PropagateDownloadFile/FileSystem: Preserve permissions for symlinks

This commit will preserve the permissions set on the symlink if a new
file is downloaded in the same place and the symlink is overwritten.
That fixes a bug which caused the downloaded file to become unreadable
if the symlink was broken and no permissions could be retrieved.

src/libsync/filesystem.cpp

@@ -14,6 +14,8 @@
 
 #include "filesystem.h"
 
+#include <filesystem>
+
 #include "common/utility.h"
 #include <QFile>
 #include <QFileInfo>
@@ -140,6 +142,31 @@ qint64 FileSystem::getSize(const QString &filename)
     return info.size();
 }
 
+QFile::Permissions FileSystem::getPermissions(const QString &filename)
+{
+    using qtStdFilePermissionsMapping = std::pair<QFile::Permission, std::filesystem::perms>;
+    constexpr std::array<qtStdFilePermissionsMapping, 9> qtStdFilePermissionsMappings = {
+        std::make_pair(QFileDevice::ReadOwner, std::filesystem::perms::owner_read),
+        {QFileDevice::WriteOwner, std::filesystem::perms::owner_write},
+        {QFileDevice::ExeOwner, std::filesystem::perms::owner_exec},
+        {QFileDevice::ReadGroup, std::filesystem::perms::group_read},
+        {QFileDevice::WriteGroup, std::filesystem::perms::group_write},
+        {QFileDevice::ExeGroup, std::filesystem::perms::group_exec},
+        {QFileDevice::ReadOther, std::filesystem::perms::others_read},
+        {QFileDevice::WriteOther, std::filesystem::perms::others_write},
+        {QFileDevice::ExeOther, std::filesystem::perms::others_exec}};
+
+    auto fileStatus = std::filesystem::symlink_status(filename.toStdString());
+    auto permissions = fileStatus.permissions();
+
+    QFile::Permissions resultPermissions;
+    for (auto [qtPermissionFlag, stdPermissionFlag] : qtStdFilePermissionsMappings) {
+        auto isPermissionSet = (permissions & stdPermissionFlag) != std::filesystem::perms::none;
+        resultPermissions.setFlag(qtPermissionFlag, isPermissionSet);
+    }
+    return resultPermissions;
+}
+
 // Code inspired from Qt5's QDir::removeRecursively
 bool FileSystem::removeRecursively(const QString &path, const std::function<void(const QString &path, bool isDir)> &onDeleted, QStringList *errors)
 {

src/libsync/filesystem.h

@@ -63,6 +63,13 @@ namespace FileSystem {
      */
     qint64 OWNCLOUDSYNC_EXPORT getSize(const QString &filename);
 
+    /**
+     * @brief Get permissions for a file
+     *
+     * If the file is a symlink, the permissions for the symlink are returned.
+     */
+    QFile::Permissions OWNCLOUDSYNC_EXPORT getPermissions(const QString &filename);
+
     /**
      * @brief Retrieve a file inode with csync
      */

src/libsync/propagatedownload.cpp

@@ -1108,7 +1108,8 @@ namespace { // Anonymous namespace for the recall feature
     static void preserveGroupOwnership(const QString &fileName, const QFileInfo &fi)
     {
 #ifdef Q_OS_UNIX
-        int chownErr = chown(fileName.toLocal8Bit().constData(), -1, fi.groupId());
+        int chownErr = fchownat(AT_FDCWD, fileName.toLocal8Bit().constData(), -1,
+                                fi.groupId(), AT_SYMLINK_NOFOLLOW);
         if (chownErr) {
             // TODO: Consider further error handling!
             qCWarning(lcPropagateDownload) << QString("preserveGroupOwnership: chown error %1: setting group %2 failed on file %3").arg(chownErr).arg(fi.groupId()).arg(fileName);
@@ -1220,11 +1221,11 @@ void PropagateDownloadFile::downloadFinished()
     auto previousFileExists = FileSystem::fileExists(filename) && _item->_instruction != CSYNC_INSTRUCTION_CASE_CLASH_CONFLICT;
     if (previousFileExists) {
         // Preserve the existing file permissions.
-        const auto existingFile = QFileInfo{filename};
-        if (existingFile.permissions() != _tmpFile.permissions()) {
-            _tmpFile.setPermissions(existingFile.permissions());
+        auto previousPermissions = FileSystem::getPermissions(filename);
+        if (previousPermissions != FileSystem::getPermissions(_tmpFile.fileName())) {
+            _tmpFile.setPermissions(previousPermissions);
         }
-        preserveGroupOwnership(_tmpFile.fileName(), existingFile);
+        preserveGroupOwnership(_tmpFile.fileName(), QFileInfo(filename));
 
         // Make the file a hydrated placeholder if possible
         const auto result = propagator()->syncOptions()._vfs->convertToPlaceholder(_tmpFile.fileName(), *_item, filename);