Effort/Make VCL Thread-Transparent

[1], [2]

Status: in progress

VCL provides the OOo base widget set and windowing abstraction. It manages the event loop and dispatches the events. It also owns the display connection and manages various resources, e.g. fonts.

VCL hampers thread transparency in the following ways

• It is Thread Affine under Windows - bascially letting shine through the Win32 thread affinity regarding window messages, window construction and window destruction.
• It provides the Solar MutEx, which needs to be acquired befor VCL can be called.
• It completely releases the Solar MutEx in some sitations, without any hints at the API.

VCLs internals are not protected against concurrent access, except by the Solar MutEx, which needs to be locked from the outside. VCLs API only provides blocking or polling functions for accessing the event loop, which is especially bad as it does not offer a way, to avoid long acquired MutExes, while waiting for events, except by actively polling.

To make VCL thread transparent, we propose, to

• remove the Solar MutEx completely and declare VCL to be Thread Unsafe, and to
• encapsulate calls to the Thread Affine Win32 API, in a way, that this thread affinity is not visible from the outside.

The goal being that VCL behaves as an ordinary library, not showing any threading behavior at all.

To ensure short MutEx acquiration times, we propose to

• add a system handle providing API, so that we can poll/select on the handle in an outer event loop, and only need to enter VCL in case of an event.

Side effects

• Some Win32 based OOo components (DDE) expect their initialising thread eventually to enter the VCL event loop, to dispatch messages for objects created during this initialization. These implementations need to be adapted, to either delegate Win32 thread affine calls into VCLs new internal thread, or to create a thread for dispatching purposes by their own.
• With the removal of the Solar MutEx, VCL can not release a protecting MutEx anymore, which it currently does when executing a dialog. That means, that a protecting MutEx will stay acuired, basically disallowing other threads to enter VCL. This is needs to be addressed by making the office dialogs asynchron.

Time Frame

At least Removing the Solar MutEx and Encapsulating the inherited Win32 Thread Affinity are going to depend on Uno/Effort/Creating the Uno Threading Framework, therefor VCL Thread Transparency will earliest be available after the UTF.

Dependencies

Task	State
Make Dialogs Asynchronous	90%
Creating a Handle API	90%
Removing the Solar MutEx	0%
Encapsulating the inherited Win32 Thread Affinity	90%
Fix DDE support to not rely on main thread	0%