The ShardRegistry schedules itself to perform _internalReload()s periodically, using the TaskExecutor. These jobs are dispatched to NetworkInterfaceASIO, and will eventually run on its thread.
From _internalReload(), we call reload(), which tries to getAllShards() from the ShardingCatalogClientImpl. getAllShards() makes a Fetcher instance, which launches networking work, and then waits for it to join(). However, when we run this, we are already on NetworkInterfaceASIO's thread. This breaks the contract that callbacks to NetworkInterfaceASIO may not perform blocking work. Worse, when these calls are issued through the same TaskExecutor, the thread will deadlock.