Existent Files

libs/storage/Tsavorite/cs/src/devices/AzureStorageDevice/AzureStorageDevice.cs

@@ -440,29 +440,56 @@ public override unsafe void ReadAsync(int segmentId, ulong sourceAddress, IntPtr
             // Lazily cache the blob entry for the segment being read
             if (!blobs.TryGetValue(segmentId, out BlobEntry blobEntry))
             {
-                var blobClients = pageBlobDirectory.GetPageBlobClient(GetSegmentBlobName(segmentId));
-                var entry = new BlobEntry(blobClients, blobClients.Default.GetProperties().Value.ETag, this);
-                blobs.TryAdd(segmentId, entry);
+                BlobEntry entry = new(this);
+                if (blobs.TryAdd(segmentId, entry))
+                {
+                    var pageBlob = pageBlobDirectory.GetPageBlobClient(GetSegmentBlobName(segmentId));
+
+                    // If segment size is -1 we use a default
+                    var size = segmentSize == -1 ? MAX_PAGEBLOB_SIZE : segmentSize;
+
+                    // If no blob exists for the segment, we must first create the segment asynchronouly. (Create call takes ~70 ms by measurement)
+                    // After creation is done, we can call read.
+                    _ = entry.CreateAsync(size, pageBlob);
+                }
+                // Otherwise, some other thread beat us to it. Okay to use their blobs.
                 blobEntry = blobs[segmentId];
             }
 
-            ReadFromBlobUnsafeAsync(blobEntry.PageBlob, (long)sourceAddress, (long)destinationAddress, readLength, id)
-                  .ContinueWith((Task t) =>
-                  {
-                      if (pendingReadWriteOperations.TryRemove(id, out ReadWriteRequestInfo request))
-                      {
-                          if (t.IsFaulted)
-                          {
-                              BlobManager?.StorageTracer?.TsavoriteStorageProgress($"StorageOpReturned AzureStorageDevice.ReadAsync id={id} (Failure)");
-                              request.Callback(uint.MaxValue, request.NumBytes, request.Context);
-                          }
-                          else
-                          {
-                              BlobManager?.StorageTracer?.TsavoriteStorageProgress($"StorageOpReturned AzureStorageDevice.ReadAsync id={id}");
-                              request.Callback(0, request.NumBytes, request.Context);
-                          }
-                      }
-                  }, TaskContinuationOptions.ExecuteSynchronously);
+            TryReadAsync(blobEntry, (long)sourceAddress, (long)destinationAddress, readLength, id);
+        }
+
+        void TryReadAsync(BlobEntry blobEntry, long sourceAddress, long destinationAddress, uint readLength, long id)
+        {
+            // If pageBlob is null, it means the blob has not been created yet. We should wait for it to be created.
+            if (blobEntry.PageBlob.Default == null &&
+                blobEntry.TryQueueAction(() => ReadFromBlobAsync(blobEntry, sourceAddress, destinationAddress, readLength, id)))
+            {
+                return;
+            }
+            // Otherwise, we can proceed with the read.
+            ReadFromBlobAsync(blobEntry, sourceAddress, destinationAddress, readLength, id);
+        }
+
+        unsafe void ReadFromBlobAsync(BlobEntry blobEntry, long sourceAddress, long destinationAddress, uint readLength, long id)
+        {
+            ReadFromBlobUnsafeAsync(blobEntry, sourceAddress, destinationAddress, readLength, id)
+                .ContinueWith((Task t) =>
+                {
+                    if (pendingReadWriteOperations.TryRemove(id, out ReadWriteRequestInfo request))
+                    {
+                        if (t.IsFaulted)
+                        {
+                            BlobManager?.StorageTracer?.TsavoriteStorageProgress($"StorageOpReturned AzureStorageDevice.ReadAsync id={id} (Failure)");
+                            request.Callback(uint.MaxValue, request.NumBytes, request.Context);
+                        }
+                        else
+                        {
+                            BlobManager?.StorageTracer?.TsavoriteStorageProgress($"StorageOpReturned AzureStorageDevice.ReadAsync id={id} for blob={blobEntry.ETag}");
+                            request.Callback(0, request.NumBytes, request.Context);
+                        }
+                    }
+                }, TaskContinuationOptions.ExecuteSynchronously);
         }
 
         /// <summary>
@@ -557,12 +584,12 @@ await BlobManager.PerformWithRetriesAsync(
             }
         }
 
-        unsafe Task ReadFromBlobUnsafeAsync(BlobUtilsV12.PageBlobClients blob, long sourceAddress, long destinationAddress, uint readLength, long id)
+        unsafe Task ReadFromBlobUnsafeAsync(BlobEntry blob, long sourceAddress, long destinationAddress, uint readLength, long id)
         {
             return ReadFromBlobAsync(new UnmanagedMemoryStream((byte*)destinationAddress, readLength, readLength, FileAccess.Write), blob, sourceAddress, readLength, id);
         }
 
-        async Task ReadFromBlobAsync(UnmanagedMemoryStream stream, BlobUtilsV12.PageBlobClients blob, long sourceAddress, uint readLength, long id)
+        async Task ReadFromBlobAsync(UnmanagedMemoryStream stream, BlobEntry blob, long sourceAddress, uint readLength, long id)
         {
             using (stream)
             {
@@ -577,7 +604,7 @@ await BlobManager.PerformWithRetriesAsync(
                         "PageBlobClient.DownloadStreamingAsync",
                         "ReadFromDevice",
                         $"id={id} readLength={length} sourceAddress={sourceAddress + offset}",
-                        blob.Default.Name,
+                        blob.PageBlob.Default.Name,
                         1000 + (int)length / 1000,
                         true,
                         async (numAttempts) =>
@@ -589,7 +616,7 @@ await BlobManager.PerformWithRetriesAsync(
 
                             if (length > 0)
                             {
-                                var client = (numAttempts > 1 || length == MAX_DOWNLOAD_SIZE) ? blob.Default : blob.Aggressive;
+                                var client = (numAttempts > 1 || length == MAX_DOWNLOAD_SIZE) ? blob.PageBlob.Default : blob.PageBlob.Aggressive;
 
                                 var response = await client.DownloadStreamingAsync(
                                     range: new Azure.HttpRange(sourceAddress + offset, length),