LibTTF: Memory map TTF fonts instead of reading them into heap memory

All GUI applications currently load all TTF fonts on startup
(to populate the Gfx::FontDatabase. This could probably be smarter.)

Before this patch, everyone would open the files and read them into
heap-allocated storage. Now we simply mmap() them instead. :^)

Meta/Lagom/Fuzzers/FuzzTTF.cpp

@@ -8,9 +8,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
-extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
+extern "C" int LLVMFuzzerTestOneInput(u8 const* data, size_t size)
 {
-    ByteBuffer font_data = ByteBuffer::copy(data, size);
-    (void)TTF::Font::try_load_from_memory(font_data);
+    (void)TTF::Font::try_load_from_externally_owned_memory({ data, size });
     return 0;
 }

Userland/Libraries/LibTTF/Font.cpp

@@ -1,11 +1,12 @@
 /*
  * Copyright (c) 2020, Srimanta Barua <srimanta.barua1@gmail.com>
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include "AK/ByteBuffer.h"
 #include <AK/Checked.h>
+#include <AK/MappedFile.h>
 #include <AK/Utf32View.h>
 #include <AK/Utf8View.h>
 #include <LibCore/File.h>
@@ -15,6 +16,7 @@
 #include <LibTTF/Tables.h>
 #include <LibTextCodec/Decoder.h>
 #include <math.h>
+#include <sys/mman.h>
 
 namespace TTF {
 
@@ -207,19 +209,20 @@ GlyphHorizontalMetrics Hmtx::get_glyph_horizontal_metrics(u32 glyph_id) const
 
 Result<NonnullRefPtr<Font>, String> Font::try_load_from_file(String path, unsigned index)
 {
-    auto file_or_error = Core::File::open(move(path), Core::OpenMode::ReadOnly);
+    auto file_or_error = MappedFile::map(path);
     if (file_or_error.is_error())
-        return file_or_error.error();
+        return String { file_or_error.error().string() };
 
-    auto file = file_or_error.value();
-    if (!file->open(Core::OpenMode::ReadOnly))
-        return String { "Could not open file" };
-
-    auto buffer = file->read_all();
-    return try_load_from_memory(buffer, index);
+    auto& file = *file_or_error.value();
+    auto result = try_load_from_externally_owned_memory(file.bytes(), index);
+    if (result.is_error())
+        return result.error();
+    auto& font = *result.value();
+    font.m_mapped_file = file_or_error.release_value();
+    return result;
 }
 
-Result<NonnullRefPtr<Font>, String> Font::try_load_from_memory(ByteBuffer& buffer, unsigned index)
+Result<NonnullRefPtr<Font>, String> Font::try_load_from_externally_owned_memory(ReadonlyBytes buffer, unsigned index)
 {
     if (buffer.size() < 4)
         return String { "Font file too small" };
@@ -243,7 +246,7 @@ Result<NonnullRefPtr<Font>, String> Font::try_load_from_memory(ByteBuffer& buffe
 }
 
 // FIXME: "loca" and "glyf" are not available for CFF fonts.
-Result<NonnullRefPtr<Font>, String> Font::try_load_from_offset(ByteBuffer&& buffer, u32 offset)
+Result<NonnullRefPtr<Font>, String> Font::try_load_from_offset(ReadonlyBytes buffer, u32 offset)
 {
     if (Checked<u32>::addition_would_overflow(offset, (u32)Sizes::OffsetTable))
         return String { "Invalid offset in font header" };

Userland/Libraries/LibTTF/Font.h

@@ -6,7 +6,6 @@
 
 #pragma once
 
-#include <AK/ByteBuffer.h>
 #include <AK/HashMap.h>
 #include <AK/Noncopyable.h>
 #include <AK/RefCounted.h>
@@ -47,7 +46,7 @@ class Font : public RefCounted<Font> {
 
 public:
     static Result<NonnullRefPtr<Font>, String> try_load_from_file(String path, unsigned index = 0);
-    static Result<NonnullRefPtr<Font>, String> try_load_from_memory(ByteBuffer&, unsigned index = 0);
+    static Result<NonnullRefPtr<Font>, String> try_load_from_externally_owned_memory(ReadonlyBytes bytes, unsigned index = 0);
 
     ScaledFontMetrics metrics(float x_scale, float y_scale) const;
     ScaledGlyphMetrics glyph_metrics(u32 glyph_id, float x_scale, float y_scale) const;
@@ -72,9 +71,10 @@ private:
         TableRecord = 16,
     };
 
-    static Result<NonnullRefPtr<Font>, String> try_load_from_offset(ByteBuffer&&, unsigned index = 0);
-    Font(ByteBuffer&& buffer, Head&& head, Name&& name, Hhea&& hhea, Maxp&& maxp, Hmtx&& hmtx, Cmap&& cmap, Loca&& loca, Glyf&& glyf)
-        : m_buffer(move(buffer))
+    static Result<NonnullRefPtr<Font>, String> try_load_from_offset(ReadonlyBytes, unsigned index = 0);
+
+    Font(ReadonlyBytes bytes, Head&& head, Name&& name, Hhea&& hhea, Maxp&& maxp, Hmtx&& hmtx, Cmap&& cmap, Loca&& loca, Glyf&& glyf)
+        : m_buffer(move(bytes))
         , m_head(move(head))
         , m_name(move(name))
         , m_hhea(move(hhea))
@@ -86,8 +86,10 @@ private:
     {
     }
 
-    // This owns the font data
-    ByteBuffer m_buffer;
+    RefPtr<MappedFile> m_mapped_file;
+
+    ReadonlyBytes m_buffer;
+
     // These are stateful wrappers around non-owning slices
     Head m_head;
     Name m_name;