QFileSelector: Preserve path to root directory

While QFileSelector is documented to work on files, select accepts
arbitrary URLs. Moreover, the QML engine can end up intercepting
arbitrary (user provided) URLs, including to directories.
Prior to this change, passing "file:///" or "/" to the function would
break: We would temporarily get a "//" path, which is invalid, and thus
we would return an invalid in the end.
Prevent this by only appending a slash to the path when it doesn't have
one.

Fixes: QTBUG-85410
Pick-to: 6.2
Change-Id: I1d2807a9d225df611c3a5e871e3c1d90a6a25953
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/io/qfileselector.cpp

@@ -278,7 +278,10 @@ QString QFileSelectorPrivate::select(const QString &filePath) const
     Q_Q(const QFileSelector);
     QFileInfo fi(filePath);
 
-    QString ret = selectionHelper(fi.path().isEmpty() ? QString() : fi.path() + QLatin1Char('/'),
+    QString pathString;
+    if (auto path = fi.path(); !path.isEmpty())
+        pathString = path.endsWith(u'/') ? path : path + u'/';
+    QString ret = selectionHelper(pathString,
             fi.fileName(), q->allSelectors());
 
     if (!ret.isEmpty())

tests/auto/corelib/io/qfileselector/tst_qfileselector.cpp

@@ -215,6 +215,9 @@ void tst_QFileSelector::urlConvenience_data()
     strUrlWithFragment = QString("file:") + testWithQueryAndFragment;
     QTest::newRow("file with query and fragment too") << QUrl(strUrlWithFragment) << (QStringList()) << QUrl(strUrlWithFragment);
 
+    // preserve path to root
+    QTest::newRow("path to root") << QUrl("file:///") << (QStringList()) << QUrl("file:///");
+
     // http://qt-project.org/images/qtdn/sprites-combined-latest.png is chosen as a representative real world URL
     // But note that this test is checking that http urls are NOT selected so it shouldn't be checked
     QUrl testHttpUrl("http://qt-project.org/images/sprites-combined-latest.png");