investigate if we need to fix flow control

[komuw]

see: sanic-org/sanic#1176

[komuw]

We are already practicing good flow control practices.
We are using[1] a streamWriter[2].
When we want to send data to SMSC, we call writer.write() followed by await wariter.drain();

naz/naz/client.py

Lines 883 to 884 in 945a568

	self.writer.write(msg)
	await self.writer.drain()

The documentation for streamwriter[2] .write method says;

Write data to the stream.     
This method is not subject to flow control. Calls to `.write()` should be followed by `drain()`

The documentation for streamwriter[2] .drain method says;

This is a flow control method that interacts with the underlying IO write buffer.     
When the size of the buffer reaches the high watermark,     
`drain()` blocks until the size of the buffer is drained down to the low watermark      
and writing can be resumed.           
When there is nothing to wait for, the drain() returns immediately.

When the size of the buffer reaches the high watermark, writer.drain() blocks until the size of the buffer is drained down to the low watermark and writing can be resumed.

So by doing;

naz/naz/client.py

Lines 883 to 884 in 945a568

	self.writer.write(msg)
	await self.writer.drain()

we are practising flow control.

The only thing we are not doing is setting our own buffer sizes. We are relying on the default buffer sizes(which I think are pretty okay).

1. The default writer buffer sizes are:

low_buffer_watermark_in_bytes, high_buffer_watermark_in_bytes = writer.transport.get_write_buffer_limits()
(16384, 65536) # ie, 16KB and 65KB

2. The default reader buffer sizes are, 64 KiB - [3]

ref:

1. naz/naz/client.py

   Lines 343 to 345 in 945a568

   	reader, writer = await asyncio.open_connection(
   	self.smsc_host, self.smsc_port, loop=self.async_loop
   	)

2. https://docs.python.org/3/library/asyncio-stream.html#streamwriter
3. https://docs.python.org/3/library/asyncio-stream.html#asyncio.open_connection