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>

7 files changed, 30 insertions, 14 deletions

diff --git a/src/gui/text/qfontengine.cpp b/src/gui/text/qfontengine.cpp
index 83e64a51a6..d654772e86 100644
--- a/src/gui/text/qfontengine.cpp
+++ b/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();
diff --git a/src/gui/text/qfontengine_ft.cpp b/src/gui/text/qfontengine_ft.cpp
index c13f60ff69..43e24b3943 100644
--- a/src/gui/text/qfontengine_ft.cpp
+++ b/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;
diff --git a/src/gui/text/qfontengine_p.h b/src/gui/text/qfontengine_p.h
index 0bfb9e70e2..72f5c092dd 100644
--- a/src/gui/text/qfontengine_p.h
+++ b/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) {}
diff --git a/src/gui/text/qharfbuzzng.cpp b/src/gui/text/qharfbuzzng.cpp
index c09f27b665..31bb6f38f2 100644
--- a/src/gui/text/qharfbuzzng.cpp
+++ b/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);
diff --git a/src/platformsupport/fontdatabases/mac/qfontengine_coretext.mm b/src/platformsupport/fontdatabases/mac/qfontengine_coretext.mm
index 4fe78cb568..7ec59f552f 100644
--- a/src/platformsupport/fontdatabases/mac/qfontengine_coretext.mm
+++ b/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;
 }
 
diff --git a/src/plugins/platforms/windows/qwindowsfontengine.cpp b/src/plugins/platforms/windows/qwindowsfontengine.cpp
index 86fcb666b0..79615e79ec 100644
--- a/src/plugins/platforms/windows/qwindowsfontengine.cpp
+++ b/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;
 }
 
diff --git a/src/plugins/platforms/windows/qwindowsfontenginedirectwrite.cpp b/src/plugins/platforms/windows/qwindowsfontenginedirectwrite.cpp
index 1c5e4508ac..158eee6b38 100644
--- a/src/plugins/platforms/windows/qwindowsfontenginedirectwrite.cpp
+++ b/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 {