moveToTrash/Unix: avoid creating too many QStorageInfo

QStorageInfo is great, but rather expensive, so this introduces a faster
check by stat()ing the source file and $HOME, to see if they are the
same device, saving us two or three QStorageInfo constructions. That is
a necessary condition: if they aren't the same device, we know rename()
into $HOME/.local/share/Trash will fail.

But it's not a sufficient condition: they need to be the same mount
point and that's something only QStorageInfo will give us. Strictly
speaking, the only way to be sure that you can rename() into the trash
path is to, well, attempt it (as usual, something for a later commit).

Change-Id: I9d43e5b91eb142d6945cfffd1786c474cac25083
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>

src/corelib/io/qfilesystemengine_unix.cpp

@@ -1194,7 +1194,7 @@ bool QFileSystemEngine::moveFileToTrash(const QFileSystemEntry &, QFileSystemEnt
     Implementing as per https://specifications.freedesktop.org/trash-spec/trashspec-1.0.html
 */
 
-static QString freeDesktopTrashLocation(const QFileSystemEntry &source, QSystemError &error)
+static auto freeDesktopTrashLocation(const QFileSystemEntry &source, QSystemError &error)
 {
     auto makeTrashDir = [](const QDir &topDir, const QString &trashDir, QSystemError &error) {
         auto ownerPerms = QFileDevice::ReadOwner
@@ -1212,19 +1212,33 @@ static QString freeDesktopTrashLocation(const QFileSystemEntry &source, QSystemE
             return targetDir;
         return QString();
     };
+    struct R {
+        QString trashDir;
+        qsizetype volumePrefixLength = 0;
+    } r;
 
-    if (QFileSystemMetaData md; !QFileSystemEngine::fillMetaData(source, md, QFileSystemMetaData::ExistsAttribute)
+    // first, check if they are in the same device
-            || !md.exists()) {
+    QString homePath = QFileSystemEngine::homePath();
-        error = QSystemError(ENOENT, QSystemError::StandardLibraryError);
+    const QString sourcePath = source.filePath();
-        return QString();
+    QT_STATBUF sourceInfo, homeInfo;
+    if (QT_STAT(QFile::encodeName(sourcePath), &sourceInfo) != 0 ||
+            QT_STAT(QFile::encodeName(homePath), &homeInfo) != 0) {
+        error = QSystemError(errno, QSystemError::StandardLibraryError);
+        return r;
     }
 
-    QString trash;
-    const QString sourcePath = source.filePath();
     const QStorageInfo sourceStorage(sourcePath);
+    bool isHomeVolume = false;
+    if (sourceInfo.st_dev == homeInfo.st_dev) {
+        // being the same device is not enough for rename(): they must be the
+        // same mount, so we need QStorageInfo to compare
+        isHomeVolume = sourceStorage == QStorageInfo(QFileSystemEngine::homePath());
+    }
+
+    QString &trash = r.trashDir;
     const QStorageInfo homeStorage(QDir::home());
     // We support trashing of files outside the users home partition
-    if (sourceStorage != homeStorage) {
+    if (!isHomeVolume) {
         const auto dotTrash = "/.Trash"_L1;
         QFileSystemEntry dotTrashDir(sourceStorage.rootPath() + dotTrash);
 
@@ -1277,6 +1291,14 @@ static QString freeDesktopTrashLocation(const QFileSystemEntry &source, QSystemE
             const QString userTrashDir = dotTrash + u'-' + userID;
             trash = makeTrashDir(QDir(sourceStorage.rootPath() + userTrashDir), QString(), error);
         }
+
+        if (!trash.isEmpty()) {
+            r.volumePrefixLength = sourceStorage.rootPath().size();
+            if (r.volumePrefixLength == 1)
+                r.volumePrefixLength = 0;      // isRoot
+            else
+                ++r.volumePrefixLength;        // to include the slash
+        }
     }
     /*
         "If both (1) and (2) fail [...], the implementation MUST either trash the
@@ -1297,7 +1319,7 @@ static QString freeDesktopTrashLocation(const QFileSystemEntry &source, QSystemE
         }
     }
 
-    return trash;
+    return r;
 }
 
 //static
@@ -1311,7 +1333,7 @@ bool QFileSystemEngine::moveFileToTrash(const QFileSystemEntry &source,
         }
         return absoluteName(source);
     }();
-    QString trashPath = freeDesktopTrashLocation(sourcePath, error);
+    auto [trashPath, volumePrefixLength] = freeDesktopTrashLocation(sourcePath, error);
     if (trashPath.isEmpty())
         return false;
     QDir trashDir(trashPath);
@@ -1360,14 +1382,6 @@ bool QFileSystemEngine::moveFileToTrash(const QFileSystemEntry &source,
         infoFile.setFileName(infoFileName);
     }
 
-    QString pathForInfo = sourcePath.filePath();
-    const QStorageInfo storageInfo(pathForInfo);
-    if (storageInfo.isValid() && storageInfo.rootPath() != rootPath() && storageInfo != QStorageInfo(QDir::home())) {
-        pathForInfo = std::move(pathForInfo).mid(storageInfo.rootPath().length());
-        if (pathForInfo.front() == u'/')
-            pathForInfo = pathForInfo.mid(1);
-    }
-
     /*
         We might fail to rename if source and target are on different file systems.
         In that case, we don't try further, i.e. copying and removing the original
@@ -1382,7 +1396,7 @@ bool QFileSystemEngine::moveFileToTrash(const QFileSystemEntry &source,
 
     QByteArray info =
             "[Trash Info]\n"
-            "Path=" + QUrl::toPercentEncoding(pathForInfo, "/") + "\n"
+            "Path=" + QUrl::toPercentEncoding(sourcePath.filePath().mid(volumePrefixLength), "/") + "\n"
             "DeletionDate=" + QDateTime::currentDateTime().toString(Qt::ISODate).toUtf8()
             + "\n";
     infoFile.write(info);