Unify QFontEngine::getSfntTableData() behavior on all platforms

Being a most significant method in the font API,
getSfntTableData() must behave in exactly the same way on all platforms.
Briefly, it must return true if the table exists in the font,
despite the other params, and always stores the table data length
in 'length' param, thus reporting the amount of bytes actually
needed to store the table data in a buffer.

Change-Id: I7a15465020c1ea818ea46a05ea3b9b7e1cd60d14
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/gui/text/qfontengine.cpp

@@ -937,14 +937,31 @@ void QFontEngine::getUnscaledGlyph(glyph_t glyph, QPainterPath *path, glyph_metr
     addGlyphsToPath(&glyph, &p, 1, path, QFlag(0));
 }
 
+/*!
+    Returns \c true if the font table idetified by \a tag exists in the font;
+    returns \c false otherwise.
+
+    If \a buffer is NULL, stores the size of the buffer required for the font table data,
+    in bytes, in \a length. If \a buffer is not NULL and the capacity
+    of the buffer, passed in \a length, is sufficient to store the font table data,
+    also copies the font table data to \a buffer.
+
+    Note: returning \c false when the font table exists could lead to an undefined behavior.
+*/
+bool QFontEngine::getSfntTableData(uint tag, uchar *buffer, uint *length) const
+{
+    Q_UNUSED(tag)
+    Q_UNUSED(buffer)
+    Q_UNUSED(length)
+    return false;
+}
+
 QByteArray QFontEngine::getSfntTable(uint tag) const
 {
     QByteArray table;
     uint len = 0;
     if (!getSfntTableData(tag, 0, &len))
         return table;
-    if (!len)
-        return table;
     table.resize(len);
     if (!getSfntTableData(tag, reinterpret_cast<uchar *>(table.data()), &len))
         return QByteArray();

src/gui/text/qfontengine_ft.cpp

@@ -125,6 +125,7 @@ static bool ft_getSfntTable(void *user_data, uint tag, uchar *buffer, uint *leng
         FT_ULong len = *length;
         result = FT_Load_Sfnt_Table(face, tag, 0, buffer, &len) == FT_Err_Ok;
         *length = len;
+        Q_ASSERT(!result || int(*length) > 0);
     }
 
     return result;

src/gui/text/qfontengine_p.h

@@ -144,8 +144,8 @@ public:
     };
     virtual Properties properties() const;
     virtual void getUnscaledGlyph(glyph_t glyph, QPainterPath *path, glyph_metrics_t *metrics);
-    QByteArray getSfntTable(uint /*tag*/) const;
-    virtual bool getSfntTableData(uint /*tag*/, uchar * /*buffer*/, uint * /*length*/) const { return false; }
+    QByteArray getSfntTable(uint tag) const;
+    virtual bool getSfntTableData(uint tag, uchar *buffer, uint *length) const;
 
     struct FaceId {
         FaceId() : index(0), encoding(0) {}

src/gui/text/qharfbuzzng.cpp

@@ -625,7 +625,7 @@ _hb_qt_reference_table(hb_face_t * /*face*/, hb_tag_t tag, void *user_data)
     Q_ASSERT(get_font_table);
 
     uint length = 0;
-    if (Q_UNLIKELY(!get_font_table(data->user_data, tag, 0, &length) || length == 0))
+    if (Q_UNLIKELY(!get_font_table(data->user_data, tag, 0, &length)))
         return hb_blob_get_empty();
 
     char *buffer = (char *)malloc(length);

src/platformsupport/fontdatabases/mac/qfontengine_coretext.mm

@@ -659,16 +659,13 @@ bool QCoreTextFontEngine::canRender(const QChar *string, int len)
 bool QCoreTextFontEngine::getSfntTableData(uint tag, uchar *buffer, uint *length) const
 {
     QCFType<CFDataRef> table = CTFontCopyTable(ctfont, tag, 0);
-    if (!table || !length)
+    if (!table)
         return false;
     CFIndex tableLength = CFDataGetLength(table);
-    int availableLength = *length;
-    *length = tableLength;
-    if (buffer) {
-        if (tableLength > availableLength)
-            return false;
+    if (buffer && int(*length) >= tableLength)
         CFDataGetBytes(table, CFRangeMake(0, tableLength), buffer);
-    }
+    *length = tableLength;
+    Q_ASSERT(int(*length) > 0);
     return true;
 }
 

src/plugins/platforms/windows/qwindowsfontengine.cpp

@@ -1032,6 +1032,7 @@ bool QWindowsFontEngine::getSfntTableData(uint tag, uchar *buffer, uint *length)
     SelectObject(hdc, hfont);
     DWORD t = qbswap<quint32>(tag);
     *length = GetFontData(hdc, t, 0, buffer, *length);
+    Q_ASSERT(*length == GDI_ERROR || int(*length) > 0);
     return *length != GDI_ERROR;
 }
 

src/plugins/platforms/windows/qwindowsfontenginedirectwrite.cpp

@@ -282,8 +282,8 @@ bool QWindowsFontEngineDirectWrite::getSfntTableData(uint tag, uchar *buffer, ui
             ret = true;
             if (buffer && *length >= tableSize)
                 memcpy(buffer, tableData, tableSize);
-            else
-                *length = tableSize;
+            *length = tableSize;
+            Q_ASSERT(int(*length) > 0);
         }
         m_directWriteFontFace->ReleaseFontTable(tableContext);
     } else {