Kernel: Introduce the DeviceManagement singleton

This singleton simplifies many aspects that we struggled with before:
1. There's no need to make derived classes of Device expose the
constructor as public anymore. The singleton is a friend of them, so he
can call the constructor. This solves the issue with try_create_device
helper neatly, hopefully for good.
2. Getting a reference of the NullDevice is now being done from this
singleton, which means that NullDevice no longer needs to use its own
singleton, and we can apply the try_create_device helper on it too :)
3. We can now defer registration completely after the Device constructor
which means the Device constructor is merely assigning the major and
minor numbers of the Device, and the try_create_device helper ensures it
calls the after_inserting method immediately after construction. This
creates a great opportunity to make registration more OOM-safe.

1 files changed, 3 insertions, 2 deletions

diff --git a/Kernel/Devices/FullDevice.h b/Kernel/Devices/FullDevice.h
index 3a3f7e09b9..2336e48cff 100644
--- a/Kernel/Devices/FullDevice.h
+++ b/Kernel/Devices/FullDevice.h
@@ -12,14 +12,15 @@ namespace Kernel {
 
 class FullDevice final : public CharacterDevice {
     AK_MAKE_ETERNAL
+    friend class DeviceManagement;
+
 public:
     static NonnullRefPtr<FullDevice> must_create();
     virtual ~FullDevice() override;
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     FullDevice();
 
-private:
     // ^CharacterDevice
     virtual KResultOr<size_t> read(OpenFileDescription&, u64, UserOrKernelBuffer&, size_t) override;
     virtual KResultOr<size_t> write(OpenFileDescription&, u64, const UserOrKernelBuffer&, size_t) override;