Remove false Q_UNREACHABLE from shaping code

This was added by 9ff76c27b9031ae7c49c4c9e8b5a3bea1e0e3c78 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
Change-Id: Ia0dd6a04844c9be90dcab6c464bebe339a3dab11
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
(cherry picked from commit fccd419dd632306a4bd85928223e0a56a59510ef)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 11 insertions, 2 deletions

diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index d3a28d8d02..c76abc5424 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/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);