[Bug 1896295] Re: WIN32: IOC scan loops hang

[mdavidsaver via Core-talk <core-talk at aps.anl.gov>]

> ... I thought the code that used the monotonic clock in the internal
timer queues never got into a released version ...

dbScan.c was the one usage I still had confidence in, so I left it.
sigh...

epicsTimeDiffInSeconds() only tries to handle rollover >= `ULONG_MAX/2`.
Using it with a time source which rolls over sooner is wrong, and likely
unfixable.

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https://bugs.launchpad.net/bugs/1896295

Title:
  WIN32: IOC scan loops hang

Status in EPICS Base:
  New

Bug description:
  The IOC scan loops hung on all IOCs on several computers, on
  investigation this looks to be caused by the monotonic clock wrapping.
  The values of next and now in dbScan.c / periodicTask() are consistent
  with GetTickCount() wrapping (which happens every 49.7 days), which I
  believe lead to epicsEventWaitWithTimeout() being called with a large
  delay. I was initially confused by the debugger saying a delay of 0
  had been passed to epicsEventWaitWithTimeout(), but I cannot see how
  this is possible (the minimum delay should be penalty) so I assume the
  debugger is reporting incorrectly due to the code being in release
  mode.

  The epics Monotonic time functions should not normally be using
  GetTickCount(), however it looks like the logic in the test for
  whether to use GetPerformanceCounter() is inverted and so
  GetTickCount() is always used. I will push a patch to fix this, and
  also to use GetTickCount64() as a fallback instead (which wraps on a
  much longer timescale).

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