From 095cacd668f9bc30f3b239e4a016bac6af697d42 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kai=20K=C3=B6hne?= <kai.koehne@qt.io>
Date: Tue, 29 Apr 2025 16:09:45 +0200
Subject: [PATCH] Do not treat the installation of an empty QTranslator as an
 error

The return value of installTranslator() should indicate whether
the _registration_ was successful. Apart from merely logging
the result, the only sensible action the calling code might
take is to delete the QTranslator object that failed to register.

The old behavior of installTranslator() for empty QTranslator
objects was to store the pointer for further use, but still return
false. This might lead to hard-to-detect memory corruption.

The patch changes behavior to _not_ consider installing empty
QTranslator objects as an error.

This has the following reasons:

* As translations are often done later in the SW development process,
it is common to replace empty .qm files with translated
ones in a very late stage. But having different code paths based on
this can be surprising.

* The other parts of the QTranslator toolchain do not treat
empty translations in a specific way, either.

While at it, also remove the now unused QT_NO_TRANSLATION_BUILDER
macro (left over from Qt 4).

[ChangeLog][Core][Behavior Change] QCoreApplication::installTranslator()
will now return true even for empty translators
(QTranslator::isEmpty()). This makes it explicit that empty QTranslator
objects are registered and queried, and therefore deleting the object
prematurely might result in undefined behavior.

Change-Id: Ia0531afcf5c72aa837406cf5de2cf73052014445
Reviewed-by: Masoud Jami <masoud.jami@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
---
 src/corelib/kernel/qcoreapplication.cpp       | 10 +++++---
 .../kernel/qtranslator/tst_qtranslator.cpp    | 25 +++++++++++++++++++
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/src/corelib/kernel/qcoreapplication.cpp b/src/corelib/kernel/qcoreapplication.cpp
index 6cf355aeaa9..180b8955906 100644
--- a/src/corelib/kernel/qcoreapplication.cpp
+++ b/src/corelib/kernel/qcoreapplication.cpp
@@ -2165,9 +2165,12 @@ void QCoreApplicationPrivate::quit()
     generated by \QD provide a \c retranslateUi() function that can be
     called.
 
-    The function returns \c true on success and false on failure.
+    The function returns \c true on success and \c false on failure.
 
     \note QCoreApplication does \e not take ownership of \a translationFile.
+    It is the responsibility of the application to ensure that, if the
+    function returned \c true, the \a translationFile object is alive until
+    either \l removeTranslator() is called for it, or the application exits.
 
     \sa removeTranslator(), translate(), QTranslator::load(),
         {Writing Source Code for Translation#Prepare for Dynamic Language Changes}{Prepare for Dynamic Language Changes}
@@ -2180,16 +2183,15 @@ bool QCoreApplication::installTranslator(QTranslator *translationFile)
 
     if (!QCoreApplicationPrivate::checkInstance("installTranslator"))
         return false;
+
     QCoreApplicationPrivate *d = self->d_func();
     {
         QWriteLocker locker(&d->translateMutex);
         d->translators.prepend(translationFile);
     }
 
-#ifndef QT_NO_TRANSLATION_BUILDER
     if (translationFile->isEmpty())
-        return false;
-#endif
+        return true;
 
 #ifndef QT_NO_QOBJECT
     QEvent ev(QEvent::LanguageChange);
diff --git a/tests/auto/corelib/kernel/qtranslator/tst_qtranslator.cpp b/tests/auto/corelib/kernel/qtranslator/tst_qtranslator.cpp
index eedf59a7c99..66dcac85f6b 100644
--- a/tests/auto/corelib/kernel/qtranslator/tst_qtranslator.cpp
+++ b/tests/auto/corelib/kernel/qtranslator/tst_qtranslator.cpp
@@ -75,6 +75,7 @@ private slots:
     void loadLocale_data();
     void loadLocale();
     void threadLoad();
+    void install();
     void testLanguageChange();
     void plural();
     void translate_qm_file_generated_with_msgfmt();
@@ -285,6 +286,30 @@ void tst_QTranslator::threadLoad()
     QVERIFY(thread.wait(10 * 1000));
 }
 
+void tst_QTranslator::install()
+{
+    {
+        // normal translation
+        QTranslator tor;
+        QVERIFY(tor.load("hellotr_la.qm"));
+        QCOMPARE(qApp->installTranslator(&tor), true);
+        QCOMPARE(qApp->removeTranslator(&tor), true);
+    }
+    {
+        // empty translation
+        QTranslator tor;
+        QVERIFY(tor.load("hellotr_empty.qm"));
+        QCOMPARE(qApp->installTranslator(&tor), true);
+        QCOMPARE(qApp->removeTranslator(&tor), true);
+    }
+    {
+        // nullptr
+        QCOMPARE(qApp->installTranslator(nullptr), false);
+        QCOMPARE(qApp->removeTranslator(nullptr), false);
+    }
+
+}
+
 void tst_QTranslator::testLanguageChange()
 {
     languageChangeEventCounter = 0;