LibGfx: restructure Bitmap ctor to expect an alloc'd backing store

Moves Bitmap backing store creation to the static create() methods.
This backing store is then passed into the Bitmap constructor. This
allows us correctly return nullptr from create() in the event that
memory allocation fails.

Libraries/LibGfx/Bitmap.cpp

@@ -26,6 +26,7 @@
 
 #include <AK/Checked.h>
 #include <AK/Memory.h>
+#include <AK/Optional.h>
 #include <AK/SharedBuffer.h>
 #include <AK/String.h>
 #include <LibGfx/BMPLoader.h>
@@ -44,6 +45,12 @@
 
 namespace Gfx {
 
+struct BackingStore {
+    void* data { nullptr };
+    size_t pitch { 0 };
+    size_t size_in_bytes { 0 };
+};
+
 size_t Bitmap::minimum_pitch(size_t width, BitmapFormat format)
 {
     size_t element_size;
@@ -76,39 +83,32 @@ static bool size_would_overflow(BitmapFormat format, const IntSize& size)
 
 RefPtr<Bitmap> Bitmap::create(BitmapFormat format, const IntSize& size)
 {
-    if (size_would_overflow(format, size))
+    auto backing_store = Bitmap::allocate_backing_store(format, size, Purgeable::No);
+    if (!backing_store.has_value())
         return nullptr;
-    return adopt(*new Bitmap(format, size, Purgeable::No));
+    return adopt(*new Bitmap(format, size, Purgeable::No, backing_store.value()));
 }
 
 RefPtr<Bitmap> Bitmap::create_purgeable(BitmapFormat format, const IntSize& size)
 {
-    if (size_would_overflow(format, size))
+    auto backing_store = Bitmap::allocate_backing_store(format, size, Purgeable::Yes);
+    if (!backing_store.has_value())
         return nullptr;
-    return adopt(*new Bitmap(format, size, Purgeable::Yes));
+    return adopt(*new Bitmap(format, size, Purgeable::Yes, backing_store.value()));
 }
 
-Bitmap::Bitmap(BitmapFormat format, const IntSize& size, Purgeable purgeable)
+Bitmap::Bitmap(BitmapFormat format, const IntSize& size, Purgeable purgeable, const BackingStore& backing_store)
     : m_size(size)
-    , m_pitch(minimum_pitch(size.width(), format))
+    , m_data(backing_store.data)
+    , m_pitch(backing_store.pitch)
     , m_format(format)
     , m_purgeable(purgeable == Purgeable::Yes)
 {
     ASSERT(!m_size.is_empty());
     ASSERT(!size_would_overflow(format, size));
-    allocate_palette_from_format(format, {});
-#ifdef __serenity__
-    int map_flags = purgeable == Purgeable::Yes ? (MAP_PURGEABLE | MAP_PRIVATE) : (MAP_ANONYMOUS | MAP_PRIVATE);
-    m_data = mmap_with_name(nullptr, size_in_bytes(), PROT_READ | PROT_WRITE, map_flags, 0, 0, String::format("GraphicsBitmap [%dx%d]", width(), height()).characters());
-#else
-    int map_flags = (MAP_ANONYMOUS | MAP_PRIVATE);
-    m_data = mmap(nullptr, size_in_bytes(), PROT_READ | PROT_WRITE, map_flags, 0, 0);
-#endif
-    if (m_data == MAP_FAILED) {
-        perror("mmap");
-        ASSERT_NOT_REACHED();
-    }
     ASSERT(m_data);
+    ASSERT(backing_store.size_in_bytes == size_in_bytes());
+    allocate_palette_from_format(format, {});
     m_needs_munmap = true;
 }
 
@@ -336,6 +336,30 @@ ShareableBitmap Bitmap::to_shareable_bitmap(pid_t peer_pid) const
     return ShareableBitmap(*bitmap);
 }
 
+Optional<BackingStore> Bitmap::allocate_backing_store(BitmapFormat format, const IntSize& size, Purgeable purgeable)
+{
+    if (size_would_overflow(format, size))
+        return {};
+
+    const auto pitch = minimum_pitch(size.width(), format);
+    const auto data_size_in_bytes = size_in_bytes(pitch, size.height());
+
+    void* data = nullptr;
+#ifdef __serenity__
+    int map_flags = purgeable == Purgeable::Yes ? (MAP_PURGEABLE | MAP_PRIVATE) : (MAP_ANONYMOUS | MAP_PRIVATE);
+    data = mmap_with_name(nullptr, data_size_in_bytes, PROT_READ | PROT_WRITE, map_flags, 0, 0, String::format("GraphicsBitmap [%dx%d]", size.width(), size.height()).characters());
+#else
+    UNUSED_PARAM(purgeable);
+    int map_flags = (MAP_ANONYMOUS | MAP_PRIVATE);
+    data = mmap(nullptr, data_size_in_bytes, PROT_READ | PROT_WRITE, map_flags, 0, 0);
+#endif
+    if (data == MAP_FAILED) {
+        perror("mmap");
+        return {};
+    }
+    return { { data, pitch, data_size_in_bytes } };
+}
+
 void Bitmap::allocate_palette_from_format(BitmapFormat format, const Vector<RGBA32>& source_palette)
 {
     size_t size = palette_size(format);

Libraries/LibGfx/Bitmap.h

@@ -79,6 +79,8 @@ static StorageFormat determine_storage_format(BitmapFormat format)
     }
 }
 
+struct BackingStore;
+
 enum RotationDirection {
     Left,
     Right
@@ -192,7 +194,8 @@ public:
 
     void set_mmap_name(const StringView&);
 
-    size_t size_in_bytes() const { return m_pitch * m_size.height(); }
+    static constexpr size_t size_in_bytes(size_t pitch, int height) { return pitch * height; }
+    size_t size_in_bytes() const { return size_in_bytes(m_pitch, height()); }
 
     Color palette_color(u8 index) const { return Color::from_rgba(m_palette[index]); }
     void set_palette_color(u8 index, Color color) { m_palette[index] = color.value(); }
@@ -223,10 +226,12 @@ private:
         No,
         Yes
     };
-    Bitmap(BitmapFormat, const IntSize&, Purgeable);
+    Bitmap(BitmapFormat, const IntSize&, Purgeable, const BackingStore&);
     Bitmap(BitmapFormat, const IntSize&, size_t pitch, void*);
     Bitmap(BitmapFormat, NonnullRefPtr<SharedBuffer>&&, const IntSize&, const Vector<RGBA32>& palette);
 
+    static Optional<BackingStore> allocate_backing_store(BitmapFormat, const IntSize&, Purgeable);
+
     void allocate_palette_from_format(BitmapFormat, const Vector<RGBA32>& source_palette);
 
     IntSize m_size;