Android: Defer initialization of logging rules until qApp construction

On Android, we load the application library, and its dependencies (Qt),
on Android's main thread (thread 0), and then spin up a secondary
thread (thread 1), that we call main() on.

If any QObject is constructed during loading of the application library
or any of Qt's libraries, via static initializers or constructor
functions, we will set QCoreApplicationPrivate::theMainThread to
thread 0, which will confuse Qt later on when it's being run on
thread 1, and will result in a warning during QCoreApplication
construction:

 QApplication was not created in the main() thread

This situation can easily lead to a crash as well.

Unfortunately logging via qDebug/qCDebug and friends will trigger
this too, as they internally use QObject.

Fixing the root cause of this is under investigation, but for now
we will partially revert fa2a653b3b934783 for Android. The effect
is that any qCDebug with a "qt.*" category before qApp construction
will turn into a no-op, like it was before fa2a653b3b934783.

This patch does not cover the case of a regular qDebug, or a qCDebug
with a non-Qt category. Those will still produce the same symptom,
as before fa2a653b3b934783.

Task-number: QTBUG-65863
Change-Id: I95675731d233244530d0a2a1c82a9578d5599775
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 538b1b50764fb3a1898d425a7155319afbcf3b25)

1 files changed, 8 insertions, 0 deletions

diff --git a/src/corelib/kernel/qcoreapplication.cpp b/src/corelib/kernel/qcoreapplication.cpp
index 12c2669f36..c8b130bbb9 100644
--- a/src/corelib/kernel/qcoreapplication.cpp
+++ b/src/corelib/kernel/qcoreapplication.cpp
@@ -776,6 +776,14 @@ void QCoreApplicationPrivate::init()
     if (!coreappdata()->applicationVersionSet)
         coreappdata()->applicationVersion = appVersion();
 
+#if defined(Q_OS_ANDROID)
+    // We've deferred initializing the logging registry due to not being
+    // able to guarantee that logging happened on the same thread as the
+    // Qt main thread, but now that the Qt main thread is set up, we can
+    // enable categorized logging.
+    QLoggingRegistry::instance()->initializeRules();
+#endif
+
 #if QT_CONFIG(library)
     // Reset the lib paths, so that they will be recomputed, taking the availability of argv[0]
     // into account. If necessary, recompute right away and replay the manual changes on top of the