qt: Avoid race on QtTransferable member

As Michael Stahl pointed out in [1], there is a data
race on `QtTransferable::m_bProvideUTF16FromOtherEncoding`.

Adjust the code a bit to no longer make use of the
member and drop it.

The QtClipboard case was fine because both methods making
use of the member always run in the main thread with the
SolarMutex held.

For anything else, the `m_pMimeData` doesn't change
don't change, so access to that member doesn't need
to be guarded by a mutex and thus dropping
`QtTransferable::m_bProvideUTF16FromOtherEncoding`
should be sufficient to address that race at least.
(Another one will be addressed separately.)

[1] https://gerrit.libreoffice.org/c/core/+/166140/comment/bc1c9f11_6ad630b7

Change-Id: Iaf2fb460b129493f5627c95b6968aa57da368b4c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166749
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>
Tested-by: Jenkins
(cherry picked from commit 8939999deef4f77f19d7b2d31df09260a34affe1)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166891
(cherry picked from commit 28b2020f276f12561252ea3a42555bfe88a0c1b6)

2 files changed, 17 insertions, 19 deletions

diff --git a/vcl/inc/qt5/QtTransferable.hxx b/vcl/inc/qt5/QtTransferable.hxx
index 38cb4d92005b..c58490e90460 100644
--- a/vcl/inc/qt5/QtTransferable.hxx
+++ b/vcl/inc/qt5/QtTransferable.hxx
@@ -34,7 +34,6 @@ class QtTransferable : public cppu::WeakImplHelper<css::datatransfer::XTransfera
     QtTransferable(const QtTransferable&) = delete;
 
     const QMimeData* m_pMimeData;
-    bool m_bProvideUTF16FromOtherEncoding;
 
 protected:
     /** Sets new mime data.
diff --git a/vcl/qt5/QtTransferable.cxx b/vcl/qt5/QtTransferable.cxx
index 18c2583e2e63..a6902824ab3a 100644
--- a/vcl/qt5/QtTransferable.cxx
+++ b/vcl/qt5/QtTransferable.cxx
@@ -43,7 +43,6 @@ static bool lcl_textMimeInfo(std::u16string_view rMimeString, bool& bHaveNoChars
 
 QtTransferable::QtTransferable(const QMimeData* pMimeData)
     : m_pMimeData(pMimeData)
-    , m_bProvideUTF16FromOtherEncoding(false)
 {
     assert(pMimeData);
 }
@@ -94,8 +93,10 @@ css::uno::Sequence<css::datatransfer::DataFlavor> SAL_CALL QtTransferable::getTr
         nMimeTypeCount++;
     }
 
-    m_bProvideUTF16FromOtherEncoding = (bHaveNoCharset || bHaveUTF8) && !bHaveUTF16;
-    if (m_bProvideUTF16FromOtherEncoding)
+    // in case of text/plain data, but no UTF-16 encoded one,
+    // QtTransferable::getTransferData converts from existing encoding to UTF-16
+    const bool bProvideUTF16FromOtherEncoding = (bHaveNoCharset || bHaveUTF8) && !bHaveUTF16;
+    if (bProvideUTF16FromOtherEncoding)
     {
         aFlavor.MimeType = "text/plain;charset=utf-16";
         aFlavor.DataType = cppu::UnoType<OUString>::get();
@@ -127,26 +128,24 @@ css::uno::Any SAL_CALL QtTransferable::getTransferData(const css::datatransfer::
     if (rFlavor.MimeType == "text/plain;charset=utf-16")
     {
         OUString aString;
-        if (m_bProvideUTF16FromOtherEncoding)
-        {
-            if (m_pMimeData->hasFormat("text/plain;charset=utf-8"))
-            {
-                QByteArray aByteData(m_pMimeData->data(QStringLiteral("text/plain;charset=utf-8")));
-                aString = OUString::fromUtf8(reinterpret_cast<const char*>(aByteData.data()));
-            }
-            else
-            {
-                QByteArray aByteData(m_pMimeData->data(QStringLiteral("text/plain")));
-                aString = OUString(reinterpret_cast<const char*>(aByteData.data()),
-                                   aByteData.size(), osl_getThreadTextEncoding());
-            }
-        }
-        else
+        // use existing UTF-16 encoded text/plain or convert to UTF-16 as needed
+        if (m_pMimeData->hasFormat("text/plain;charset=utf-16"))
         {
             QByteArray aByteData(m_pMimeData->data(toQString(rFlavor.MimeType)));
             aString = OUString(reinterpret_cast<const sal_Unicode*>(aByteData.data()),
                                aByteData.size() / 2);
         }
+        else if (m_pMimeData->hasFormat("text/plain;charset=utf-8"))
+        {
+            QByteArray aByteData(m_pMimeData->data(QStringLiteral("text/plain;charset=utf-8")));
+            aString = OUString::fromUtf8(reinterpret_cast<const char*>(aByteData.data()));
+        }
+        else
+        {
+            QByteArray aByteData(m_pMimeData->data(QStringLiteral("text/plain")));
+            aString = OUString(reinterpret_cast<const char*>(aByteData.data()), aByteData.size(),
+                               osl_getThreadTextEncoding());
+        }
         aAny <<= aString;
     }
     else