[LyX/master] Do not use a static variable as QTextLayout cache

[Jean-Marc Lasgouttes]

commit 91b385166de84677197f326702f0e95a5d649091
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Fri Dec 11 16:33:34 2015 +0100

    Do not use a static variable as QTextLayout cache
    
    It is a bad idea to have a QObject variable that oulives the main 
QApplication object. See for example:
    https://www.ics.com/designpatterns/book/globals.html
    
    Here the QTextLayout object was static to the anonymous namespace 
getTextLayout function, and got destroyed after the freetype renderer had been 
disposed of by QApplication.
    
    This causes segmentation faults when quitting LyX on some systems.
    
    This patch moves the cache together with other GuiFontMetrics caches. It 
means that one will have one such QTextLayout per font type, but this will not 
change much.

diff --git a/src/frontends/qt4/GuiFontMetrics.cpp 
b/src/frontends/qt4/GuiFontMetrics.cpp
index 7cf841f..c58be2a 100644
--- a/src/frontends/qt4/GuiFontMetrics.cpp
+++ b/src/frontends/qt4/GuiFontMetrics.cpp
@@ -23,8 +23,6 @@
 
 #include "support/lassert.h"
 
-#include <QTextLayout>
-
 using namespace std;
 using namespace lyx::support;
 
@@ -56,7 +54,8 @@ inline QChar const ucs4_to_qchar(char_type const ucs4)
 // Limit strwidth_cache_ size to 512kB of string data
 GuiFontMetrics::GuiFontMetrics(QFont const & font)
        : font_(font), metrics_(font, 0),
-         strwidth_cache_(1 << 19)
+         strwidth_cache_(1 << 19),
+         tl_cache_rtl_(false), tl_cache_wordspacing_(-1.0)
 {
 }
 
@@ -170,36 +169,27 @@ int GuiFontMetrics::signedWidth(docstring const & s) const
                return width(s);
 }
 
-namespace {
 
-QTextLayout const & getTextLayout(docstring const & s, QFont font,
-                                  bool const rtl, double const wordspacing)
+QTextLayout const &
+GuiFontMetrics::getTextLayout(docstring const & s, QFont font,
+                              bool const rtl, double const wordspacing) const
 {
-       static docstring old_s;
-       static QFont old_font;
-       static bool old_rtl = false;
-       // this invalid value is to make sure that it is reset on first try.
-       static double old_wordspacing = -1.0;
-       // This one is our trivial cache
-       static QTextLayout tl;
-       if (s != old_s || font != old_font || rtl != old_rtl
-           || wordspacing != old_wordspacing) {
-               tl.setText(toqstr(s));
+       if (s != tl_cache_s_ || font != tl_cache_font_ || rtl != tl_cache_rtl_
+           || wordspacing != tl_cache_wordspacing_) {
+               tl_cache_.setText(toqstr(s));
                font.setWordSpacing(wordspacing);
-               tl.setFont(font);
+               tl_cache_.setFont(font);
                // Note that both setFlags and the enums are undocumented
-               tl.setFlags(rtl ? Qt::TextForceRightToLeft : 
Qt::TextForceLeftToRight);
-               tl.beginLayout();
-               tl.createLine();
-               tl.endLayout();
-               old_s = s;
-               old_font = font;
-               old_rtl = rtl;
-               old_wordspacing = wordspacing;
+               tl_cache_.setFlags(rtl ? Qt::TextForceRightToLeft : 
Qt::TextForceLeftToRight);
+               tl_cache_.beginLayout();
+               tl_cache_.createLine();
+               tl_cache_.endLayout();
+               tl_cache_s_ = s;
+               tl_cache_font_ = font;
+               tl_cache_rtl_ = rtl;
+               tl_cache_wordspacing_ = wordspacing;
        }
-       return tl;
-}
-
+       return tl_cache_;
 }
 
 
diff --git a/src/frontends/qt4/GuiFontMetrics.h 
b/src/frontends/qt4/GuiFontMetrics.h
index 1e813f6..8c8daab 100644
--- a/src/frontends/qt4/GuiFontMetrics.h
+++ b/src/frontends/qt4/GuiFontMetrics.h
@@ -21,6 +21,7 @@
 #include <QFont>
 #include <QFontMetrics>
 #include <QHash>
+#include <QTextLayout>
 
 namespace lyx {
 namespace frontend {
@@ -63,6 +64,11 @@ public:
        int width(QString const & str) const;
 
 private:
+
+       QTextLayout const &
+       getTextLayout(docstring const & s, QFont font,
+                     bool const rtl, double const wordspacing) const;
+
        /// The font
        QFont font_;
 
@@ -86,6 +92,14 @@ private:
 
        /// Cache of char right bearings
        mutable QHash<char_type, int> rbearing_cache_;
+
+       // A trivial QTextLayout cache
+       mutable QTextLayout tl_cache_;
+       mutable docstring tl_cache_s_;
+       mutable QFont tl_cache_font_;
+       mutable bool tl_cache_rtl_;
+       mutable double tl_cache_wordspacing_;
+
 };
 
 } // namespace frontend