QSaveFile: follow symbolic links

[ChangeLog][QtCore][QSaveFile] Now follows symbolic links while writing to
a link instead of replacing the link with the contents.

Change-Id: I5afd519cb9f96ae68fa4c23c33a18de75671a301
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/io/qsavefile.cpp

@@ -205,14 +205,26 @@ bool QSaveFile::open(OpenMode mode)
         d->writeError = QFileDevice::WriteError;
         return false;
     }
-    d->fileEngine = new QTemporaryFileEngine(d->fileName);
+
+    // Resolve symlinks. Don't use QFileInfo::canonicalFilePath so it still give the expected
+    // target even if the file does not exist
+    d->finalFileName = d->fileName;
+    if (existingFile.isSymLink()) {
+        int maxDepth = 128;
+        while (--maxDepth && existingFile.isSymLink())
+            existingFile.setFile(existingFile.symLinkTarget());
+        if (maxDepth > 0)
+            d->finalFileName = existingFile.filePath();
+    }
+
+    d->fileEngine = new QTemporaryFileEngine(d->finalFileName);
     // Same as in QFile: QIODevice provides the buffering, so there's no need to request it from the file engine.
     if (!d->fileEngine->open(mode | QIODevice::Unbuffered)) {
         QFileDevice::FileError err = d->fileEngine->error();
 #ifdef Q_OS_UNIX
         if (d->directWriteFallback && err == QFileDevice::OpenError && errno == EACCES) {
             delete d->fileEngine;
-            d->fileEngine = QAbstractFileEngine::create(d->fileName);
+            d->fileEngine = QAbstractFileEngine::create(d->finalFileName);
             if (d->fileEngine->open(mode | QIODevice::Unbuffered)) {
                 d->useTemporaryFile = false;
                 QFileDevice::open(mode);
@@ -285,7 +297,7 @@ bool QSaveFile::commit()
         // atomically replace old file with new file
         // Can't use QFile::rename for that, must use the file engine directly
         Q_ASSERT(d->fileEngine);
-        if (!d->fileEngine->renameOverwrite(d->fileName)) {
+        if (!d->fileEngine->renameOverwrite(d->finalFileName)) {
             d->setError(d->fileEngine->error(), d->fileEngine->errorString());
             d->fileEngine->remove();
             delete d->fileEngine;

src/corelib/io/qsavefile_p.h

@@ -70,6 +70,7 @@ protected:
     ~QSaveFilePrivate();
 
     QString fileName;
+    QString finalFileName; // fileName with symbolic links resolved
 
     QFileDevice::FileError writeError;
 

tests/auto/corelib/io/qsavefile/tst_qsavefile.cpp

@@ -93,6 +93,7 @@ private slots:
     void transactionalWriteNoPermissionsOnFile();
     void transactionalWriteCanceled();
     void transactionalWriteErrorRenaming();
+    void symlink();
 };
 
 static inline QByteArray msgCannotOpen(const QFileDevice &f)
@@ -340,5 +341,118 @@ void tst_QSaveFile::transactionalWriteErrorRenaming()
     QCOMPARE(file.error(), QFile::RenameError);
 }
 
+void tst_QSaveFile::symlink()
+{
+#ifdef Q_OS_UNIX
+    QByteArray someData = "some data";
+    QTemporaryDir dir;
+    QVERIFY(dir.isValid());
+
+    const QString targetFile = dir.path() + QLatin1String("/outfile");
+    const QString linkFile = dir.path() + QLatin1String("/linkfile");
+    {
+        QFile file(targetFile);
+        QVERIFY2(file.open(QIODevice::WriteOnly), msgCannotOpen(file).constData());
+        QCOMPARE(file.write("Hello"), Q_INT64_C(5));
+        file.close();
+    }
+
+    QVERIFY(QFile::link(targetFile, linkFile));
+
+    QString canonical = QFileInfo(linkFile).canonicalFilePath();
+    QCOMPARE(canonical, QFileInfo(targetFile).canonicalFilePath());
+
+    // Try saving into it
+    {
+        QSaveFile saveFile(linkFile);
+        QVERIFY(saveFile.open(QIODevice::WriteOnly));
+        QCOMPARE(saveFile.write(someData), someData.size());
+        saveFile.commit();
+
+        //Check that the linkFile is still a link and still has the same canonical path
+        QFileInfo info(linkFile);
+        QVERIFY(info.isSymLink());
+        QCOMPARE(QFileInfo(linkFile).canonicalFilePath(), canonical);
+
+        QFile file(targetFile);
+        QVERIFY2(file.open(QIODevice::ReadOnly), msgCannotOpen(file).constData());
+        QCOMPARE(file.readAll(), someData);
+        file.remove();
+    }
+
+    // Save into a symbolic link that point to a removed file
+    someData = "more stuff";
+    {
+        QSaveFile saveFile(linkFile);
+        QVERIFY(saveFile.open(QIODevice::WriteOnly));
+        QCOMPARE(saveFile.write(someData), someData.size());
+        saveFile.commit();
+
+        QFileInfo info(linkFile);
+        QVERIFY(info.isSymLink());
+        QCOMPARE(QFileInfo(linkFile).canonicalFilePath(), canonical);
+
+        QFile file(targetFile);
+        QVERIFY2(file.open(QIODevice::ReadOnly), msgCannotOpen(file).constData());
+        QCOMPARE(file.readAll(), someData);
+    }
+
+    // link to a link in another directory
+    QTemporaryDir dir2;
+    QVERIFY(dir2.isValid());
+
+    const QString linkFile2 = dir2.path() + QLatin1String("/linkfile");
+    QVERIFY(QFile::link(linkFile, linkFile2));
+    QCOMPARE(QFileInfo(linkFile2).canonicalFilePath(), canonical);
+
+
+    someData = "hello everyone";
+
+    {
+        QSaveFile saveFile(linkFile2);
+        QVERIFY(saveFile.open(QIODevice::WriteOnly));
+        QCOMPARE(saveFile.write(someData), someData.size());
+        saveFile.commit();
+
+        QFile file(targetFile);
+        QVERIFY2(file.open(QIODevice::ReadOnly), msgCannotOpen(file).constData());
+        QCOMPARE(file.readAll(), someData);
+    }
+
+    //cyclic link
+    const QString cyclicLink = dir.path() + QLatin1String("/cyclic");
+    QVERIFY(QFile::link(cyclicLink, cyclicLink));
+    {
+        QSaveFile saveFile(cyclicLink);
+        QVERIFY(saveFile.open(QIODevice::WriteOnly));
+        QCOMPARE(saveFile.write(someData), someData.size());
+        saveFile.commit();
+
+        QFile file(cyclicLink);
+        QVERIFY2(file.open(QIODevice::ReadOnly), msgCannotOpen(file).constData());
+        QCOMPARE(file.readAll(), someData);
+    }
+
+    //cyclic link2
+    QVERIFY(QFile::link(cyclicLink + QLatin1Char('1'), cyclicLink + QLatin1Char('2')));
+    QVERIFY(QFile::link(cyclicLink + QLatin1Char('2'), cyclicLink + QLatin1Char('1')));
+
+    {
+        QSaveFile saveFile(cyclicLink + QLatin1Char('1'));
+        QVERIFY(saveFile.open(QIODevice::WriteOnly));
+        QCOMPARE(saveFile.write(someData), someData.size());
+        saveFile.commit();
+
+        // the explicit file becomes a file instead of a link
+        QVERIFY(!QFileInfo(cyclicLink + QLatin1Char('1')).isSymLink());
+        QVERIFY(QFileInfo(cyclicLink + QLatin1Char('2')).isSymLink());
+
+        QFile file(cyclicLink + QLatin1Char('1'));
+        QVERIFY2(file.open(QIODevice::ReadOnly), msgCannotOpen(file).constData());
+        QCOMPARE(file.readAll(), someData);
+    }
+#endif
+}
+
 QTEST_MAIN(tst_QSaveFile)
 #include "tst_qsavefile.moc"