Simplify system locale management

Have QSystemLocale manage a stack, so that tests can install an
over-ride for the actual system-specific one reliably and restore the
system-specific one when finished. Leave the QSystemLocaleSingleton
out of the stack, all the same. In the process, mark the QDoc comments
for QSystemLocale all as \internal, since this is not public API.

Change-Id: I8faed49780215e42f32be10cf936c32bb46105bf
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/text/qlocale.cpp

@@ -695,18 +695,28 @@ static QLocalePrivate *c_private()
 */
 
 /*!
-  Constructs a QSystemLocale object.
+    \internal
+    Constructs a QSystemLocale object.
 
-  The constructor will automatically install this object as the system locale,
-  if there's not one active.  It also resets the flag that'll prompt
-  QLocale::system() to re-initialize its data, so that instantiating a
-  QSystemLocale transiently (doesn't install the transient as system locale if
-  there was one already and) triggers an update to the system locale's data.
+    The constructor will automatically install this object as the system locale.
+    It and the destructor maintain a stack of system locales, with the
+    most-recently-created instance (that hasn't yet been deleted) used as the
+    system locale. This is only intended as a way to let a platform plugin
+    install its own system locale, overriding what might otherwise be provided
+    for its class of platform (as Android does, differing from Linux), and to
+    let tests transiently over-ride the system or plugin-supplied one. As such,
+    there should not be diverse threads creating and destroying QSystemLocale
+    instances concurrently, so no attempt is made at thread-safety in managing
+    the stack.
+
+    This constructor also resets the flag that'll prompt QLocale::system() to
+    re-initialize its data, so that instantiating a QSystemLocale (even
+    transiently) triggers a refresh of the system locale's data. This is
+    exploited by some test code.
 */
-QSystemLocale::QSystemLocale()
+QSystemLocale::QSystemLocale() : next(_systemLocale)
 {
-    if (!_systemLocale)
-        _systemLocale = this;
+    _systemLocale = this;
 
     systemLocaleData.m_language_id = 0;
 }
@@ -718,14 +728,21 @@ QSystemLocale::QSystemLocale(bool)
 { }
 
 /*!
-  Deletes the object.
+    \internal
+    Deletes the object.
 */
 QSystemLocale::~QSystemLocale()
 {
     if (_systemLocale == this) {
-        _systemLocale = nullptr;
+        _systemLocale = next;
 
+        // Change to system locale => force refresh.
         systemLocaleData.m_language_id = 0;
+    } else {
+        for (QSystemLocale *p = _systemLocale; p; p = p->next) {
+            if (p->next == this)
+                p->next = next;
+        }
     }
 }
 

src/corelib/text/qlocale_p.h

@@ -36,6 +36,7 @@ struct QLocaleData;
 // Subclassed by Android platform plugin:
 class Q_CORE_EXPORT QSystemLocale
 {
+    QSystemLocale *next = nullptr; // Maintains a stack.
 public:
     QSystemLocale();
     virtual ~QSystemLocale();

tests/auto/corelib/text/qlocale/tst_qlocale.cpp

@@ -3258,9 +3258,6 @@ private:
 
 void tst_QLocale::systemLocale_data()
 {
-#ifdef Q_OS_ANDROID
-    QSKIP("Android already has a QSystemLocale installed, we can't override it");
-#endif
     // Test uses MySystemLocale, so is platform-independent.
     QTest::addColumn<QString>("name");
     QTest::addColumn<QLocale::Language>("language");