If you can't even read, what's the point in being able to open?
You need the open to succeed because you need its returned file descriptor as
the first argument to the system call to test if the file is locked.
The main issue here seems to be mandatory versus advisory locking.
Windows uses mandatory locking, while Unix uses advisory locking (by default).
The Windows locking behaviour described above is actually similar to that of
(SVR3/4-based) Unix systems that support mandatory locking (via the hack of
turning on the file's set-group-ID bit and turning off its group-execute bit).
On such Unix systems, if a file is mandatory write-locked, the open succeeds,
while a non-blocking read fails with EAGAIN and a blocking read blocks until the write-lock is removed.