On Tue, Aug 02, 2022 at 12:13:07AM -0400, Jeff King wrote:

  - more importantly, I'm not sure of the portability implications of
    the fix. This is our first use of O_NONBLOCK outside of the
    compat/simple-ipc unix-socket code. Do we need to abstract this
    behind a compat/ layer for Windows?

So I think the answer is pretty clearly "yes", from the Windows CI
results:

  run-command.c:1429:18: 'O_NONBLOCK' undeclared (first use in this function)
         flags |= O_NONBLOCK;
                  ^~~~~~~~~~

It looks like we'd have the option of either adding F_GETFL/F_SETFL
support to compat/mingw.c's fake fcntl() function, or adding a
compat/nonblock.c that abstracts the whole thing. I'd prefer the latter,
as it makes the interface a bit narrower.

But I'm not sure what should go on the Windows side of that #ifdef.
Unlike some other spots, I don't think we can just make it a noop, or
Windows will be subject to the same deadlock (unless for some reason its
write() does behave differently?).

-Peff