event/MultiSocketMonitor: add workaround for /dev/null

The ALSA "null" driver opens /dev/null and returns the file handle
from snd_pcm_poll_descriptors(), but /dev/null cannot be used with
epoll, the epoll_ctl() system call returns -EPERM.  This means that
the ALSA output hangs, eventually freezing the whole MPD process.

This commit adds a workaround to the MultiSocketMonitor class which is
used by the ALSA output plugin.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/695

NEWS

@@ -1,4 +1,6 @@
 ver 0.21.18 (not yet released)
+* output
+  - alsa: fix hang bug with ALSA "null" outputs
 * reduce unnecessary CPU wakeups
 
 ver 0.21.17 (2019/12/16)

src/event/MultiSocketMonitor.cxx

@@ -22,6 +22,10 @@
 
 #include <algorithm>
 
+#ifdef USE_EPOLL
+#include <errno.h>
+#endif
+
 #ifndef _WIN32
 #include <poll.h>
 #endif
@@ -37,6 +41,9 @@ MultiSocketMonitor::Reset() noexcept
 	assert(GetEventLoop().IsInside());
 
 	fds.clear();
+#ifdef USE_EPOLL
+	always_ready_fds.clear();
+#endif
 	IdleMonitor::Cancel();
 	timeout_event.Cancel();
 	ready = refresh = false;
@@ -49,6 +56,13 @@ MultiSocketMonitor::AddSocket(SocketDescriptor fd, unsigned events) noexcept
 	bool success = fds.front().Schedule(events);
 	if (!success) {
 		fds.pop_front();
+
+#ifdef USE_EPOLL
+		if (errno == EPERM)
+			/* not supported by epoll (e.g. "/dev/null"):
+			   add it to the "always ready" list */
+			always_ready_fds.push_front({fd, events});
+#endif
 	}
 
 	return success;
@@ -60,6 +74,9 @@ MultiSocketMonitor::ClearSocketList() noexcept
 	assert(GetEventLoop().IsInside());
 
 	fds.clear();
+#ifdef USE_EPOLL
+	always_ready_fds.clear();
+#endif
 }
 
 #ifndef _WIN32
@@ -67,6 +84,10 @@ MultiSocketMonitor::ClearSocketList() noexcept
 void
 MultiSocketMonitor::ReplaceSocketList(pollfd *pfds, unsigned n) noexcept
 {
+#ifdef USE_EPOLL
+	always_ready_fds.clear();
+#endif
+
 	pollfd *const end = pfds + n;
 
 	UpdateSocketList([pfds, end](SocketDescriptor fd) -> unsigned {
@@ -89,7 +110,20 @@ MultiSocketMonitor::ReplaceSocketList(pollfd *pfds, unsigned n) noexcept
 void
 MultiSocketMonitor::Prepare() noexcept
 {
-	const auto timeout = PrepareSockets();
+	auto timeout = PrepareSockets();
+
+#ifdef USE_EPOLL
+	if (!always_ready_fds.empty()) {
+		/* if there was at least one file descriptor not
+		   supported by epoll, install a very short timeout
+		   because we assume it's always ready */
+		constexpr std::chrono::steady_clock::duration ready_timeout =
+			std::chrono::milliseconds(1);
+		if (timeout < timeout.zero() || timeout > ready_timeout)
+			timeout = ready_timeout;
+	}
+#endif
+
 	if (timeout >= timeout.zero())
 		timeout_event.Schedule(timeout);
 	else

src/event/MultiSocketMonitor.hxx

@@ -102,6 +102,21 @@ class MultiSocketMonitor : IdleMonitor
 
 	std::forward_list<SingleFD> fds;
 
+#ifdef USE_EPOLL
+	struct AlwaysReady {
+		const SocketDescriptor fd;
+		const unsigned revents;
+	};
+
+	/**
+	 * A list of file descriptors which are always ready.  This is
+	 * a kludge needed because the ALSA output plugin gives us a
+	 * file descriptor to /dev/null, which is incompatible with
+	 * epoll (epoll_ctl() returns -EPERM).
+	 */
+	std::forward_list<AlwaysReady> always_ready_fds;
+#endif
+
 public:
 	static constexpr unsigned READ = SocketMonitor::READ;
 	static constexpr unsigned WRITE = SocketMonitor::WRITE;
@@ -198,6 +213,11 @@ public:
 				i.ClearReturnedEvents();
 			}
 		}
+
+#ifdef USE_EPOLL
+		for (const auto &i : always_ready_fds)
+			f(i.fd, i.revents);
+#endif
 	}
 
 protected: