Make sure num_glyphs pass to HarfBuzz is large enough

(Backport from master)

Currently we only pass the num_glyphs for the run to
HB_ShapeItem, but it can be less then the string length
for this run because of Unicode surrogates. Thus, we need
to pass at least the length of that run as num_glyphs to
HB (given that we have enough space allocated because for
the entire string), if that's still not enough, we will
do ensureSpace again according to the num_glyphs returned
by HB and move remaining glyphs backwards.

Task-number: QTBUG-15679
Reviewed-by: Lars Knoll

2 files changed, 24 insertions, 1 deletions

diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index b511b5a..4378c62 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -1233,6 +1233,8 @@ void QTextEngine::shapeTextWithHarfbuzz(int item) const
             shaper_item.num_glyphs -= itemBoundaries[k + 1];
         }
         shaper_item.initialGlyphCount = shaper_item.num_glyphs;
+        if (shaper_item.num_glyphs < shaper_item.item.length)
+            shaper_item.num_glyphs = shaper_item.item.length;
 
         QFontEngine *actualFontEngine = font;
         uint engineIdx = 0;
@@ -1257,7 +1259,8 @@ void QTextEngine::shapeTextWithHarfbuzz(int item) const
             }
 
             const QGlyphLayout g = availableGlyphs(&si).mid(glyph_pos);
-            moveGlyphData(g.mid(shaper_item.num_glyphs), g.mid(shaper_item.initialGlyphCount), remaining_glyphs);
+            if (shaper_item.num_glyphs > shaper_item.item.length)
+                moveGlyphData(g.mid(shaper_item.num_glyphs), g.mid(shaper_item.initialGlyphCount), remaining_glyphs);
 
             shaper_item.glyphs = g.glyphs;
             shaper_item.attributes = g.attributes;
diff --git a/tests/auto/qtextlayout/tst_qtextlayout.cpp b/tests/auto/qtextlayout/tst_qtextlayout.cpp
index 2d15566..85e6616 100644
--- a/tests/auto/qtextlayout/tst_qtextlayout.cpp
+++ b/tests/auto/qtextlayout/tst_qtextlayout.cpp
@@ -125,6 +125,7 @@ private slots:
     void textWidthVsWIdth();
     void textWidthWithStackedTextEngine();
     void textWidthWithLineSeparator();
+    void textWithSurrogates_qtbug15679();
 
 private:
     QFont testFont;
@@ -1416,5 +1417,24 @@ void tst_QTextLayout::textWidthWithLineSeparator()
     QCOMPARE(line1.naturalTextWidth(), line2.naturalTextWidth());
 }
 
+void tst_QTextLayout::textWithSurrogates_qtbug15679()
+{
+    QString str = QString::fromUtf8("🀀a🀀");
+    QTextLayout layout(str);
+    layout.beginLayout();
+    QTextLine line = layout.createLine();
+    layout.endLayout();
+
+    qreal x[6];
+    for (int i = 0; i < 6; i++)
+        x[i] = line.cursorToX(i);
+
+    // If the first and third character are using the same
+    // font, they must have the same advance (since they
+    // are surrogate pairs, we need to add two for each
+    // character)
+    QCOMPARE(x[2] - x[0], x[5] - x[3]);
+}
+
 QTEST_MAIN(tst_QTextLayout)
 #include "tst_qtextlayout.moc"