QEvent: document and check when registerEventTypes() returns -1

In tst_QEvent, add a check that exhausts the available event type ids and verifies
that registerEventTypes() returns -1 in that case, as documented.

I haven't found a way to test the other case in which -1 is returned.

Since this test operates on a global write-only registry the new test case needs
to keep track of whether the earlier test cases have run successfully. If they
didn't, skip this test case.

Change-Id: I68ea9d17d10dcec22175994aba269dd09c9adf43
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qcoreevent.cpp

@@ -406,6 +406,9 @@ Q_GLOBAL_STATIC(QEventUserEventRegistration, userEventRegistrationHelper)
     between QEvent::User and QEvent::MaxUser that has not yet been
     registered. The \a hint is ignored if its value is not between
     QEvent::User and QEvent::MaxUser.
+
+    Returns -1 if all available values are already taken or the
+    program is shutting down.
 */
 int QEvent::registerEventType(int hint)
 {

tests/auto/gui/kernel/qevent/tst_qevent.cpp

@@ -55,9 +55,14 @@ public:
 private slots:
     void registerEventType_data();
     void registerEventType();
+    void exhaustEventTypeRegistration(); // keep behind registerEventType() test
+
+private:
+    bool registerEventTypeSucceeded; // track success of registerEventType for use by exhaustEventTypeRegistration()
 };
 
 tst_QEvent::tst_QEvent()
+    : registerEventTypeSucceeded(true)
 { }
 
 tst_QEvent::~tst_QEvent()
@@ -82,9 +87,30 @@ void tst_QEvent::registerEventType_data()
 
 void tst_QEvent::registerEventType()
 {
+    const bool oldRegisterEventTypeSucceeded = registerEventTypeSucceeded;
+    registerEventTypeSucceeded = false;
     QFETCH(int, hint);
     QFETCH(int, expected);
     QCOMPARE(QEvent::registerEventType(hint), expected);
+    registerEventTypeSucceeded = oldRegisterEventTypeSucceeded;
+}
+
+void tst_QEvent::exhaustEventTypeRegistration()
+{
+    if (!registerEventTypeSucceeded)
+        QSKIP("requires the previous test (registerEventType) to have finished successfully");
+
+    int i = QEvent::User;
+    int result;
+    while ((result = QEvent::registerEventType(i)) == i)
+        ++i;
+    QCOMPARE(i, int(QEvent::User + 1000));
+    QCOMPARE(result, int(QEvent::MaxUser - 4));
+    i = QEvent::User + 1001;
+    while ((result = QEvent::registerEventType(i)) == i)
+        ++i;
+    QCOMPARE(result, -1);
+    QCOMPARE(i, int(QEvent::MaxUser - 4));
 }
 
 QTEST_MAIN(tst_QEvent)