QFile::rename: avoid two stat(2)/CreateFile in a row

QFileSystemEngine::id() will stat(2)/CreateFile in order to get the ID
of the file anyway, so we don't need to use QFile::exists() to check if
the destination exists. Instead, rely on id() returning a null value to
indicate error. On Windows, it's possible that the calls to either
GetFileInformationByHandle or GetFileInformationByHandleEx might fail,
but we ignore those.

Change-Id: I1eba2b016de74620bfc8fffd14ccaebcbed64419
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/corelib/io/qfile.cpp b/src/corelib/io/qfile.cpp
index 06d706b915..a840307145 100644
--- a/src/corelib/io/qfile.cpp
+++ b/src/corelib/io/qfile.cpp
@@ -567,9 +567,11 @@ QFile::rename(const QString &newName)
     }
     // If the file exists and it is a case-changing rename ("foo" -> "Foo"),
     // compare Ids to make sure it really is a different file.
-    if (QFile::exists(newName)) {
-        if (d->fileName.compare(newName, Qt::CaseInsensitive)
-            || QFileSystemEngine::id(QFileSystemEntry(d->fileName)) != QFileSystemEngine::id(QFileSystemEntry(newName))) {
+    // Note: this does not take file engines into account.
+    QByteArray targetId = QFileSystemEngine::id(QFileSystemEntry(newName));
+    if (!targetId.isNull()) {
+        QByteArray fileId = QFileSystemEngine::id(QFileSystemEntry(d->fileName));
+        if (fileId != targetId || d->fileName.compare(newName, Qt::CaseInsensitive)) {
             // ### Race condition. If a file is moved in after this, it /will/ be
             // overwritten. On Unix, the proper solution is to use hardlinks:
             // return ::link(old, new) && ::remove(old);