[Libreoffice-bugs] [Bug 140813] New: Use GetUpdatedClipboardFormats Vista API to minimize clipboard-related overhead

[bugzilla-daemon at bugs.documentfoundation.org]

https://bugs.documentfoundation.org/show_bug.cgi?id=140813

            Bug ID: 140813
           Summary: Use GetUpdatedClipboardFormats Vista API to minimize
                    clipboard-related overhead
           Product: LibreOffice
           Version: unspecified
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: medium
         Component: LibreOffice
          Assignee: libreoffice-bugs at lists.freedesktop.org
          Reporter: mikekaganski at hotmail.com
                CC: glogow at fbihome.de

Currently LibreOffice uses a dedicated thread to respond to clipboard change
notifications (see CMtaOleClipboard in vcl/win/dtrans/MtaOleClipb.cxx), which
obtains an IDataObject (using OleGetClipboard) for the updated clipboard data,
and stores it for possible further use. The object is wrapped into an
appartment-neutral object in CWinClipboard::getContents
(vcl/win/dtrans/WinClipboard.cxx). This object is later queried for available
formats when deciding if Paste should be enabled, and which paste options to
provide. Often, this is the only information needed, because not every data in
clipboard ends up in LibreOffice.

This is (a) rather expensive (obtaining a COM object, creating an
appartment-neutral wrapper over it, having to properly release it at the next
clipboard update - for each clipboard event, regardless if we will use the data
or not), and (b) prone to errors (see e.g. tdf#136175; also there are hangs
sometimes in unit testing, caused by deadlocking of released COM object waiting
for a thread that is waiting for its release).

Since Windows Vista, there's GetUpdatedClipboardFormats function [1] created to
list available formats in the system clipboard without opening it, and so
without race conditions [2] [3]. We could possibly reimplement the thread to
only get the list of available formats using this function, and only obtain the
COM object when actually pasting something. That would require that the
XTransferable returned from CWinClipboard::getContents would be able to ask for
the IDataObject only when needed.

[1]
https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getupdatedclipboardformats
[2]
https://techcommunity.microsoft.com/t5/microsoft-security-and/why-does-my-shared-clipboard-not-work-part-2/ba-p/246465
[3] https://support.citrix.com/article/CTX216506

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