QMimeType: Use default key as fallback for comment() property

When QMimeProvider parses the shared mime database xml files,
it will read the <comment> element for mime comment and treat the
`xml:lang` attribute as locale language string. When no `xml:lang`
attr is provided, QMimeProvider will read the value and treat it as
a en_US locale string as the default key.

When we call QMimeType::comment(), it will try to get the locale
comment string with the default language (QLocale().name()), once
it can't find a matched result, it should return the default key
(which QMimeProvider set it as en_US locale before) as fallback.

Task-number: QTBUG-71314
Change-Id: I444f8159d6f19dfef6338cd79312f608d8f13394
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/mimetypes/qmimeprovider.cpp

@@ -502,7 +502,7 @@ void QMimeBinaryProvider::loadMimeTypePrivate(QMimeTypePrivate &data)
                     QString lang = xml.attributes().value(QLatin1String("xml:lang")).toString();
                     const QString text = xml.readElementText();
                     if (lang.isEmpty()) {
-                        lang = QLatin1String("en_US");
+                        lang = QLatin1String("default"); // no locale attribute provided, treat it as default.
                     }
                     data.localeComments.insert(lang, text);
                     continue; // we called readElementText, so we're at the EndElement already.

src/corelib/mimetypes/qmimetype.cpp

@@ -258,6 +258,7 @@ QString QMimeType::comment() const
     QStringList languageList;
     languageList << QLocale().name();
     languageList << QLocale().uiLanguages();
+    languageList << QLatin1String("default"); // use the default locale if possible.
     for (const QString &language : qAsConst(languageList)) {
         const QString lang = language == QLatin1String("C") ? QLatin1String("en_US") : language;
         const QString comm = d->localeComments.value(lang);

src/corelib/mimetypes/qmimetypeparser.cpp

@@ -248,11 +248,11 @@ bool QMimeTypeParserBase::parse(QIODevice *dev, const QString &fileName, QString
             }
                 break;
             case ParseComment: {
-                // comments have locale attributes. We want the default, English one
+                // comments have locale attributes.
                 QString locale = atts.value(QLatin1String(localeAttributeC)).toString();
                 const QString comment = reader.readElementText();
                 if (locale.isEmpty())
-                    locale = QString::fromLatin1("en_US");
+                    locale = QString::fromLatin1("default");
                 data.localeComments.insert(locale, comment);
             }
                 break;

tests/auto/corelib/mimetypes/qmimedatabase/tst_qmimedatabase.cpp

@@ -992,6 +992,20 @@ void tst_QMimeDatabase::installNewGlobalMimeType()
     const QString fooTestFile2 = QLatin1String(RESOURCE_PREFIX "magic-and-hierarchy2.foo");
     QCOMPARE(db.mimeTypeForFile(fooTestFile2).name(), QString::fromLatin1("application/vnd.qnx.bar-descriptor"));
 
+    // Test if we can use the default comment
+    {
+        struct RestoreLocale
+        {
+            ~RestoreLocale() { QLocale::setDefault(QLocale::c()); }
+        } restoreLocale;
+
+        QLocale::setDefault(QLocale("zh_CN"));
+        QMimeType suseymp = db.mimeTypeForName("text/x-suse-ymp");
+        QVERIFY(suseymp.isValid());
+        QCOMPARE(suseymp.comment(),
+                 QString::fromLatin1("YaST Meta Package"));
+    }
+
     // Now test removing the mimetype definitions again
     for (int i = 0; i < m_additionalMimeFileNames.size(); ++i)
         QFile::remove(destDir + m_additionalMimeFileNames.at(i));