Module calloop::sources::ping::eventfd

Eventfd based implementation of the ping event source.

Implementation notes

The eventfd is a much lighter signalling mechanism provided by the Linux kernel. Rather than write an arbitrary sequence of bytes, it only has a 64-bit counter.

To avoid closing the eventfd early, we wrap it in a RAII-style closer CloseOnDrop in make_ping(). When all the senders are dropped, another wrapper FlagOnDrop handles signalling this to the event source, which is the sole owner of the eventfd itself. The senders have weak references to the eventfd, and if the source is dropped before the senders, they will simply not do anything (except log a message).

To differentiate between regular ping events and close ping events, we add 2 to the counter for regular events and 1 for close events. In the source we can then check the LSB and if it’s set, we know it was a close event. This only works if a close event never fires more than once.

Structs

• ArcAsFd 🔒
• FlagOnDrop 🔒
  This manages signalling to the PingSource when it’s dropped. There should only ever be one of these per PingSource.
• Ping
• PingSource

Constants

• INCREMENT_CLOSE 🔒
• INCREMENT_PING 🔒

Functions

• drain_ping 🔒
• make_ping
• send_ping 🔒