Avoid loading and keeping unused fallback font engines

When we request glyphs from fallback fonts, we would potentially
load all fonts on the system into memory. This is especially true
for glyphs that are not supported by any font (or by the last in
the list) in any "Common" script (which e.g. includes CJK).

This would make any application which tried to display unsupported
glyphs use huge amounts of memory for keeping unused fonts cached,
only limited by the number of fonts on the system.

The patch contains two solutions: First, before loading the font,
the multi font engine will be asked whether it needs to be tried
for the given character. By default, this will always be true, so
all fonts will be tried, but with the new font config multi engine
in the platform plugin, it will ask FontConfig whether the font
contains a glyph for the character.

Should the font be loaded and still fail to resolve the character
(which could be the case for other platforms), we will simply
delete it again immediately instead keeping it cached.

Change-Id: I92dfb39289a359f49caa02c2caf8baf66098fb59
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

9 files changed, 74 insertions, 4 deletions

diff --git a/src/gui/text/qfont.cpp b/src/gui/text/qfont.cpp
index a347c62629..ee833a06cf 100644
--- a/src/gui/text/qfont.cpp
+++ b/src/gui/text/qfont.cpp
@@ -2729,6 +2729,20 @@ QFontEngine *QFontCache::findEngine(const Key &key)
     return it.value().data;
 }
 
+void QFontCache::removeEngine(QFontEngine *engine)
+{
+    EngineCache::iterator it = engineCache.begin();
+    while (it != engineCache.end()) {
+        if (it.value().data == engine) {
+            it = engineCache.erase(it);
+            if (--engine->cache_count == 0)
+                decreaseCost(engine->cache_cost);
+        } else {
+            ++it;
+        }
+    }
+}
+
 void QFontCache::insertEngine(const Key &key, QFontEngine *engine)
 {
     FC_DEBUG("QFontCache: inserting new engine %p", engine);
diff --git a/src/gui/text/qfont_p.h b/src/gui/text/qfont_p.h
index e26a98aaf2..d10249201a 100644
--- a/src/gui/text/qfont_p.h
+++ b/src/gui/text/qfont_p.h
@@ -243,6 +243,7 @@ public:
 
     QFontEngine *findEngine(const Key &key);
     void insertEngine(const Key &key, QFontEngine *engine);
+    void removeEngine(QFontEngine *engine);
 
 
     private:
diff --git a/src/gui/text/qfontdatabase_qpa.cpp b/src/gui/text/qfontdatabase_qpa.cpp
index 2a9c37fb13..1a1f08f73d 100644
--- a/src/gui/text/qfontdatabase_qpa.cpp
+++ b/src/gui/text/qfontdatabase_qpa.cpp
@@ -206,7 +206,8 @@ QFontEngine *loadEngine(int script, const QFontDef &request,
         if (family && !family->fallbackFamilies.isEmpty())
             fallbacks = family->fallbackFamilies;
 
-        engine = new QFontEngineMultiQPA(engine, script, fallbacks);
+        QPlatformFontDatabase *pfdb = QGuiApplicationPrivate::platformIntegration()->fontDatabase();
+        engine = pfdb->fontEngineMulti(engine, QUnicodeTables::Script(script), fallbacks);
 
         // Cache Multi font engine as well in case we got the FT single
         // font engine when we are actually looking for a Multi one
diff --git a/src/gui/text/qfontengine.cpp b/src/gui/text/qfontengine.cpp
index 156a4a1e59..a084a3dd8c 100644
--- a/src/gui/text/qfontengine.cpp
+++ b/src/gui/text/qfontengine.cpp
@@ -1379,18 +1379,24 @@ bool QFontEngineMulti::stringToCMap(const QChar *str, int len,
     int glyph_pos = 0;
     for (int i = 0; i < len; ++i) {
         bool surrogate = (str[i].isHighSurrogate() && i < len-1 && str[i+1].isLowSurrogate());
-
+        uint ucs4 = surrogate ? QChar::surrogateToUcs4(str[i], str[i+1]) : str[i].unicode();
         if (glyphs->glyphs[glyph_pos] == 0 && str[i].category() != QChar::Separator_Line) {
             QGlyphLayoutInstance tmp = glyphs->instance(glyph_pos);
-            for (int x = 1; x < engines.size(); ++x) {
+            for (int x=1; x < engines.size(); ++x) {
+                if (!shouldLoadFontEngineForCharacter(x, ucs4))
+                    continue;
+
                 QFontEngine *engine = engines.at(x);
+                bool deleteThisEngine = false;
                 if (!engine) {
                     const_cast<QFontEngineMulti *>(this)->loadEngine(x);
                     engine = engines.at(x);
+                    deleteThisEngine = true;
                 }
                 Q_ASSERT(engine != 0);
                 if (engine->type() == Box)
                     continue;
+
                 glyphs->advances_x[glyph_pos] = glyphs->advances_y[glyph_pos] = 0;
                 glyphs->offsets[glyph_pos] = QFixedPoint();
                 int num = 2;
@@ -1401,13 +1407,17 @@ bool QFontEngineMulti::stringToCMap(const QChar *str, int len,
                     // set the high byte to indicate which engine the glyph came from
                     glyphs->glyphs[glyph_pos] |= (x << 24);
                     break;
+                } else if (deleteThisEngine) {
+                    const_cast<QFontEngineMulti *>(this)->unloadEngine(x);
                 }
             }
+
             // ensure we use metrics from the 1st font when we use the fallback image.
             if (!glyphs->glyphs[glyph_pos]) {
                 glyphs->setInstance(glyph_pos, tmp);
             }
         }
+
         if (surrogate)
             ++i;
         ++glyph_pos;
@@ -1418,6 +1428,29 @@ bool QFontEngineMulti::stringToCMap(const QChar *str, int len,
     return true;
 }
 
+bool QFontEngineMulti::shouldLoadFontEngineForCharacter(int at, uint ucs4) const
+{
+    Q_UNUSED(at);
+    Q_UNUSED(ucs4);
+    return true;
+}
+
+void QFontEngineMulti::unloadEngine(int at)
+{
+    QFontEngine *fontEngine = engines.at(at);
+    if (fontEngine == 0)
+        return;
+
+    // If there are other references to the engine, keep it around and keep the reference
+    if (fontEngine->ref.load() == 1) {
+        QFontCache::instance()->removeEngine(fontEngine);
+        if (fontEngine->cache_count == 0) {
+            delete fontEngine;
+            engines[at] = 0;
+        }
+    }
+}
+
 glyph_metrics_t QFontEngineMulti::boundingBox(const QGlyphLayout &glyphs)
 {
     if (glyphs.numGlyphs <= 0)
diff --git a/src/gui/text/qfontengine_ft_p.h b/src/gui/text/qfontengine_ft_p.h
index a9e67c10e8..83b51685c0 100644
--- a/src/gui/text/qfontengine_ft_p.h
+++ b/src/gui/text/qfontengine_ft_p.h
@@ -328,6 +328,7 @@ protected:
 private:
     friend class QFontEngineFTRawFont;
     friend class QFontconfigDatabase;
+    friend class QFontEngineMultiFontConfig;
 
     int loadFlags(QGlyphSet *set, GlyphFormat format, int flags, bool &hsubpixel, int &vfactor) const;
 
diff --git a/src/gui/text/qfontengine_p.h b/src/gui/text/qfontengine_p.h
index c0bd1afb80..023882d560 100644
--- a/src/gui/text/qfontengine_p.h
+++ b/src/gui/text/qfontengine_p.h
@@ -392,11 +392,14 @@ public:
             loadEngine(at);
     }
 
+    virtual bool shouldLoadFontEngineForCharacter(int at, uint ucs4) const;
+
 protected:
     friend class QPSPrintEnginePrivate;
     friend class QPSPrintEngineFontMulti;
     friend class QRawFont;
     virtual void loadEngine(int at) = 0;
+    virtual void unloadEngine(int at);
     QVector<QFontEngine *> engines;
 };
 
diff --git a/src/gui/text/qfontengine_qpa_p.h b/src/gui/text/qfontengine_qpa_p.h
index 6a73b9d309..ed2e071ac2 100644
--- a/src/gui/text/qfontengine_qpa_p.h
+++ b/src/gui/text/qfontengine_qpa_p.h
@@ -243,13 +243,16 @@ struct QPAGenerator
     QFontEngine *fe;
 };
 
-class QFontEngineMultiQPA : public QFontEngineMulti
+class Q_GUI_EXPORT QFontEngineMultiQPA : public QFontEngineMulti
 {
 public:
     QFontEngineMultiQPA(QFontEngine *fe, int script, const QStringList &fallbacks);
 
     void loadEngine(int at);
 
+    int fallbackFamilyCount() const { return fallbackFamilies.size(); }
+    QString fallbackFamilyAt(int at) const { return fallbackFamilies.at(at); }
+
 private:
     QStringList fallbackFamilies;
     int script;
diff --git a/src/gui/text/qplatformfontdatabase_qpa.cpp b/src/gui/text/qplatformfontdatabase_qpa.cpp
index f9cc97cbc7..8fcf421330 100644
--- a/src/gui/text/qplatformfontdatabase_qpa.cpp
+++ b/src/gui/text/qplatformfontdatabase_qpa.cpp
@@ -271,6 +271,18 @@ void QPlatformFontDatabase::populateFontDatabase()
 }
 
 /*!
+    Returns a multi font engine in the specified \a script to encapsulate \a fontEngine with the
+    option to fall back to to the fonts given by \a fallbacks if \a fontEngine does not support
+    a certain character.
+*/
+QFontEngineMulti *QPlatformFontDatabase::fontEngineMulti(QFontEngine *fontEngine,
+                                                         QUnicodeTables::Script script,
+                                                         const QStringList &fallbacks)
+{
+    return new QFontEngineMultiQPA(fontEngine, script, fallbacks);
+}
+
+/*!
     Returns the font engine that can be used to render the font described by
     the font definition, \a fontDef, in the specified \a script.
 */
diff --git a/src/gui/text/qplatformfontdatabase_qpa.h b/src/gui/text/qplatformfontdatabase_qpa.h
index 151442c5e1..6a58a3106c 100644
--- a/src/gui/text/qplatformfontdatabase_qpa.h
+++ b/src/gui/text/qplatformfontdatabase_qpa.h
@@ -82,12 +82,14 @@ Q_GUI_EXPORT bool operator==(const QSupportedWritingSystems &, const QSupportedW
 Q_GUI_EXPORT bool operator!=(const QSupportedWritingSystems &, const QSupportedWritingSystems &);
 
 class QFontRequestPrivate;
+class QFontEngineMulti;
 
 class Q_GUI_EXPORT QPlatformFontDatabase
 {
 public:
     virtual ~QPlatformFontDatabase();
     virtual void populateFontDatabase();
+    virtual QFontEngineMulti *fontEngineMulti(QFontEngine *fontEngine, QUnicodeTables::Script script, const QStringList &fallbacks);
     virtual QFontEngine *fontEngine(const QFontDef &fontDef, QUnicodeTables::Script script, void *handle);
     virtual QStringList fallbacksForFamily(const QString family, const QFont::Style &style, const QFont::StyleHint &styleHint, const QUnicodeTables::Script &script) const;
     virtual QStringList addApplicationFont(const QByteArray &fontData, const QString &fileName);