I/O manager: relying solely upon kqueue is not a safe way to go

[Andreas Voellmy]

Hi PHO,

On Fri, Mar 15, 2013 at 3:54 PM, PHO <pho at cielonegro.org> wrote:
>
> I wrote a small program to test if a given stdin is supported by
> kqueue, poll and select:
> https://gist.github.com/phonohawk/5169980#file-kqueue-poll-select-cpp
>
> MacOS X 10.5.8 is hopelessly broken. We can't use anything other than
> select(2) for tty and other devices:
>
> https://gist.github.com/phonohawk/5169980#file-powerpc-apple-darwin9-8-0-txt
>
>
I just ran your program on OS X 10.8.2. It works fine with tty

$ ./a.out
Type of stdin: tty

Checking if kqueue(2) works...
Great. kevent(2) successfully handled your stdin.

Checking if poll(2) works...
Good. poll(2) successfully handled your stdin.

Checking if select(2) works...
OK. select(2) successfully handled your stdin.

and fails with /dev/random, as you report:

$ ./a.out < /dev/random
Type of stdin: device

Checking if kqueue(2) works...
kevent(2) ifself succeeded but delivered us an EV_ERROR. This is how the
old I/O manager was fooled because it wasn't (and the newer one still
don't) check this situation: Invalid argument
kevent(2) expectedly failed now. This time we didn't pass a non-NULL
incoming event buffer. This is how the new I/O manager fails on this
platform.: Invalid argument

Checking if poll(2) works...
poll(2) itself succeeded but delivered us a POLLNVAL event. It can't handle
your stdin.

Checking if select(2) works...
OK. select(2) successfully handled your stdin.

Note also that no implementations of kqueue
> support monitoring writability of regular file writes, which also
> endangers the current use of kqueue-based I/O manager namely
> "threadWaitWrite".
>

Are you sure about this? Do you have any example program demonstrating
this? I just took your kqueue-poll-select.cpp program and tested
registering a file for a write event (i.e. changing EVFILT_READ to
EVFILT_WRITE, POLLIN to POLLOUT, and use writefds rather than readfds) and
it works fine on a file:

$ ./a.out < foo
Type of stdin: regular

Checking if kqueue(2) works...
Great. kevent(2) successfully handled your stdin.

Checking if poll(2) works...
Good. poll(2) successfully handled your stdin.

Checking if select(2) works...
OK. select(2) successfully handled your stdin.

-Andi
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