QFSFileEngine: make rename() on Unix not overwrite

The rename(2) system call overwrites, so instead of using it, we try to
use the link/unlink pair. This works for regular cases, but can fail if
trying to change case in case-insensitive filesystems, if we're
operating on a non-Unix filesystem (FAT) or, on Linux, if the file
doesn't belong to the calling user (BSDs permit this). For those cases,
we fall back to rename(2).

That means there's a race condition if a new file is created there. But
we at least reduce the likelihood of that happening for regular files.

Change-Id: I1eba2b016de74620bfc8fffd14ccb38fd929e5aa
Reviewed-by: David Faure <david.faure@kdab.com>

1 files changed, 6 insertions, 6 deletions

diff --git a/src/corelib/io/qfile.cpp b/src/corelib/io/qfile.cpp
index 2cfb718932..ce99ac96c0 100644
--- a/src/corelib/io/qfile.cpp
+++ b/src/corelib/io/qfile.cpp
@@ -566,18 +566,18 @@ QFile::rename(const QString &newName)
         d->setError(QFile::RenameError, tr("Source file does not exist."));
         return false;
     }
+
     // If the file exists and it is a case-changing rename ("foo" -> "Foo"),
     // compare Ids to make sure it really is a different file.
     // Note: this does not take file engines into account.
+    bool changingCase = false;
     QByteArray targetId = QFileSystemEngine::id(QFileSystemEntry(newName));
     if (!targetId.isNull()) {
         QByteArray fileId = d->fileEngine ?
                     d->fileEngine->id() :
                     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);
+        changingCase = (fileId == targetId && d->fileName.compare(newName, Qt::CaseInsensitive) == 0);
+        if (!changingCase) {
             d->setError(QFile::RenameError, tr("Destination file exists"));
             return false;
         }
@@ -593,7 +593,7 @@ QFile::rename(const QString &newName)
             return false;
         }
         tempFile.close();
-        if (!d->engine()->rename(tempFile.fileName())) {
+        if (!d->engine()->renameOverwrite(tempFile.fileName())) {
             d->setError(QFile::RenameError, tr("Error while renaming."));
             return false;
         }
@@ -616,7 +616,7 @@ QFile::rename(const QString &newName)
     unsetError();
     close();
     if(error() == QFile::NoError) {
-        if (d->engine()->rename(newName)) {
+        if (changingCase ? d->engine()->renameOverwrite(newName) : d->engine()->rename(newName)) {
             unsetError();
             // engine was able to handle the new name so we just reset it
             d->fileEngine->setFileName(newName);