Performance fix for slcan

[aaronfultonnz]

Describe the bug

There is a performance issue with the way the reads are implemented in slcan.py.

_read() reads serial data one byte at a time via read(1), even when multiple bytes (or complete frames) are already buffered in the OS serial receive buffer. Each read(1) incurs a syscall and, when no data is available, blocks for up to the serial timeout (default 1ms). A typical SLCAN frame is ~27 bytes, so each frame requires 27 separate read(1) calls minimum. This causes per-frame read latency of 50–850ms instead of <1ms (particularly on windows which seem to have sub-optimal serial support).

I'm using CAN-TP over CAN-FD, so I'm pushing this to the limits, which is why the performance issue has shown up.

Expected behavior

The serial is read as fast as possible

Additional context

OS and version: Windows 11
Python version: 3.10
python-can version: 4.6.1
python-can interface/s (if applicable): Canable2 running SL CAN firmware with FD support

The Fix

I'll submit a pull request with a suggested fix that alters the serial read behaviour to be significantly faster.