gems: TrueType VFS plugin

This commit introduces a VFS plugin that exposes the glyphs and
metadata of a TrueType font as a pseudo file system. The TTF font data
is obtained from the VFS. The resulting pseudo file system is a
directory that contains the files 'glyphs', 'baseline', 'max_width',
and 'max_height'.

The counter part of the plugin is the 'Vfs_font' class that implements
the 'Text_painter::Font' interface by accessing the pseudo file system
as provided by the TTF VFS plugin.

Fixes #2740

repos/gems/include/gems/cached_font.h

@@ -0,0 +1,270 @@
+/*
+ * \brief  Glyph cache
+ * \author Norman Feske
+ * \date   2018-03-27
+ */
+
+/*
+ * Copyright (C) 2018 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+#ifndef _INCLUDE__GEMS__CACHED_FONT_T_
+#define _INCLUDE__GEMS__CACHED_FONT_T_
+
+#include <base/output.h>
+#include <base/allocator.h>
+#include <nitpicker_gfx/text_painter.h>
+
+namespace Genode { class Cached_font; }
+
+
+class Genode::Cached_font : public Text_painter::Font
+{
+	public:
+
+		struct Stats
+		{
+			unsigned misses;
+			unsigned hits;
+			unsigned consumed_bytes;
+
+			void print(Output &out) const
+			{
+				Genode::print(out, "used: ", consumed_bytes/1024, " KiB, "
+				              "hits: ", hits, ", misses: ", misses);
+			}
+		};
+
+	private:
+
+		typedef Text_painter::Area  Area;
+		typedef Text_painter::Font  Font;
+		typedef Text_painter::Glyph Glyph;
+
+		struct Time { unsigned value; };
+
+		Allocator    &_alloc;
+		Font   const &_font;
+		size_t const  _limit;
+		Time  mutable _now { 0 };
+		Stats mutable _stats { };
+
+		class Cached_glyph : Avl_node<Cached_glyph>
+		{
+			private:
+
+				friend class Avl_node<Cached_glyph>;
+				friend class Avl_tree<Cached_glyph>;
+				friend class Cached_font;
+
+				Codepoint const _codepoint;
+				Glyph     const _glyph;
+				Time            _last_used;
+
+				Glyph::Opacity _values[];
+
+				bool _higher(Codepoint const other) const
+				{
+					return _codepoint.value > other.value;
+				}
+
+				unsigned _importance(Time now) const
+				{
+					return now.value - _last_used.value;
+				}
+
+			public:
+
+				Cached_glyph(Codepoint c, Glyph const &glyph, Time now)
+				:
+					_codepoint(c),
+					_glyph({ .width   = glyph.width,
+					         .height  = glyph.height,
+					         .vpos    = glyph.vpos,
+					         .advance = glyph.advance,
+					         .values  = _values }),
+					_last_used(now)
+				{
+					for (unsigned i = 0; i < glyph.num_values(); i++)
+						_values[i] = glyph.values[i];
+				}
+
+				/**
+				 * Avl_node interface
+				 */
+				bool higher(Cached_glyph *c) { return _higher(c->_codepoint); }
+
+				Cached_glyph *find_by_codepoint(Codepoint requested)
+				{
+					if (_codepoint.value == requested.value) return this;
+
+					Cached_glyph *c = Avl_node<Cached_glyph>::child(_higher(requested));
+
+					return c ? c->find_by_codepoint(requested) : nullptr;
+				}
+
+				Cached_glyph *find_least_recently_used(Time now)
+				{
+					Cached_glyph *result = this;
+
+					for (unsigned i = 0; i < 2; i++) {
+						Cached_glyph *c = Avl_node<Cached_glyph>::child(i);
+						if (c && c->_importance(now) > result->_importance(now))
+							result = c;
+					}
+					return result;
+				}
+
+				void mark_as_used(Time now) { _last_used = now; }
+
+				void apply(Font::Apply_fn const &fn) const { fn.apply(_glyph); }
+		};
+
+		Avl_tree<Cached_glyph> mutable _avl_tree { };
+
+		/**
+		 * Size of one cache entry in bytes
+		 */
+		size_t const _alloc_size = sizeof(Cached_glyph)
+		                         + 4*_font.bounding_box().count();
+
+		/**
+		 * Add cache entry for the given glyph
+		 *
+		 * \throw Out_of_ram
+		 * \throw Out_of_caps
+		 */
+		void _insert(Codepoint codepoint, Glyph const &glyph)
+		{
+			auto const cached_glyph_ptr = (Cached_glyph *)_alloc.alloc(_alloc_size);
+
+			_stats.consumed_bytes += _alloc_size;
+
+			memset(cached_glyph_ptr, 0, _alloc_size);
+
+			construct_at<Cached_glyph>(cached_glyph_ptr, codepoint, glyph, _now);
+
+			_avl_tree.insert(cached_glyph_ptr);
+		}
+
+		/**
+		 * Evict glyph from cache
+		 */
+		void _remove(Cached_glyph &glyph)
+		{
+			_avl_tree.remove(&glyph);
+
+			glyph.~Cached_glyph();
+
+			_alloc.free(&glyph, _alloc_size);
+
+			_stats.consumed_bytes -= _alloc_size;
+		}
+
+		Cached_glyph *_find_by_codepoint(Codepoint codepoint)
+		{
+			if (!_avl_tree.first())
+				return nullptr;
+
+			return _avl_tree.first()->find_by_codepoint(codepoint);
+		}
+
+		/**
+		 * Evice least recently used glyph from cache
+		 *
+		 * \return true if a glyph was released
+		 */
+		bool _remove_least_recently_used()
+		{
+			if (!_avl_tree.first())
+				return false;
+
+			Cached_glyph *glyph = _avl_tree.first()->find_least_recently_used(_now);
+			if (!glyph)
+				return false; /* this should never happen */
+
+			_remove(*glyph);
+			_stats.misses++;
+			return true;
+		}
+
+		void _remove_all()
+		{
+			while (Cached_glyph *glyph_ptr = _avl_tree.first())
+				_remove(*glyph_ptr);
+		}
+
+	public:
+
+		struct Limit { size_t value; };
+
+		/**
+		 * Constructor
+		 *
+		 * \param alloc  backing store for cached glyphs
+		 * \param font   original (uncached) font
+		 * \param limit  maximum cache size in bytes
+		 */
+		Cached_font(Allocator &alloc, Font const &font, Limit limit)
+		:
+			_alloc(alloc), _font(font), _limit(limit.value)
+		{ }
+
+		~Cached_font() { _remove_all(); }
+
+		Stats stats() const { return _stats; }
+
+		void _apply_glyph(Codepoint c, Apply_fn const &fn) const override
+		{
+			_now.value++;
+
+			/*
+			 * Try to lookup glyph from the cache. If it is missing, fill cache
+			 * with requested glyph and repeat the lookup. When under memory
+			 * pressure, flush least recently used glyphs from cache.
+			 *
+			 * Even though '_apply_glyph' is a const method, the internal cache
+			 * and stats must of course be mutated. Hence the 'const_cast'.
+			 */
+			Cached_font &mutable_this = const_cast<Cached_font &>(*this);
+
+			/* retry once after handling a cache miss */
+			for (int i = 0; i < 2; i++) {
+
+				if (Cached_glyph *glyph_ptr = mutable_this._find_by_codepoint(c)) {
+					glyph_ptr->apply(fn);
+					glyph_ptr->mark_as_used(_now);
+					_stats.hits += (i == 0);
+					return;
+				}
+
+				while (_stats.consumed_bytes + _alloc_size > _limit)
+					if (!mutable_this._remove_least_recently_used())
+						break;
+
+				_font.apply_glyph(c, [&] (Glyph const &glyph) {
+					mutable_this._insert(c, glyph); });
+			}
+		}
+
+		Advance_info advance_info(Codepoint c) const override
+		{
+			unsigned                      width = 0;
+			Text_painter::Fixpoint_number advance { 0 };
+
+			/* go through the '_apply_glyph' cache-fill mechanism */
+			Font::apply_glyph(c, [&] (Glyph const &glyph) {
+				width = glyph.width, advance = glyph.advance; });
+
+			return Advance_info { .width = width, .advance = advance };
+		}
+
+		unsigned baseline() const override { return _font.baseline(); }
+
+		Area bounding_box() const override { return _font.bounding_box(); }
+};
+
+#endif /* _INCLUDE__GEMS__CACHED_FONT_T_ */

repos/gems/include/gems/vfs_font.h

@@ -0,0 +1,169 @@
+/*
+ * \brief  Implementation of 'Text_painter::Font' for VFS-mounted fonts
+ * \author Norman Feske
+ * \date   2018-03-26
+ */
+
+/*
+ * Copyright (C) 2018 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+#ifndef _INCLUDE__GEMS__VFS_FONT_T_
+#define _INCLUDE__GEMS__VFS_FONT_T_
+
+#include <gems/vfs.h>
+#include <nitpicker_gfx/text_painter.h>
+
+namespace Genode { class Vfs_font; }
+
+
+class Genode::Vfs_font : public Text_painter::Font
+{
+	public:
+
+		typedef Glyph_painter::Glyph Glyph;
+
+		static constexpr Vfs::file_size GLYPH_SLOT_BYTES = 64*1024;
+
+		class Glyph_header
+		{
+			private:
+
+				uint8_t _width              = 0;
+				uint8_t _height             = 0;
+				uint8_t _vpos               = 0;
+				int8_t  _advance_decimal    = 0;
+				uint8_t _advance_fractional = 0;
+				uint8_t _reserved[3] { };
+
+				Glyph::Opacity _values[];
+
+				float _advance() const
+				{
+					float value = 256.0*_advance_decimal + _advance_fractional;
+					return value/256;
+				}
+
+			public:
+
+				Glyph_header(Glyph const &glyph)
+				:
+					_width ((uint8_t)min(255U, glyph.width)),
+					_height((uint8_t)min(255U, glyph.height)),
+					_vpos  ((uint8_t)min(255U, glyph.vpos)),
+					_advance_decimal((int8_t)max(-127, min(127, glyph.advance.decimal()))),
+					_advance_fractional((uint8_t)glyph.advance.value & 0xff)
+				{ }
+
+				Glyph_header() { }
+
+				Glyph glyph() const { return Glyph { .width   = _width,
+				                                     .height  = _height,
+				                                     .vpos    = _vpos,
+				                                     .advance = _advance(),
+				                                     .values  = _values }; }
+
+		} __attribute__((packed));
+
+	private:
+
+		typedef Text_painter::Codepoint Codepoint;
+		typedef Text_painter::Area      Area;
+		typedef Directory::Path         Path;
+
+		Directory const _font_dir;
+		unsigned  const _baseline;
+		Area      const _bounding_box;
+
+		struct Glyph_buffer
+		{
+			Allocator &_alloc;
+
+			unsigned const num_bytes;
+
+			Glyph_header &header;
+
+			Glyph_buffer(Allocator &alloc, Area size)
+			:
+				_alloc(alloc), num_bytes(sizeof(Glyph_header) + size.count()*4),
+				header(*(Glyph_header *)alloc.alloc(num_bytes))
+			{ }
+
+			~Glyph_buffer() { _alloc.free(&header, num_bytes); }
+
+			char *ptr() { return (char *)&header; }
+		};
+
+		Glyph_buffer mutable _buffer;
+
+		Readonly_file _glyphs_file;
+
+		template <typename T, unsigned MAX_LEN = 128>
+		static T _value_from_file(Directory const &dir, Path const &path,
+		                          T const &default_value)
+		{
+			T result = default_value;
+			try {
+				Readonly_file const file(dir, path);
+				char buf[MAX_LEN + 1] { };
+				if (file.read(buf, sizeof(buf)) <= MAX_LEN)
+					if (ascii_to(buf, result))
+						return result;
+			}
+			catch (Readonly_file::Open_failed) { }
+			return default_value;
+		}
+
+		static Readonly_file::At _file_pos(Codepoint c)
+		{
+			return Readonly_file::At{(Vfs::file_size)c.value*GLYPH_SLOT_BYTES};
+		}
+
+	public:
+
+		struct Unavailable : Exception { };
+
+		/**
+		 * Constructor
+		 *
+		 * \param alloc  allocator for glyph buffer
+		 * \param dir    directory
+		 * \param path   path to font
+		 *
+		 * \throw Unavailable  unable to obtain font data
+		 */
+		Vfs_font(Allocator &alloc, Directory const &dir, Path const &path)
+		:
+			_font_dir(dir, path),
+			_baseline(_value_from_file(_font_dir, "baseline", 0U)),
+			_bounding_box(_value_from_file(_font_dir, "max_width",  0U),
+			              _value_from_file(_font_dir, "max_height", 0U)),
+			_buffer(alloc, _bounding_box),
+			_glyphs_file(_font_dir, "glyphs")
+		{ }
+
+		void _apply_glyph(Codepoint c, Apply_fn const &fn) const override
+		{
+			_glyphs_file.read(_file_pos(c), _buffer.ptr(), _buffer.num_bytes);
+
+			fn.apply(_buffer.header.glyph());
+		}
+
+		Advance_info advance_info(Codepoint c) const override
+		{
+			_glyphs_file.read(_file_pos(c), _buffer.ptr(), sizeof(Glyph_header));
+
+			Glyph const glyph = _buffer.header.glyph();
+
+			return Advance_info { .width = glyph.width, .advance = glyph.advance };
+		}
+
+		unsigned baseline() const override { return _baseline; }
+
+		Area bounding_box() const override { return _bounding_box; }
+};
+
+#endif /* _INCLUDE__GEMS__TTF_FONT_T_ */

repos/gems/lib/mk/vfs_ttf.mk

@@ -0,0 +1,9 @@
+SRC_CC = vfs.cc
+
+INC_DIR += $(REP_DIR)/src/lib/vfs/ttf
+
+LIBS  += ttf_font
+
+vpath %.cc $(REP_DIR)/src/lib/vfs/ttf
+
+SHARED_LIB = yes

repos/gems/recipes/src/vfs_ttf/content.mk

@@ -0,0 +1,16 @@
+MIRROR_FROM_REP_DIR := lib/mk/vfs_ttf.mk lib/mk/ttf_font.mk \
+                       src/lib/vfs/ttf src/lib/ttf_font
+
+STB_PORT_DIR := $(call port_dir,$(GENODE_DIR)/repos/libports/ports/stb)
+
+content: $(MIRROR_FROM_REP_DIR) include/stb_truetype.h LICENSE
+
+include/stb_truetype.h:
+	mkdir -p $(dir $@)
+	cp -r $(STB_PORT_DIR)/include/stb_truetype.h $@
+
+$(MIRROR_FROM_REP_DIR):
+	$(mirror_from_rep_dir)
+
+LICENSE:
+	cp $(GENODE_DIR)/LICENSE $@

repos/gems/recipes/src/vfs_ttf/hash

@@ -0,0 +1 @@
+2018-03-30 89b39a8703dbb8b098068d2ec4f73b4705debb44

repos/gems/recipes/src/vfs_ttf/used_apis

@@ -0,0 +1,7 @@
+base
+os
+so
+libc
+vfs
+gems
+nitpicker_gfx

repos/gems/src/lib/vfs/ttf/glyphs_file_system.h

@@ -0,0 +1,154 @@
+/*
+ * \brief  Glyphs file system
+ * \author Norman Feske
+ * \date   2018-03-26
+ */
+
+/*
+ * Copyright (C) 2018 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+#ifndef _GLYPHS_FILE_SYSTEM_H_
+#define _GLYPHS_FILE_SYSTEM_H_
+
+/* Genode includes */
+#include <vfs/single_file_system.h>
+#include <nitpicker_gfx/text_painter.h>
+
+/* gems includes */
+#include <gems/vfs_font.h>
+
+namespace Vfs {
+
+	using namespace Genode;
+
+	class Glyphs_file_system;
+
+	typedef Text_painter::Font     Font;
+	typedef Vfs_font::Glyph_header Glyph_header;
+}
+
+
+class Vfs::Glyphs_file_system : public Vfs::Single_file_system
+{
+	private:
+
+		static constexpr unsigned  UNICODE_MAX = 0x10ffff;
+
+		static constexpr file_size FILE_SIZE = Vfs_font::GLYPH_SLOT_BYTES*(UNICODE_MAX + 1);
+
+		Font const &_font;
+
+		struct Vfs_handle : Single_vfs_handle
+		{
+			Font const &_font;
+
+			Vfs_handle(Directory_service &ds,
+			           File_io_service   &fs,
+			           Allocator         &alloc,
+			           Font        const &font)
+			:
+				Single_vfs_handle(ds, fs, alloc, 0), _font(font)
+			{ }
+
+			Read_result read(char *dst, file_size count,
+			                 file_size &out_count) override
+			{
+				out_count = 0;
+
+				if (seek() > FILE_SIZE)
+					return READ_ERR_INVALID;
+
+				Codepoint const codepoint { (uint32_t)(seek() / Vfs_font::GLYPH_SLOT_BYTES) };
+
+				file_size byte_offset = seek() % Vfs_font::GLYPH_SLOT_BYTES;
+
+				_font.apply_glyph(codepoint, [&] (Glyph_painter::Glyph const &glyph) {
+
+					if (byte_offset < sizeof(Glyph_header)) {
+
+						Glyph_header const header(glyph);
+
+						char const * const src = (char const *)&header + byte_offset;
+						size_t const len = min(sizeof(header) - byte_offset, count);
+						memcpy(dst, src, len);
+
+						dst         += len;
+						byte_offset += len;
+						count       -= len;
+						out_count   += len;
+					}
+
+					/*
+					 * Given that 'byte_offset' is at least 'sizeof(header)',
+					 * continue working with 'alpha_offset', which is the first
+					 * offset of interest within the array of alpha values.
+					 */
+					size_t const alpha_offset = byte_offset - sizeof(Glyph_header);
+					size_t const alpha_values_len = 4*glyph.width*glyph.height;
+
+					if (alpha_offset < alpha_values_len) {
+						char const * const src = (char const *)glyph.values + alpha_offset;
+						size_t const len = min(alpha_values_len - alpha_offset, count);
+						memcpy(dst, src, len);
+						out_count += len;
+					}
+				});
+
+				return READ_OK;
+			}
+
+			Write_result write(char const *, file_size, file_size &) override
+			{
+				return WRITE_ERR_IO;
+			}
+
+			bool read_ready() override { return true; }
+		};
+
+	public:
+
+		Glyphs_file_system(Font const &font)
+		:
+			Single_file_system(NODE_TYPE_CHAR_DEVICE, type(), Xml_node("<glyphs/>")),
+			_font(font)
+		{ }
+
+		static char const *type_name() { return "glyphs"; }
+
+		char const *type() override { return type_name(); }
+
+
+		/*********************************
+		 ** Directory-service interface **
+		 *********************************/
+
+		Open_result open(char const  *path, unsigned,
+		                 Vfs::Vfs_handle **out_handle,
+		                 Allocator   &alloc) override
+		{
+			if (!_single_file(path))
+				return OPEN_ERR_UNACCESSIBLE;
+
+			try {
+				*out_handle = new (alloc)
+					Vfs_handle(*this, *this, alloc, _font);
+				return OPEN_OK;
+			}
+			catch (Genode::Out_of_ram)  { return OPEN_ERR_OUT_OF_RAM; }
+			catch (Genode::Out_of_caps) { return OPEN_ERR_OUT_OF_CAPS; }
+		}
+
+		Stat_result stat(char const *path, Stat &out) override
+		{
+			Stat_result result = Single_file_system::stat(path, out);
+			out.mode |= 0444;
+			out.size = FILE_SIZE;
+			return result;
+		}
+};
+
+#endif /* _GLYPHS_FILE_SYSTEM_H_ */

repos/gems/src/lib/vfs/ttf/target.mk

@@ -0,0 +1,2 @@
+TARGET = dummy-vfs_ttf
+LIBS = vfs_ttf

repos/gems/src/lib/vfs/ttf/vfs.cc

@@ -0,0 +1,173 @@
+/*
+ * \brief  Truetype font file system
+ * \author Norman Feske
+ * \date   2018-03-07
+ */
+
+/*
+ * Copyright (C) 2018 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+/* Genode includes */
+#include <vfs/dir_file_system.h>
+#include <vfs/readonly_value_file_system.h>
+
+/* gems includes */
+#include <gems/ttf_font.h>
+#include <gems/vfs.h>
+#include <gems/cached_font.h>
+
+/* local includes */
+#include <glyphs_file_system.h>
+
+namespace Vfs_ttf {
+
+	using namespace Vfs;
+	using namespace Genode;
+
+	class Font_from_file;
+	class Local_factory;
+	class File_system;
+
+	struct Dummy_io_response_handler : Vfs::Io_response_handler
+	{
+		void handle_io_response(Vfs::Vfs_handle::Context *) override { };
+	};
+
+	typedef Text_painter::Font Font;
+}
+
+
+struct Vfs_ttf::Font_from_file
+{
+	typedef Directory::Path Path;
+
+	Directory    const _dir;
+	File_content const _content;
+
+	Constructible<Ttf_font const> _font { };
+
+	/*
+	 * Each slot of the glyphs file is 64 KiB, which limits the maximum glyph
+	 * size to 128x128. We cap the size at 100px to prevent cut-off glyphs.
+	 */
+	static constexpr float MAX_SIZE_PX = 100.0;
+
+	Font_from_file(Vfs::File_system &root, Entrypoint &ep, Allocator &alloc,
+	               Path const &file_path, float px)
+	:
+		_dir(root, ep, alloc),
+		_content(alloc, _dir, file_path, File_content::Limit{10*1024*1024})
+	{
+		_content.bytes([&] (char const *ptr, size_t) {
+			_font.construct(alloc, ptr, min(px, MAX_SIZE_PX)); });
+	}
+
+	Font const &font() const { return *_font; }
+};
+
+
+struct Vfs_ttf::Local_factory : File_system_factory
+{
+	Font_from_file                       _font;
+	Cached_font::Limit                   _cache_limit;
+	Cached_font                          _cached_font;
+	Glyphs_file_system                   _glyphs_fs;
+	Readonly_value_file_system<unsigned> _baseline_fs;
+	Readonly_value_file_system<unsigned> _max_width_fs;
+	Readonly_value_file_system<unsigned> _max_height_fs;
+
+	Local_factory(Env &env, Allocator &alloc, Xml_node node,
+	              Vfs::File_system &root_dir)
+	:
+		_font(root_dir, env.ep(), alloc,
+		      node.attribute_value("path", Directory::Path()),
+		      node.attribute_value("size_px", 16.0)),
+		_cache_limit({node.attribute_value("cache", Number_of_bytes())}),
+		_cached_font(alloc, _font.font(), _cache_limit),
+		_glyphs_fs    (_cached_font),
+		_baseline_fs  ("baseline",   _font.font().baseline()),
+		_max_width_fs ("max_width",  _font.font().bounding_box().w()),
+		_max_height_fs("max_height", _font.font().bounding_box().h())
+	{ }
+
+	Vfs::File_system *create(Env &, Allocator &, Xml_node node,
+	                         Io_response_handler &, Vfs::File_system &) override
+	{
+		if (node.has_type(Glyphs_file_system::type_name()))
+			return &_glyphs_fs;
+
+		if (node.has_type(Readonly_value_file_system<unsigned>::type_name()))
+			return _baseline_fs.matches(node)   ? &_baseline_fs
+			     : _max_width_fs.matches(node)  ? &_max_width_fs
+			     : _max_height_fs.matches(node) ? &_max_height_fs
+			     : nullptr;
+
+		return nullptr;
+	}
+};
+
+
+class Vfs_ttf::File_system : private Local_factory,
+                             private Dummy_io_response_handler,
+                             public  Vfs::Dir_file_system
+{
+	private:
+
+		typedef String<200> Config;
+		static Config _config(Xml_node node)
+		{
+			char buf[Config::capacity()] { };
+
+			Xml_generator xml(buf, sizeof(buf), "dir", [&] () {
+				typedef String<64> Name;
+				xml.attribute("name", node.attribute_value("name", Name()));
+				xml.node("glyphs", [&] () { });
+				xml.node("readonly_value", [&] () { xml.attribute("name", "baseline");   });
+				xml.node("readonly_value", [&] () { xml.attribute("name", "max_width");  });
+				xml.node("readonly_value", [&] () { xml.attribute("name", "max_height"); });
+			});
+			return Config(Cstring(buf));
+		}
+
+	public:
+
+		File_system(Env &env, Allocator &alloc, Xml_node node,
+		            Vfs::File_system &root_dir)
+		:
+			Local_factory(env, alloc, node, root_dir),
+			Vfs::Dir_file_system(env, alloc,
+			                     Xml_node(_config(node).string()),
+			                     *this, *this, root_dir)
+		{ }
+
+		char const *type() override { return "ttf"; }
+};
+
+
+/**************************
+ ** VFS plugin interface **
+ **************************/
+
+extern "C" Vfs::File_system_factory *vfs_file_system_factory(void)
+{
+	struct Factory : Vfs::File_system_factory
+	{
+		Vfs::File_system *create(Genode::Env &env, Genode::Allocator &alloc,
+		                         Genode::Xml_node node,
+		                         Vfs::Io_response_handler &,
+		                         Vfs::File_system &root_dir) override
+		{
+			try {
+				return new (alloc) Vfs_ttf::File_system(env, alloc, node, root_dir); }
+			catch (...) { }
+			return nullptr;
+		}
+	};
+
+	static Factory factory;
+	return &factory;
+}