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@xxxxxxxxxxxxxxxx> - Josh Triplett
