event/MultiSocketMonitor: add method ClearSocketList()

src/event/MultiSocketMonitor.cxx

@@ -19,6 +19,7 @@
 
 #include "config.h"
 #include "MultiSocketMonitor.hxx"
+#include "Loop.hxx"
 
 #include <algorithm>
 
@@ -35,6 +36,14 @@ MultiSocketMonitor::~MultiSocketMonitor()
 	// TODO
 }
 
+void
+MultiSocketMonitor::ClearSocketList()
+{
+	assert(GetEventLoop().IsInsideOrNull());
+
+	fds.clear();
+}
+
 #ifndef WIN32
 
 void

src/event/MultiSocketMonitor.hxx

@@ -126,6 +126,11 @@ public:
 	}
 
 	/**
+	 * Remove all sockets.
+	 */
+	void ClearSocketList();
+
+	/**
 	 * Update the known sockets by invoking the given function for
 	 * each one; its return value is the events bit mask.  A
 	 * return value of 0 means the socket will be removed from the

src/mixer/AlsaMixerPlugin.cxx

@@ -91,8 +91,10 @@ static constexpr Domain alsa_mixer_domain("alsa_mixer");
 int
 AlsaMixerMonitor::PrepareSockets()
 {
-	if (mixer == nullptr)
+	if (mixer == nullptr) {
+		ClearSocketList();
 		return -1;
+	}
 
 	int count = snd_mixer_poll_descriptors_count(mixer);
 	if (count < 0)