Re: [PATCH v2 0/4] Support non-blocking pidfds

[Christian Brauner <christian.brauner@ubuntu.com>]

On Thu, Sep 03, 2020 at 04:58:55PM -0700, Josh Triplett wrote:
> On Wed, Sep 02, 2020 at 12:21:26PM +0200, Christian Brauner wrote:
> > Hi,
> > 
> > Passing a non-blocking pidfd to waitid() currently has no effect, i.e.
> > is not supported. There are users which would like to use waitid() on
> > pidfds that are O_NONBLOCK and mix it with pidfds that are blocking and
> > both pass them to waitid().
> > The expected behavior is to have waitid() return -EAGAIN for
> > non-blocking pidfds and to block for blocking pidfds without needing to
> > perform any additional checks for flags set on the pidfd before passing
> > it to waitid().
> > Non-blocking pidfds will return EAGAIN from waitid() when no child
> > process is ready yet. Returning -EAGAIN for non-blocking pidfds makes it
> > easier for event loops that handle EAGAIN specially.
> > 
> > It also makes the API more consistent and uniform. In essence, waitid()
> > is treated like a read on a non-blocking pidfd or a recvmsg() on a
> > non-blocking socket.
> > With the addition of support for non-blocking pidfds we support the same
> > functionality that sockets do. For sockets() recvmsg() supports
> > MSG_DONTWAIT for pidfds waitid() supports WNOHANG. Both flags are
> > per-call options. In contrast non-blocking pidfds and non-blocking
> > sockets are a setting on an open file description affecting all threads
> > in the calling process as well as other processes that hold file
> > descriptors referring to the same open file description. Both behaviors,
> > per call and per open file description, have genuine use-cases.
> > 
> > A concrete use-case that was brought on-list (see [1]) was Josh's async
> > pidfd library. Ever since the introduction of pidfds and more advanced
> > async io various programming languages such as Rust have grown support
> > for async event libraries. These libraries are created to help build
> > epoll-based event loops around file descriptors. A common pattern is to
> > automatically make all file descriptors they manage to O_NONBLOCK.
> > 
> > For such libraries the EAGAIN error code is treated specially. When a
> > function is called that returns EAGAIN the function isn't called again
> > until the event loop indicates the the file descriptor is ready.
> > Supporting EAGAIN when waiting on pidfds makes such libraries just work
> > with little effort.
> 
> Thanks for the patch series, Christian!
> 
> This will make it much easier to use pidfd in non-blocking event loops.
> 
> Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Thank you and thanks for your input on a bunch of other stuff as well. :)

Christian