qt: Guard clipboard mime data with SolarMutex

Most of the access to the QtClipboardTransferable
mime data happens exclusively on the main thread,
with the solar mutex held.

However, `mimeData()`, called from `QtClipboard::getContents`
didn't ensure that yet, so as Michael Stahl pointed out in [1],

commit 1db5b87fe69c2375f1d66974dafcd563303c76db
Author: Michael Weghorn <m.weghorn@posteo.de>
Date:   Tue Feb 13 13:23:17 2024 +0100

tdf#156562 qt: Sync with system clipboard content if necessary

introduced a data race by allowing to set new mime data.

Introduce a new
`QtClipboardTransferable::hasMimeData(const QMimeData* pMimeData)`
that guards access to the mime data with the solar mutext as well
and use that instead, so all access to the `QtClipboardTransferable`
mime data is now guarded by the solar mutex.

Also add an explicit note for the mime data getter/setter in the
`QtTransferable` base class that subclasses allowing to update
mime data are responsible for preventing data races.

[1] https://gerrit.libreoffice.org/c/core/+/166141/comment/fe75f418_40c1b622

Change-Id: I01dbbb0b37a4c6ad06b4d3001ecce8b0260eb32e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166750
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>
(cherry picked from commit 621cfc0e4120ab2b381b54268fe39bd19257df9b)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166892
(cherry picked from commit 8fbff7fa22cc50132f09442e338fc71434c4a77a)

3 files changed, 18 insertions, 2 deletions

diff --git a/vcl/inc/qt5/QtTransferable.hxx b/vcl/inc/qt5/QtTransferable.hxx
index c58490e90460..5687fa06df52 100644
--- a/vcl/inc/qt5/QtTransferable.hxx
+++ b/vcl/inc/qt5/QtTransferable.hxx
@@ -40,12 +40,17 @@ protected:
      *  Since data flavors supported by this class depend on the mime data,
      *  results from previous calls to the public methods of this
      *  class are no longer valid after setting new mime data using this method.
+     *
+     *  Subclasses that set new mime data must ensure that no data race exists
+     *  on m_pMimeData.
+     *  (For the current only subclass doing so, QtClipboardTransferable, all access
+     *  to m_pMimeData happens with the SolarMutex held.)
      */
     void setMimeData(const QMimeData* pMimeData) { m_pMimeData = pMimeData; }
+    const QMimeData* mimeData() const { return m_pMimeData; }
 
 public:
     QtTransferable(const QMimeData* pMimeData);
-    const QMimeData* mimeData() const { return m_pMimeData; }
 
     css::uno::Sequence<css::datatransfer::DataFlavor> SAL_CALL getTransferDataFlavors() override;
     sal_Bool SAL_CALL isDataFlavorSupported(const css::datatransfer::DataFlavor& rFlavor) override;
@@ -74,6 +79,9 @@ class QtClipboardTransferable final : public QtTransferable
 public:
     explicit QtClipboardTransferable(const QClipboard::Mode aMode, const QMimeData* pMimeData);
 
+    // whether pMimeData are the current mime data
+    bool hasMimeData(const QMimeData* pMimeData) const;
+
     // these are the same then QtTransferable, except they go through RunInMainThread
     css::uno::Sequence<css::datatransfer::DataFlavor> SAL_CALL getTransferDataFlavors() override;
     sal_Bool SAL_CALL isDataFlavorSupported(const css::datatransfer::DataFlavor& rFlavor) override;
diff --git a/vcl/qt5/QtClipboard.cxx b/vcl/qt5/QtClipboard.cxx
index e9eb476fb253..ea05784bbfb2 100644
--- a/vcl/qt5/QtClipboard.cxx
+++ b/vcl/qt5/QtClipboard.cxx
@@ -103,7 +103,7 @@ css::uno::Reference<css::datatransfer::XTransferable> QtClipboard::getContents()
     {
         const auto* pTrans = dynamic_cast<QtClipboardTransferable*>(m_aContents.get());
         assert(pTrans);
-        if (pTrans && pTrans->mimeData() == pMimeData)
+        if (pTrans && pTrans->hasMimeData(pMimeData))
             return m_aContents;
     }
 
diff --git a/vcl/qt5/QtTransferable.cxx b/vcl/qt5/QtTransferable.cxx
index a6902824ab3a..1aec5da27843 100644
--- a/vcl/qt5/QtTransferable.cxx
+++ b/vcl/qt5/QtTransferable.cxx
@@ -13,6 +13,7 @@
 #include <comphelper/sequence.hxx>
 #include <sal/log.hxx>
 #include <o3tl/string_view.hxx>
+#include <tools/debug.hxx>
 
 #include <QtWidgets/QApplication>
 
@@ -173,10 +174,17 @@ void QtClipboardTransferable::ensureConsistencyWithSystemClipboard()
     {
         SAL_WARN("vcl.qt", "In flight clipboard change detected - updating mime data with current "
                            "clipboard contents.");
+        DBG_TESTSOLARMUTEX();
         setMimeData(pCurrentClipboardData);
     }
 }
 
+bool QtClipboardTransferable::hasMimeData(const QMimeData* pMimeData) const
+{
+    SolarMutexGuard aGuard;
+    return QtTransferable::mimeData() == pMimeData;
+}
+
 css::uno::Any SAL_CALL
 QtClipboardTransferable::getTransferData(const css::datatransfer::DataFlavor& rFlavor)
 {