If logging is enabled, either ereport() or fprintf() might stomp on errno
internally, causing this function to return the wrong result. That might
only end in a misleading error report, but in any code that's examining
errno to decide what to do next, the consequences could be far graver.
This has been broken since the very first version of this file in 2006
... it's a bit astonishing that we didn't identify this long ago.
Reported by Amit Kapila, though this isn't his proposed fix.
o remove many WIN32_CLIENT_ONLY defines
o add WIN32_ONLY_COMPILER define
o add 3rd argument to open() for portability
o add include/port/win32_msvc directory for
system includes
Magnus Hagander
