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.

19 files changed, 603 insertions, 24 deletions

diff --git a/Makefile.am b/Makefile.am
index 83a61fbc9..7ee53c812 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -579,6 +579,9 @@ libdb_plugins_a_SOURCES = \
 	src/db/plugins/simple/Song.hxx \
 	src/db/plugins/simple/SongSort.cxx \
 	src/db/plugins/simple/SongSort.hxx \
+	src/db/plugins/simple/Mount.cxx \
+	src/db/plugins/simple/Mount.hxx \
+	src/db/plugins/simple/PrefixedLightSong.hxx \
 	src/db/plugins/simple/SimpleDatabasePlugin.cxx \
 	src/db/plugins/simple/SimpleDatabasePlugin.hxx
 
diff --git a/src/Main.cxx b/src/Main.cxx
index 61e5fb2b6..b26be9135 100644
--- a/src/Main.cxx
+++ b/src/Main.cxx
@@ -214,7 +214,7 @@ glue_db_init_and_load(void)
 
 	SimpleDatabase &db = *(SimpleDatabase *)instance->database;
 	instance->update = new UpdateService(*instance->event_loop, db,
-					     *instance->storage,
+					     static_cast<CompositeStorage &>(*instance->storage),
 					     *instance);
 
 	/* run database update after daemonization? */
diff --git a/src/command/CommandError.cxx b/src/command/CommandError.cxx
index c94ffea63..89085fc68 100644
--- a/src/command/CommandError.cxx
+++ b/src/command/CommandError.cxx
@@ -112,6 +112,10 @@ print_error(Client &client, const Error &error)
 		case DB_NOT_FOUND:
 			command_error(client, ACK_ERROR_NO_EXIST, "Not found");
 			return CommandResult::ERROR;
+
+		case DB_CONFLICT:
+			command_error(client, ACK_ERROR_ARG, "Conflict");
+			return CommandResult::ERROR;
 		}
 #endif
 	} else if (error.IsDomain(errno_domain)) {
diff --git a/src/command/StorageCommands.cxx b/src/command/StorageCommands.cxx
index 92d235f4c..f0698f04b 100644
--- a/src/command/StorageCommands.cxx
+++ b/src/command/StorageCommands.cxx
@@ -29,6 +29,8 @@
 #include "Instance.hxx"
 #include "storage/Registry.hxx"
 #include "storage/CompositeStorage.hxx"
+#include "db/plugins/simple/SimpleDatabasePlugin.hxx"
+#include "db/update/Service.hxx"
 #include "Idle.hxx"
 
 static void
@@ -98,6 +100,16 @@ handle_mount(Client &client, gcc_unused int argc, char *argv[])
 		return CommandResult::ERROR;
 	}
 
+	if (strchr(local_uri, '/') != nullptr) {
+		/* allow only top-level mounts for now */
+		/* TODO: eliminate this limitation after ensuring that
+		   UpdateQueue::Erase() really gets called for every
+		   unmount, and no Directory disappears recursively
+		   during database update */
+		command_error(client, ACK_ERROR_ARG, "Bad mount point");
+		return CommandResult::ERROR;
+	}
+
 	Error error;
 	Storage *storage = CreateStorageURI(remote_uri, error);
 	if (storage == nullptr) {
@@ -111,6 +123,23 @@ handle_mount(Client &client, gcc_unused int argc, char *argv[])
 
 	composite.Mount(local_uri, storage);
 	idle_add(IDLE_MOUNT);
+
+#ifdef ENABLE_DATABASE
+	Database *_db = client.partition.instance.database;
+	if (_db != nullptr && _db->IsPlugin(simple_db_plugin)) {
+		SimpleDatabase &db = *(SimpleDatabase *)_db;
+
+		if (!db.Mount(local_uri, remote_uri, error)) {
+			composite.Unmount(local_uri);
+			return print_error(client, error);
+		}
+
+		// TODO: call Instance::OnDatabaseModified()?
+		// TODO: trigger database update?
+		idle_add(IDLE_DATABASE);
+	}
+#endif
+
 	return CommandResult::OK;
 }
 
@@ -132,11 +161,29 @@ handle_unmount(Client &client, gcc_unused int argc, char *argv[])
 		return CommandResult::ERROR;
 	}
 
+#ifdef ENABLE_DATABASE
+	if (client.partition.instance.update != nullptr)
+		/* ensure that no database update will attempt to work
+		   with the database/storage instances we're about to
+		   destroy here */
+		client.partition.instance.update->CancelMount(local_uri);
+
+	Database *_db = client.partition.instance.database;
+	if (_db != nullptr && _db->IsPlugin(simple_db_plugin)) {
+		SimpleDatabase &db = *(SimpleDatabase *)_db;
+
+		if (db.Unmount(local_uri))
+			// TODO: call Instance::OnDatabaseModified()?
+			idle_add(IDLE_DATABASE);
+	}
+#endif
+
 	if (!composite.Unmount(local_uri)) {
 		command_error(client, ACK_ERROR_ARG, "Not a mount point");
 		return CommandResult::ERROR;
 	}
 
 	idle_add(IDLE_MOUNT);
+
 	return CommandResult::OK;
 }
diff --git a/src/db/DatabaseError.hxx b/src/db/DatabaseError.hxx
index 1485a21b6..c71bbdfff 100644
--- a/src/db/DatabaseError.hxx
+++ b/src/db/DatabaseError.hxx
@@ -30,6 +30,8 @@ enum db_error {
 	DB_DISABLED,
 
 	DB_NOT_FOUND,
+
+	DB_CONFLICT,
 };
 
 extern const Domain db_domain;
diff --git a/src/db/plugins/simple/Directory.cxx b/src/db/plugins/simple/Directory.cxx
index b4255b0ac..3ac2f96a2 100644
--- a/src/db/plugins/simple/Directory.cxx
+++ b/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) {
diff --git a/src/db/plugins/simple/Directory.hxx b/src/db/plugins/simple/Directory.hxx
index 5b39231e3..029815d95 100644
--- a/src/db/plugins/simple/Directory.hxx
+++ b/src/db/plugins/simple/Directory.hxx
@@ -57,6 +57,7 @@ struct Song;
 struct db_visitor;
 class SongFilter;
 class Error;
+class Database;
 
 struct Directory {
 	/**
@@ -94,6 +95,12 @@ struct Directory {
 
 	std::string path;
 
+	/**
+	 * If this is not nullptr, then this directory does not really
+	 * exist, but is a mount point for another #Database.
+	 */
+	Database *mounted_database;
+
 public:
 	Directory(std::string &&_path_utf8, Directory *_parent);
 	~Directory();
@@ -106,6 +113,10 @@ public:
 		return new Directory(std::string(), nullptr);
 	}
 
+	bool IsMount() const {
+		return mounted_database != nullptr;
+	}
+
 	/**
 	 * Remove this #Directory object from its parent and free it.  This
 	 * must not be called with the root Directory.
diff --git a/src/db/plugins/simple/DirectorySave.cxx b/src/db/plugins/simple/DirectorySave.cxx
index 6cc5df6cb..b5989dd06 100644
--- a/src/db/plugins/simple/DirectorySave.cxx
+++ b/src/db/plugins/simple/DirectorySave.cxx
@@ -88,7 +88,8 @@ directory_save(FILE *fp, const Directory &directory)
 	directory_for_each_child(cur, directory) {
 		fprintf(fp, DIRECTORY_DIR "%s\n", cur->GetName());
 
-		directory_save(fp, *cur);
+		if (!cur->IsMount())
+			directory_save(fp, *cur);
 
 		if (ferror(fp))
 			return;
diff --git a/src/db/plugins/simple/Mount.cxx b/src/db/plugins/simple/Mount.cxx
new file mode 100644
index 000000000..96c7bbb5c
--- /dev/null
+++ b/src/db/plugins/simple/Mount.cxx
@@ -0,0 +1,96 @@
+/*
+ * Copyright (C) 2003-2014 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "Mount.hxx"
+#include "PrefixedLightSong.hxx"
+#include "db/Selection.hxx"
+#include "db/LightDirectory.hxx"
+#include "db/LightSong.hxx"
+#include "db/Interface.hxx"
+#include "fs/Traits.hxx"
+#include "util/Error.hxx"
+
+#include <string>
+
+struct PrefixedLightDirectory : LightDirectory {
+	std::string buffer;
+
+	PrefixedLightDirectory(const LightDirectory &directory,
+			       const char *base)
+		:LightDirectory(directory),
+		 buffer(IsRoot()
+			? std::string(base)
+			: PathTraitsUTF8::Build(base, uri)) {
+		uri = buffer.c_str();
+	}
+};
+
+static bool
+PrefixVisitDirectory(const char *base, const VisitDirectory &visit_directory,
+		     const LightDirectory &directory, Error &error)
+{
+	return visit_directory(PrefixedLightDirectory(directory, base), error);
+}
+
+static bool
+PrefixVisitSong(const char *base, const VisitSong &visit_song,
+		const LightSong &song, Error &error)
+{
+	return visit_song(PrefixedLightSong(song, base), error);
+}
+
+static bool
+PrefixVisitPlaylist(const char *base, const VisitPlaylist &visit_playlist,
+		    const PlaylistInfo &playlist,
+		    const LightDirectory &directory,
+		    Error &error)
+{
+	return visit_playlist(playlist,
+			      PrefixedLightDirectory(directory, base),
+			      error);
+}
+
+bool
+WalkMount(const char *base, const Database &db,
+	  bool recursive, const SongFilter *filter,
+	  const VisitDirectory &visit_directory, const VisitSong &visit_song,
+	  const VisitPlaylist &visit_playlist,
+	  Error &error)
+{
+	using namespace std::placeholders;
+
+	VisitDirectory vd;
+	if (visit_directory)
+		vd = std::bind(PrefixVisitDirectory,
+			       base, std::ref(visit_directory), _1, _2);
+
+	VisitSong vs;
+	if (visit_song)
+		vs = std::bind(PrefixVisitSong,
+			       base, std::ref(visit_song), _1, _2);
+
+	VisitPlaylist vp;
+	if (visit_playlist)
+		vp = std::bind(PrefixVisitPlaylist,
+			       base, std::ref(visit_playlist), _1, _2, _3);
+
+	return db.Visit(DatabaseSelection("", recursive, filter),
+			vd, vs, vp, error);
+}
diff --git a/src/db/plugins/simple/Mount.hxx b/src/db/plugins/simple/Mount.hxx
new file mode 100644
index 000000000..a4690114c
--- /dev/null
+++ b/src/db/plugins/simple/Mount.hxx
@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2003-2014 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_DB_SIMPLE_MOUNT_HXX
+#define MPD_DB_SIMPLE_MOUNT_HXX
+
+#include "db/Visitor.hxx"
+
+class Database;
+class SongFilter;
+class Error;
+
+bool
+WalkMount(const char *base, const Database &db,
+	  bool recursive, const SongFilter *filter,
+	  const VisitDirectory &visit_directory, const VisitSong &visit_song,
+	  const VisitPlaylist &visit_playlist,
+	  Error &error);
+
+#endif
diff --git a/src/db/plugins/simple/PrefixedLightSong.hxx b/src/db/plugins/simple/PrefixedLightSong.hxx
new file mode 100644
index 000000000..3664de001
--- /dev/null
+++ b/src/db/plugins/simple/PrefixedLightSong.hxx
@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2003-2014 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_DB_SIMPLE_PREFIXED_LIGHT_SONG_HXX
+#define MPD_DB_SIMPLE_PREFIXED_LIGHT_SONG_HXX
+
+#include "check.h"
+#include "db/LightSong.hxx"
+#include "fs/Traits.hxx"
+
+#include <string>
+
+class PrefixedLightSong : public LightSong {
+	std::string buffer;
+
+public:
+	PrefixedLightSong(const LightSong &song, const char *base)
+		:LightSong(song),
+		 buffer(PathTraitsUTF8::Build(base, GetURI().c_str())) {
+		uri = buffer.c_str();
+		directory = nullptr;
+	}
+};
+
+#endif
diff --git a/src/db/plugins/simple/SimpleDatabasePlugin.cxx b/src/db/plugins/simple/SimpleDatabasePlugin.cxx
index e83ef575b..68101f564 100644
--- a/src/db/plugins/simple/SimpleDatabasePlugin.cxx
+++ b/src/db/plugins/simple/SimpleDatabasePlugin.cxx
@@ -19,6 +19,7 @@
 
 #include "config.h"
 #include "SimpleDatabasePlugin.hxx"
+#include "PrefixedLightSong.hxx"
 #include "db/DatabasePlugin.hxx"
 #include "db/Selection.hxx"
 #include "db/Helpers.hxx"
@@ -32,6 +33,7 @@
 #include "fs/TextFile.hxx"
 #include "config/ConfigData.hxx"
 #include "fs/FileSystem.hxx"
+#include "util/CharUtil.hxx"
 #include "util/Error.hxx"
 #include "util/Domain.hxx"
 #include "Log.hxx"
@@ -42,7 +44,17 @@ static constexpr Domain simple_db_domain("simple_db");
 
 inline SimpleDatabase::SimpleDatabase()
 	:Database(simple_db_plugin),
-	 path(AllocatedPath::Null()) {}
+	 path(AllocatedPath::Null()),
+	 cache_path(AllocatedPath::Null()),
+	 prefixed_light_song(nullptr) {}
+
+inline SimpleDatabase::SimpleDatabase(AllocatedPath &&_path)
+	:Database(simple_db_plugin),
+	 path(std::move(_path)),
+	 path_utf8(path.ToUTF8()),
+	 cache_path(AllocatedPath::Null()),
+	 prefixed_light_song(nullptr) {
+}
 
 Database *
 SimpleDatabase::Create(gcc_unused EventLoop &loop,
@@ -71,6 +83,10 @@ SimpleDatabase::Configure(const config_param &param, Error &error)
 
 	path_utf8 = path.ToUTF8();
 
+	cache_path = param.GetBlockPath("cache_directory", error);
+	if (path.IsNull() && error.IsDefined())
+		return false;
+
 	return true;
 }
 
@@ -169,6 +185,8 @@ SimpleDatabase::Load(Error &error)
 bool
 SimpleDatabase::Open(Error &error)
 {
+	assert(prefixed_light_song == nullptr);
+
 	root = Directory::NewRoot();
 	mtime = 0;
 
@@ -195,6 +213,7 @@ void
 SimpleDatabase::Close()
 {
 	assert(root != nullptr);
+	assert(prefixed_light_song == nullptr);
 	assert(borrowed_song_count == 0);
 
 	delete root;
@@ -204,11 +223,27 @@ const LightSong *
 SimpleDatabase::GetSong(const char *uri, Error &error) const
 {
 	assert(root != nullptr);
+	assert(prefixed_light_song == nullptr);
 	assert(borrowed_song_count == 0);
 
 	db_lock();
 
 	auto r = root->LookupDirectory(uri);
+
+	if (r.directory->IsMount()) {
+		/* pass the request to the mounted database */
+		db_unlock();
+
+		const LightSong *song =
+			r.directory->mounted_database->GetSong(r.uri, error);
+		if (song == nullptr)
+			return nullptr;
+
+		prefixed_light_song =
+			new PrefixedLightSong(*song, r.directory->GetPath());
+		return prefixed_light_song;
+	}
+
 	if (r.uri == nullptr) {
 		/* it's a directory */
 		db_unlock();
@@ -245,11 +280,17 @@ SimpleDatabase::GetSong(const char *uri, Error &error) const
 void
 SimpleDatabase::ReturnSong(gcc_unused const LightSong *song) const
 {
-	assert(song == &light_song);
+	assert(song != nullptr);
+	assert(song == &light_song || song == prefixed_light_song);
+
+	delete prefixed_light_song;
+	prefixed_light_song = nullptr;
 
 #ifndef NDEBUG
-	assert(borrowed_song_count > 0);
-	--borrowed_song_count;
+	if (song == &light_song) {
+		assert(borrowed_song_count > 0);
+		--borrowed_song_count;
+	}
 #endif
 }
 
@@ -347,6 +388,104 @@ SimpleDatabase::Save(Error &error)
 	return true;
 }
 
+bool
+SimpleDatabase::Mount(const char *uri, Database *db, Error &error)
+{
+	assert(uri != nullptr);
+	assert(*uri != 0);
+	assert(db != nullptr);
+
+	ScopeDatabaseLock protect;
+
+	auto r = root->LookupDirectory(uri);
+	if (r.uri == nullptr) {
+		error.Format(db_domain, DB_CONFLICT,
+			     "Already exists: %s", uri);
+		return nullptr;
+	}
+
+	if (strchr(r.uri, '/') != nullptr) {
+		error.Format(db_domain, DB_NOT_FOUND,
+			     "Parent not found: %s", uri);
+		return nullptr;
+	}
+
+	Directory *mnt = r.directory->CreateChild(r.uri);
+	mnt->mounted_database = db;
+	return true;
+}
+
+static constexpr bool
+IsSafeChar(char ch)
+{
+	return IsAlphaNumericASCII(ch) || ch == '-' || ch == '_' || ch == '%';
+}
+
+static constexpr bool
+IsUnsafeChar(char ch)
+{
+	return !IsSafeChar(ch);
+}
+
+bool
+SimpleDatabase::Mount(const char *local_uri, const char *storage_uri,
+		      Error &error)
+{
+	if (cache_path.IsNull()) {
+		error.Format(db_domain, DB_NOT_FOUND,
+			     "No 'cache_directory' configured");
+		return nullptr;
+	}
+
+	std::string name(storage_uri);
+	std::replace_if(name.begin(), name.end(), IsUnsafeChar, '_');
+
+	auto db = new SimpleDatabase(AllocatedPath::Build(cache_path,
+							  name.c_str()));
+	if (!db->Open(error)) {
+		delete db;
+		return false;
+	}
+
+	// TODO: update the new database instance?
+
+	if (!Mount(local_uri, db, error)) {
+		db->Close();
+		delete db;
+		return false;
+	}
+
+	return true;
+}
+
+Database *
+SimpleDatabase::LockUmountSteal(const char *uri)
+{
+	ScopeDatabaseLock protect;
+
+	auto r = root->LookupDirectory(uri);
+	if (r.uri != nullptr || !r.directory->IsMount())
+		return nullptr;
+
+	Database *db = r.directory->mounted_database;
+	r.directory->mounted_database = nullptr;
+	r.directory->Delete();
+
+	return db;
+}
+
+bool
+SimpleDatabase::Unmount(const char *uri)
+{
+	Database *db = LockUmountSteal(uri);
+	if (db == nullptr)
+		return false;
+
+	db->Close();
+	delete db;
+	return true;
+}
+
 const DatabasePlugin simple_db_plugin = {
 	"simple",
 	DatabasePlugin::FLAG_REQUIRE_STORAGE,
diff --git a/src/db/plugins/simple/SimpleDatabasePlugin.hxx b/src/db/plugins/simple/SimpleDatabasePlugin.hxx
index 9836a6bcf..a03969f92 100644
--- a/src/db/plugins/simple/SimpleDatabasePlugin.hxx
+++ b/src/db/plugins/simple/SimpleDatabasePlugin.hxx
@@ -32,16 +32,28 @@ struct Directory;
 struct DatabasePlugin;
 class EventLoop;
 class DatabaseListener;
+class PrefixedLightSong;
 
 class SimpleDatabase : public Database {
 	AllocatedPath path;
 	std::string path_utf8;
 
+	/**
+	 * The path where cache files for Mount() are located.
+	 */
+	AllocatedPath cache_path;
+
 	Directory *root;
 
 	time_t mtime;
 
 	/**
+	 * A buffer for GetSong() when prefixing the #LightSong
+	 * instance from a mounted #Database.
+	 */
+	mutable PrefixedLightSong *prefixed_light_song;
+
+	/**
 	 * A buffer for GetSong().
 	 */
 	mutable LightSong light_song;
@@ -52,6 +64,8 @@ class SimpleDatabase : public Database {
 
 	SimpleDatabase();
 
+	SimpleDatabase(AllocatedPath &&_path);
+
 public:
 	static Database *Create(EventLoop &loop, DatabaseListener &listener,
 				const config_param &param,
@@ -73,6 +87,20 @@ public:
 		return mtime > 0;
 	}
 
+	/**
+	 * @param db the #Database to be mounted; must be "open"; on
+	 * success, this object gains ownership of the given #Database
+	 */
+	gcc_nonnull_all
+	bool Mount(const char *uri, Database *db, Error &error);
+
+	gcc_nonnull_all
+	bool Mount(const char *local_uri, const char *storage_uri,
+		   Error &error);
+
+	gcc_nonnull_all
+	bool Unmount(const char *uri);
+
 	/* virtual methods from class Database */
 	virtual bool Open(Error &error) override;
 	virtual void Close() override;
@@ -107,6 +135,8 @@ private:
 	bool Check(Error &error) const;
 
 	bool Load(Error &error);
+
+	Database *LockUmountSteal(const char *uri);
 };
 
 extern const DatabasePlugin simple_db_plugin;
diff --git a/src/db/update/Container.cxx b/src/db/update/Container.cxx
index 9f8d84839..1c420fa99 100644
--- a/src/db/update/Container.cxx
+++ b/src/db/update/Container.cxx
@@ -42,6 +42,9 @@ UpdateWalk::MakeDirectoryIfModified(Directory &parent, const char *name,
 
 	// directory exists already
 	if (directory != nullptr) {
+		if (directory->IsMount())
+			return nullptr;
+
 		if (directory->mtime == info.mtime && !walk_discard) {
 			/* not modified */
 			return nullptr;
diff --git a/src/db/update/Queue.cxx b/src/db/update/Queue.cxx
index 096a39a8c..6d6d80131 100644
--- a/src/db/update/Queue.cxx
+++ b/src/db/update/Queue.cxx
@@ -21,12 +21,13 @@
 #include "Queue.hxx"
 
 bool
-UpdateQueue::Push(const char *path, bool discard, unsigned id)
+UpdateQueue::Push(SimpleDatabase &db, Storage &storage,
+		  const char *path, bool discard, unsigned id)
 {
 	if (update_queue.size() >= MAX_UPDATE_QUEUE_SIZE)
 		return false;
 
-	update_queue.emplace_back(path, discard, id);
+	update_queue.emplace_back(db, storage, path, discard, id);
 	return true;
 }
 
@@ -40,3 +41,27 @@ UpdateQueue::Pop()
 	update_queue.pop_front();
 	return i;
 }
+
+void
+UpdateQueue::Erase(SimpleDatabase &db)
+{
+	for (auto i = update_queue.begin(), end = update_queue.end();
+	     i != end;) {
+		if (i->db == &db)
+			i = update_queue.erase(i);
+		else
+			++i;
+	}
+}
+
+void
+UpdateQueue::Erase(Storage &storage)
+{
+	for (auto i = update_queue.begin(), end = update_queue.end();
+	     i != end;) {
+		if (i->storage == &storage)
+			i = update_queue.erase(i);
+		else
+			++i;
+	}
+}
diff --git a/src/db/update/Queue.hxx b/src/db/update/Queue.hxx
index 039c62fe3..9064ea481 100644
--- a/src/db/update/Queue.hxx
+++ b/src/db/update/Queue.hxx
@@ -21,19 +21,30 @@
 #define MPD_UPDATE_QUEUE_HXX
 
 #include "check.h"
+#include "Compiler.h"
 
 #include <string>
 #include <list>
 
+class SimpleDatabase;
+class Storage;
+
 struct UpdateQueueItem {
+	SimpleDatabase *db;
+	Storage *storage;
+
 	std::string path_utf8;
 	unsigned id;
 	bool discard;
 
 	UpdateQueueItem():id(0) {}
-	UpdateQueueItem(const char *_path, bool _discard,
+
+	UpdateQueueItem(SimpleDatabase &_db,
+			Storage &_storage,
+			const char *_path, bool _discard,
 			unsigned _id)
-		:path_utf8(_path), id(_id), discard(_discard) {}
+		:db(&_db), storage(&_storage), path_utf8(_path),
+		 id(_id), discard(_discard) {}
 
 	bool IsDefined() const {
 		return id != 0;
@@ -46,13 +57,21 @@ class UpdateQueue {
 	std::list<UpdateQueueItem> update_queue;
 
 public:
-	bool Push(const char *path, bool discard, unsigned id);
+	gcc_nonnull_all
+	bool Push(SimpleDatabase &db, Storage &storage,
+		  const char *path, bool discard, unsigned id);
 
 	UpdateQueueItem Pop();
 
 	void Clear() {
 		update_queue.clear();
 	}
+
+	gcc_nonnull_all
+	void Erase(SimpleDatabase &db);
+
+	gcc_nonnull_all
+	void Erase(Storage &storage);
 };
 
 #endif
diff --git a/src/db/update/Service.cxx b/src/db/update/Service.cxx
index 2971998e4..e8a1f6b02 100644
--- a/src/db/update/Service.cxx
+++ b/src/db/update/Service.cxx
@@ -22,7 +22,10 @@
 #include "Walk.hxx"
 #include "UpdateDomain.hxx"
 #include "db/DatabaseListener.hxx"
+#include "db/DatabaseLock.hxx"
 #include "db/plugins/simple/SimpleDatabasePlugin.hxx"
+#include "db/plugins/simple/Directory.hxx"
+#include "storage/CompositeStorage.hxx"
 #include "Idle.hxx"
 #include "util/Error.hxx"
 #include "Log.hxx"
@@ -39,7 +42,7 @@
 #include <assert.h>
 
 UpdateService::UpdateService(EventLoop &_loop, SimpleDatabase &_db,
-			     Storage &_storage,
+			     CompositeStorage &_storage,
 			     DatabaseListener &_listener)
 	:DeferredMonitor(_loop),
 	 db(_db), storage(_storage),
@@ -71,6 +74,42 @@ UpdateService::CancelAllAsync()
 		walk->Cancel();
 }
 
+void
+UpdateService::CancelMount(const char *uri)
+{
+	/* determine which (mounted) database will be updated and what
+	   storage will be scanned */
+
+	db_lock();
+	const auto lr = db.GetRoot().LookupDirectory(uri);
+	db_unlock();
+
+	if (!lr.directory->IsMount())
+		return;
+
+	bool cancel_current = false;
+
+	Storage *storage2 = storage.GetMount(uri);
+	if (storage2 != nullptr) {
+		queue.Erase(*storage2);
+		cancel_current = next.IsDefined() && next.storage == storage2;
+	}
+
+	Database &_db2 = *lr.directory->mounted_database;
+	if (_db2.IsPlugin(simple_db_plugin)) {
+		SimpleDatabase &db2 = static_cast<SimpleDatabase &>(_db2);
+		queue.Erase(db2);
+		cancel_current |= next.IsDefined() && next.db == &db2;
+	}
+
+	if (cancel_current && walk != nullptr) {
+		walk->Cancel();
+
+		if (update_thread.IsDefined())
+			update_thread.Join();
+	}
+}
+
 inline void
 UpdateService::Task()
 {
@@ -84,12 +123,12 @@ UpdateService::Task()
 
 	SetThreadIdlePriority();
 
-	modified = walk->Walk(db.GetRoot(), next.path_utf8.c_str(),
+	modified = walk->Walk(next.db->GetRoot(), next.path_utf8.c_str(),
 			      next.discard);
 
-	if (modified || !db.FileExists()) {
+	if (modified || !next.db->FileExists()) {
 		Error error;
-		if (!db.Save(error))
+		if (!next.db->Save(error))
 			LogError(error, "Failed to save database");
 	}
 
@@ -120,7 +159,7 @@ UpdateService::StartThread(UpdateQueueItem &&i)
 	modified = false;
 
 	next = std::move(i);
-	walk = new UpdateWalk(GetEventLoop(), listener, storage);
+	walk = new UpdateWalk(GetEventLoop(), listener, *next.storage);
 
 	Error error;
 	if (!update_thread.Start(Task, this, error))
@@ -144,9 +183,52 @@ UpdateService::Enqueue(const char *path, bool discard)
 {
 	assert(GetEventLoop().IsInsideOrNull());
 
+	/* determine which (mounted) database will be updated and what
+	   storage will be scanned */
+	SimpleDatabase *db2;
+	Storage *storage2;
+
+	db_lock();
+	const auto lr = db.GetRoot().LookupDirectory(path);
+	db_unlock();
+	if (lr.directory->IsMount()) {
+		/* follow the mountpoint, update the mounted
+		   database */
+
+		Database &_db2 = *lr.directory->mounted_database;
+		if (!_db2.IsPlugin(simple_db_plugin))
+			/* cannot update this type of database */
+			return 0;
+
+		db2 = static_cast<SimpleDatabase *>(&_db2);
+
+		if (lr.uri == nullptr) {
+			storage2 = storage.GetMount(path);
+			path = "";
+		} else {
+			assert(lr.uri > path);
+			assert(lr.uri < path + strlen(path));
+			assert(lr.uri[-1] == '/');
+
+			const std::string mountpoint(path, lr.uri - 1);
+			storage2 = storage.GetMount(mountpoint.c_str());
+			path = lr.uri;
+		}
+	} else {
+		/* use the "root" database/storage */
+
+		db2 = &db;
+		storage2 = storage.GetMount("");
+	}
+
+	if (storage2 == nullptr)
+		/* no storage found at this mount point - should not
+		   happen */
+		return 0;
+
 	if (progress != UPDATE_PROGRESS_IDLE) {
 		const unsigned id = GenerateId();
-		if (!queue.Push(path, discard, id))
+		if (!queue.Push(*db2, *storage2, path, discard, id))
 			return 0;
 
 		update_task_id = id;
@@ -154,7 +236,7 @@ UpdateService::Enqueue(const char *path, bool discard)
 	}
 
 	const unsigned id = update_task_id = GenerateId();
-	StartThread(UpdateQueueItem(path, discard, id));
+	StartThread(UpdateQueueItem(*db2, *storage2, path, discard, id));
 
 	idle_add(IDLE_UPDATE);
 
@@ -171,7 +253,10 @@ UpdateService::RunDeferred()
 	assert(next.IsDefined());
 	assert(walk != nullptr);
 
-	update_thread.Join();
+	/* wait for thread to finish only if it wasn't cancelled by
+	   CancelMount() */
+	if (update_thread.IsDefined())
+		update_thread.Join();
 
 	delete walk;
 	walk = nullptr;
diff --git a/src/db/update/Service.hxx b/src/db/update/Service.hxx
index e42a0fa5a..cbb4a3f9d 100644
--- a/src/db/update/Service.hxx
+++ b/src/db/update/Service.hxx
@@ -28,7 +28,7 @@
 class SimpleDatabase;
 class DatabaseListener;
 class UpdateWalk;
-class Storage;
+class CompositeStorage;
 
 /**
  * This class manages the update queue and runs the update thread.
@@ -41,7 +41,7 @@ class UpdateService final : DeferredMonitor {
 	};
 
 	SimpleDatabase &db;
-	Storage &storage;
+	CompositeStorage &storage;
 
 	DatabaseListener &listener;
 
@@ -63,7 +63,7 @@ class UpdateService final : DeferredMonitor {
 
 public:
 	UpdateService(EventLoop &_loop, SimpleDatabase &_db,
-		      Storage &_storage,
+		      CompositeStorage &_storage,
 		      DatabaseListener &_listener);
 
 	~UpdateService();
@@ -92,6 +92,13 @@ public:
 	 */
 	void CancelAllAsync();
 
+	/**
+	 * Cancel all updates for the given mount point.  If an update
+	 * is already running for it, the method will wait for
+	 * cancellation to complete.
+	 */
+	void CancelMount(const char *uri);
+
 private:
 	/* virtual methods from class DeferredMonitor */
 	virtual void RunDeferred() override;
diff --git a/src/db/update/Walk.cxx b/src/db/update/Walk.cxx
index 201030f25..c329865ff 100644
--- a/src/db/update/Walk.cxx
+++ b/src/db/update/Walk.cxx
@@ -397,8 +397,12 @@ UpdateWalk::DirectoryMakeChildChecked(Directory &parent,
 	Directory *directory = parent.FindChild(name_utf8);
 	db_unlock();
 
-	if (directory != nullptr)
+	if (directory != nullptr) {
+		if (directory->IsMount())
+			directory = nullptr;
+
 		return directory;
+	}
 
 	FileInfo info;
 	if (!GetInfo(storage, uri_utf8, info) ||