event/SocketEvent: add Abandon()

src/event/SocketEvent.cxx

@@ -85,6 +85,15 @@ SocketEvent::Schedule(unsigned flags) noexcept
 	return success;
 }
 
+void
+SocketEvent::Abandon() noexcept
+{
+	if (std::exchange(scheduled_flags, 0) != 0)
+		loop.AbandonFD(fd.Get());
+
+	fd = SocketDescriptor::Undefined();
+}
+
 void
 SocketEvent::Dispatch() noexcept
 {

src/event/SocketEvent.hxx

@@ -121,6 +121,18 @@ public:
 	 */
 	void Close() noexcept;
 
+	/**
+	 * Call this instead of Cancel() to unregister this object
+	 * after the underlying socket has already been closed.  This
+	 * skips the `EPOLL_CTL_DEL` call because the kernel
+	 * automatically removes closed file descriptors from epoll.
+	 *
+	 * Doing `EPOLL_CTL_DEL` on a closed file descriptor usually
+	 * fails with `-EBADF` or could unregister a different socket
+	 * which happens to be on the same file descriptor number.
+	 */
+	void Abandon() noexcept;
+
 	unsigned GetScheduledFlags() const noexcept {
 		return scheduled_flags;
 	}