qt.conf: add private "merge_qt_conf" option

This is meant to allow the buildsystem to add additional paths (for e.g.
QML imports) while still allowing the application to pick up the paths
where e.g. Qt's plugins are located – without having to query qtpaths or
the build system to locate them first, and to then also add them to
qt.conf.
QLibraryInfo::paths will return a list containing any values provided in
qt.conf, followed by the value that would have been provided if there
were no qt.conf file.
If no value is provided at all for a given key in qt.conf,
QLibraryInfo::path will behave as if no qt.conf file was provided,
instead of using LocationInfo::defaultValue.

For now, this is considered to be internal only, and won't be
documented.

Change-Id: Ic74c66b5cbde605b43336cd88c3f909999381f46
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

src/corelib/global/qlibraryinfo.cpp

@@ -566,6 +566,17 @@ static QString normalizePath(QString ret)
     return QDir::fromNativeSeparators(ret);
 };
 
+static bool keepQtBuildDefaults()
+{
+#if QT_CONFIG(settings)
+    QSettings *config = QLibraryInfoPrivate::configuration();
+    Q_ASSERT(config != nullptr);
+    return config->value("Config/MergeQtConf", false).toBool();
+#else
+    return false;
+#endif
+}
+
 #if QT_CONFIG(settings)
 static QVariant libraryPathToValue(QLibraryInfo::LibraryPath loc)
 {
@@ -575,21 +586,26 @@ static QVariant libraryPathToValue(QLibraryInfo::LibraryPath loc)
         return value;
     QSettings *config = QLibraryInfoPrivate::configuration();
     Q_ASSERT(config != nullptr);
+    // if keepQtBuildDefaults returns true,
+    // we only consider explicit values listed in qt.conf
+    QVariant defaultValue = keepQtBuildDefaults()
+            ? QVariant()
+            : QVariant(li.defaultValue);
     config->beginGroup("Paths"_L1);
     auto cleanup = qScopeGuard([&]() { config->endGroup(); });
     if (li.fallbackKey.isNull()) {
-        value = config->value(li.key, li.defaultValue);
+        value = config->value(li.key, defaultValue);
     } else {
         value = config->value(li.key);
         if (!value.isValid())
-            value = config->value(li.fallbackKey, li.defaultValue);
+            value = config->value(li.fallbackKey, defaultValue);
     }
     return value;
 }
 #endif // settings
 
 QStringList QLibraryInfoPrivate::paths(QLibraryInfo::LibraryPath p,
-                                       QLibraryInfoPrivate::UsageMode usageMode)
+                                       UsageMode usageMode)
 {
     const QLibraryInfo::LibraryPath loc = p;
     QList<QString> ret;
@@ -611,7 +627,7 @@ QStringList QLibraryInfoPrivate::paths(QLibraryInfo::LibraryPath p,
     }
 #endif // settings
 
-    if (!fromConf) {
+    if (!fromConf || keepQtBuildDefaults()) {
         QString noConfResult;
         if (loc == QLibraryInfo::PrefixPath) {
             noConfResult = getPrefix(usageMode);

tests/auto/corelib/global/qlibraryinfo/CMakeLists.txt

@@ -20,4 +20,5 @@ qt_add_resources(tst_qlibraryinfo "qtconffiles"
         empty.qt.conf
         partial.qt.conf
         list.qt.conf
+        merge.qt.conf
 )

tests/auto/corelib/global/qlibraryinfo/merge.qt.conf

@@ -0,0 +1,5 @@
+[Paths]
+QmlImports = "/path/to/myqml"
+
+[Config]
+MergeQtConf=true

tests/auto/corelib/global/qlibraryinfo/tst_qlibraryinfo.cpp

@@ -16,6 +16,7 @@ private slots:
     void path_data();
     void path();
     void paths();
+    void merge();
 };
 
 void tst_QLibraryInfo::initTestCase()
@@ -80,6 +81,23 @@ void tst_QLibraryInfo::paths()
     QCOMPARE(values[2], baseDir + "/relativePath");
 }
 
+void tst_QLibraryInfo::merge()
+{
+    QString qtConfPath(u":/merge.qt.conf");
+    QLibraryInfoPrivate::setQtconfManualPath(&qtConfPath);
+    QLibraryInfoPrivate::reload();
+
+    QString baseDir = QCoreApplication::applicationDirPath();
+    QString docPath = QLibraryInfo::path(QLibraryInfo::DocumentationPath);
+    // we can't know where exactly the doc path points, but it should not point to ${baseDir}/doc,
+    // which would be the  behavior without merge_qt_conf
+    QCOMPARE_NE(docPath, baseDir + "/doc");
+
+    QList<QString> values = QLibraryInfo::paths(QLibraryInfo::QmlImportsPath);
+    QCOMPARE(values.size(), 2); // custom entry + Qt default entry
+    QCOMPARE(values[0], "/path/to/myqml");
+}
+
 QTEST_GUILESS_MAIN(tst_QLibraryInfo)
 
 #include "tst_qlibraryinfo.moc"