event/SocketEvent: add EPOLLRDHUP support

Author: MaxKellermann

src/event/EpollEvents.hxx

@@ -10,11 +10,12 @@ struct EpollEvents {
 	static constexpr unsigned WRITE = EPOLLOUT;
 	static constexpr unsigned ERROR = EPOLLERR;
 	static constexpr unsigned HANGUP = EPOLLHUP;
+	static constexpr unsigned READ_HANGUP = EPOLLRDHUP;
 
 	/**
 	 * A mask containing all events which indicate a dead socket
 	 * connection (i.e. error or hangup).  It may still be
 	 * possible to receive pending data from the socket buffer.
 	 */
-	static constexpr unsigned DEAD_MASK = ERROR|HANGUP;
+	static constexpr unsigned DEAD_MASK = ERROR|HANGUP|READ_HANGUP;
 };

src/event/SocketEvent.hxx

@@ -4,6 +4,7 @@
 #pragma once
 
 #include "BackendEvents.hxx"
+#include "event/Features.h" // for USE_EPOLL
 #include "net/SocketDescriptor.hxx"
 #include "util/BindMethod.hxx"
 #include "util/IntrusiveList.hxx"
@@ -175,6 +176,18 @@ public:
 		Schedule(IMPLICIT_FLAGS);
 	}
 
+#ifdef USE_EPOLL
+	/**
+	 * Cancel READ but schedule READ_HANGUP.  This is useful to be
+	 * able to detect hangup while we're not interested in reading
+	 * further data from the socket; in that case, the Linux
+	 * kernel will not report HANGUP.
+	 */
+	void CancelReadAndScheduleReadHangup() noexcept {
+		Schedule((GetScheduledFlags() & ~READ) | READ_HANGUP);
+	}
+#endif
+
 	bool IsReadPending() const noexcept {
 		return GetScheduledFlags() & READ;
 	}