Daffodil assumes the input streams are like files - reads are always blocking for either 1 or more bytes of data, or End-of-data.
People want to use Daffodil to read data from TCP/IP sockets. These can return 0 bytes from a read because there is no data available, but that does NOT mean the end of data. It's just a temporary condition. More data may come along.
Daffodil's InputSourceDataInputStream is wrapped around a regular Java input stream, and enables us to support incoming messages which do not conform to byte-boundaries.
The problem is that there's no way for users to wrap an InputSourceDataInputStream around a TCP/IP socket, and have it behave properly when a read() call temporarily says 0 bytes available. Obviously we don't want to sit in a tight loop just retrying the read until we get either some bytes or end-of-data. The right API here is that if the read() of the underlying java stream returns 0 bytes, that a hook function supplied by the API user is called. One obvious thing a user can do is put a call to Thread.yield() in the hook. (That might even want to be the default behavior if they supply no hook.) Then if they have a separate thread parsing the data with daffodil, that thread will at least yield the CPU, i.e., behave politely in a multi-threaded world. More advanced usage could start a Daffodil parse using co-routines, returning control to the caller when the parse must pause due to read() of the Java input stream returning 0 bytes.