db/update/Remove: reimplement as a non-blocking queue

This reduces overhead for two reasons:

1. calls to Remove() are non-blocking

2. RunDeferred() may work on large chunks at a time, reducing the
   number of RunDeferred() calls

src/db/update/Remove.cxx

@@ -21,47 +21,49 @@
 #include "Remove.hxx"
 #include "UpdateDomain.hxx"
 #include "db/plugins/simple/Song.hxx"
-#include "db/LightSong.hxx"
 #include "db/DatabaseListener.hxx"
 #include "Log.hxx"
 
-#include <assert.h>
-
 /**
- * Safely remove a song from the database.  This must be done in the
+ * Safely remove songs from the database.  This must be done in the
  * main task, to be sure that there is no pointer left to it.
  */
 void
 UpdateRemoveService::RunDeferred()
 {
-	assert(removed_song != nullptr);
+	/* copy the list and unlock the mutex before invoking
+	   callbacks */
+
+	std::forward_list<std::string> copy;
 
 	{
-		const auto uri = removed_song->GetURI();
+		const ScopeLock protect(mutex);
+		std::swap(uris, copy);
+	}
+
+	for (const auto &uri : copy) {
 		FormatDefault(update_domain, "removing %s", uri.c_str());
 		listener.OnDatabaseSongRemoved(uri.c_str());
 	}
 
-	/* clear "removed_song" and send signal to update thread */
-	remove_mutex.lock();
-	removed_song = nullptr;
-	remove_cond.signal();
-	remove_mutex.unlock();
+	/* note: if Remove() was called in the meantime, it saw an
+	   empty list, and scheduled another event */
 }
 
 void
 UpdateRemoveService::Remove(const Song *song)
 {
-	assert(removed_song == nullptr);
+	bool was_empty;
 
-	removed_song = song;
+	{
+		const ScopeLock protect(mutex);
+		was_empty = uris.empty();
+		uris.emplace_front(song->GetURI());
+	}
 
-	DeferredMonitor::Schedule();
-
-	remove_mutex.lock();
-
-	while (removed_song != nullptr)
-		remove_cond.wait(remove_mutex);
-
-	remove_mutex.unlock();
+	/* inject an event into the main thread, but only if the list
+	   was empty; if it was not, then that even was already
+	   pending */
+	if (was_empty)
+		DeferredMonitor::Schedule();
 }

src/db/update/Remove.hxx

@@ -23,9 +23,11 @@
 #include "check.h"
 #include "event/DeferredMonitor.hxx"
 #include "thread/Mutex.hxx"
-#include "thread/Cond.hxx"
 #include "Compiler.h"
 
+#include <forward_list>
+#include <string>
+
 struct Song;
 class DatabaseListener;
 
@@ -36,15 +38,13 @@ class DatabaseListener;
 class UpdateRemoveService final : DeferredMonitor {
 	DatabaseListener &listener;
 
-	Mutex remove_mutex;
-	Cond remove_cond;
+	Mutex mutex;
 
-	const Song *removed_song;
+	std::forward_list<std::string> uris;
 
 public:
 	UpdateRemoveService(EventLoop &_loop, DatabaseListener &_listener)
-		:DeferredMonitor(_loop), listener(_listener),
-		 removed_song(nullptr){}
+		:DeferredMonitor(_loop), listener(_listener) {}
 
 	/**
 	 * Sends a signal to the main thread which will in turn remove