IXXAT: timestamps read 1.5 x wall time plus adapter uptime (misplaced parenthesis in _timeoffset; start tick never captured in vcinpl2)

从 can/interfaces/ixxat/canlib_vcinpl.py 的第629行和第705行附近开始，然后对比 can/interfaces/ixxat/canlib_vcinpl2.py 的第735行和第843行附近，重点关注 epoch offset 的设置和 CAN_INFO_START 的处理。使用 IXXAT 流量运行提供的 bus.recv 复现，并验证时间戳与 time.time 对齐，同时硬件得出的帧间隔保持不变。

Describe the bug

On the IXXAT backend every received Message.timestamp reads about 28 years in the future, plus the adapter's uptime. Measured on a USB-to-CAN V2 compact (VCI V4 driver 4.1.264.0, fd=False, python-can 4.6.1, Windows 11, Python 3.12): the first frame after Bus(interface="ixxat", channel=0, bitrate=500000) stamped 2684545240.8 while time.time() read 1789696744.2 — a ratio of 1.5000001. The excess over 1.5 * time.time() grew from 142.4 s to 146.5 s across two opens 4.1 s apart, i.e. it is the adapter's own tick counter since power-up, never rebased.

Two lines cause it, both present on main today:

1. A misplaced parenthesis in the epoch offset, in both backends:

   • can/interfaces/ixxat/canlib_vcinpl.py line 629
   • can/interfaces/ixxat/canlib_vcinpl2.py line 735

   self._timeoffset = start_begin + (start_end - start_begin / 2)
   

   This evaluates to 1.5 * start_begin + start_end instead of the midpoint start_begin + (start_end - start_begin) / 2. Hence the 1.5 x wall-clock epoch.

2. The start tick is never captured in canlib_vcinpl2.py (line 843): the CAN_INFO_START check is an elif sibling of the CAN_MSGTYPE_INFO branch rather than nested inside it, so an INFO message is consumed by the first elif and _starttickoffset stays 0. Every timestamp then carries the adapter's uptime. canlib_vcinpl.py on main has this check nested correctly (line 705); the FD backend does not.

   elif self._message.uMsgInfo.Bits.type == constants.CAN_MSGTYPE_INFO:
       log.info(...)
   # Handle CAN start info message
   elif self._message.abData[0] == constants.CAN_INFO_START:   # never reached for INFO messages
       self._starttickoffset = self._message.dwTime
   

The spacing between frames is correct and hardware-derived (10.014 s of frame time against 10.014 s of wall time over 10 s, never a step backwards, 9 µs resolution), so this is purely the epoch.

To Reproduce

import time, can
bus = can.Bus(interface="ixxat", channel=0, bitrate=500000)
msg = bus.recv(5)
print(msg.timestamp, time.time(), msg.timestamp / time.time())
# 2684545240.8  1789696744.2  1.5000001

Any bus with traffic reproduces it; the ratio of timestamp / time.time() is 1.5 rather than 1.0.

Expected behaviour

Message.timestamp should be Unix time to within the open call, as the other backends provide, with the adapter's own tick spacing preserved.

Additional context

• python-can 4.6.1 (also present on main at the lines cited above)
• IXXAT USB-to-CAN V2 compact, HMS VCI V4 4.1.264.0, Windows 11, Python 3.12
• Working around it downstream by adding one constant settled from the first frame's arrival when the bus's offset lies outside the open call; happy to open a PR for the two one-line fixes if that is welcome.