Data from SPI not correct

[FDelporte]

As reported by Jim Darby:
I've had some very strange errors when not running as root and using pigpiod. Sometimes when you read from a SPI device you get back more data than you sent. This shouldn't be possible and I'm still investigating.

[IAmNickNack]

I have come across a similar issue when using I2C via the GPIO socket implementation over a slow wireless network. I did not observe the problem when running the same code locally on the Pi via the loopback interface.

I think the problem might stem from PiGpioPacket#decode assuming that all the data for the packet, including the payload, is already available from the socket stream.

I have been able to solve this in a local build of pi4j which, rather than assuming all the bytes currently available, depending on the command (e.g. I2CRI, I2CRD) takes packets p3 value as a hint for the content length to expect.

This works for my I2C use cases. I don't have an SPI or serial project on the go at the moment, so have not been able to validate those cases.

I'd be happy to tidy my code an raise a PR for this if that would be helpful.