Remove false Q_UNREACHABLE from shaping code

This was added by 9ff76c27b9 on
the basis that it signifies a shaping error and would later assert
or crash.

But the line is easily reachable by user code. If Harfbuzz returns
0 glyphs, it just means it is unable to shape the string, for instance
if the input string only contains default ignorables (like a ZWJ)
and does not have any appropriate glyph to use for replacement.

Qt expects there to always be at least one glyph in the output
(num_glyphs == 0 is used to indicate shaping is not yet done), so
to avoid asserts later on, we simply populate the output with a
single 0 token, which is a required entry in the font that is
reserved for representing unrepresentable characters.

This also adds a test and therefore a zero-width joiner to the test
font to reproduce the issue.

[ChangeLog][QtGui][Text] Fixed a possible crash with certain fonts
when shaping strings consisting only of control characters.

Fixes: QTBUG-89155
Pick-to: 5.15
Pick-to: 6.0
Change-Id: Ia0dd6a04844c9be90dcab6c464bebe339a3dab11
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

src/gui/text/qtextengine.cpp

@@ -1546,12 +1546,21 @@ void QTextEngine::shapeText(int item) const
 
         si.num_glyphs = glyph_pos;
     }
+
     if (Q_UNLIKELY(si.num_glyphs == 0)) {
-        Q_UNREACHABLE(); // ### report shaping errors somehow
+        if (Q_UNLIKELY(!ensureSpace(si.glyph_data_offset + 1))) {
+            qWarning() << "Unable to allocate space for place-holder glyph";
+            return;
+        }
+
+        si.num_glyphs = 1;
+
+        // Overwrite with 0 token to indicate failure
+        QGlyphLayout g = availableGlyphs(&si);
+        g.glyphs[0] = 0;
         return;
     }
 
-
     layoutData->used += si.num_glyphs;
 
     QGlyphLayout glyphs = shapedGlyphs(&si);

tests/auto/gui/text/qglyphrun/tst_qglyphrun.cpp

@@ -63,6 +63,7 @@ private slots:
     void boundingRect();
     void mixedScripts();
     void multiLineBoundingRect();
+    void defaultIgnorables();
 
 private:
     int m_testFontId;
@@ -631,6 +632,21 @@ void tst_QGlyphRun::multiLineBoundingRect()
     QVERIFY(firstLineGlyphRun.boundingRect().height() < allGlyphRun.boundingRect().height());
 }
 
+void tst_QGlyphRun::defaultIgnorables()
+{
+    QTextLayout layout;
+    layout.setFont(QFont("QtsSpecialTestFont"));
+    layout.setText(QChar(0x200D));
+    layout.beginLayout();
+    layout.createLine();
+    layout.endLayout();
+
+    QList<QGlyphRun> runs = layout.glyphRuns();
+    QCOMPARE(runs.size(), 1);
+    QCOMPARE(runs.at(0).glyphIndexes().size(), 1);
+    QCOMPARE(runs.at(0).glyphIndexes()[0], 0);
+}
+
 #endif // QT_NO_RAWFONT
 
 QTEST_MAIN(tst_QGlyphRun)