db/simple: mount points

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

src/db/plugins/simple/Directory.cxx

@@ -21,10 +21,12 @@
 #include "Directory.hxx"
 #include "SongSort.hxx"
 #include "Song.hxx"
+#include "Mount.hxx"
 #include "db/LightDirectory.hxx"
 #include "db/LightSong.hxx"
 #include "db/Uri.hxx"
 #include "db/DatabaseLock.hxx"
+#include "db/Interface.hxx"
 #include "SongFilter.hxx"
 #include "lib/icu/Collate.hxx"
 #include "fs/Traits.hxx"
@@ -42,7 +44,8 @@ extern "C" {
 Directory::Directory(std::string &&_path_utf8, Directory *_parent)
 	:parent(_parent),
 	 mtime(0), have_stat(false),
-	 path(std::move(_path_utf8))
+	 path(std::move(_path_utf8)),
+	 mounted_database(nullptr)
 {
 	INIT_LIST_HEAD(&children);
 	INIT_LIST_HEAD(&songs);
@@ -50,6 +53,8 @@ Directory::Directory(std::string &&_path_utf8, Directory *_parent)
 
 Directory::~Directory()
 {
+	delete mounted_database;
+
 	Song *song, *ns;
 	directory_for_each_song_safe(song, ns, *this)
 		song->Free();
@@ -113,6 +118,11 @@ Directory::PruneEmpty()
 
 	Directory *child, *n;
 	directory_for_each_child_safe(child, n, *this) {
+		if (child->IsMount())
+			/* never prune mount points; they're always
+			   empty by definition, but that's ok */
+			continue;
+
 		child->PruneEmpty();
 
 		if (child->IsEmpty())
@@ -233,6 +243,22 @@ Directory::Walk(bool recursive, const SongFilter *filter,
 {
 	assert(!error.IsDefined());
 
+	if (IsMount()) {
+		assert(IsEmpty());
+
+		/* TODO: eliminate this unlock/lock; it is necessary
+		   because the child's SimpleDatabasePlugin::Visit()
+		   call will lock it again */
+		db_unlock();
+		bool result = WalkMount(GetPath(), *mounted_database,
+					recursive, filter,
+					visit_directory, visit_song,
+					visit_playlist,
+					error);
+		db_lock();
+		return result;
+	}
+
 	if (visit_song) {
 		Song *song;
 		directory_for_each_song(song, *this) {