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

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