QFileSystemModel: Enable experimental code path for per-file watching

QFileSystemModel had a #ifdefed out experimental code path for watching
single files to track changes in size, which got outdated over time.

Replace the #ifdef 0 by a check for the environment variable
QT_FILESYSTEMMODEL_WATCH_FILES, fix it up and apply some fixes to
related code to make it work:
- Split file names signaled by QFileSystemWatcher on '/' always.
- Do not instantiate QDirIterator on "", which means "current directory"
  and results in mixed-up directories.
- Check on lastModified() in QExtendedInformation::operator==() so
  that changes trigger an update in _q_fileSystemChanged().
- Fix the #ifdefed out part to compile and not to add directories
  to the watcher.

[ChangeLog][QtWidgets][QFileSystemModel] It is now possible to enable
per-file watching by setting the environment variable
QT_FILESYSTEMMODEL_WATCH_FILES, allowing to track for example
changes in file size.

Task-number: QTBUG-46684
Change-Id: Ia5da9170866416c9529251f889814b23d7a7d069
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/widgets/dialogs/qfileinfogatherer.cpp

@@ -196,7 +196,7 @@ void QFileInfoGatherer::fetchExtendedInformation(const QString &path, const QStr
 */
 void QFileInfoGatherer::updateFile(const QString &filePath)
 {
-    QString dir = filePath.mid(0, filePath.lastIndexOf(QDir::separator()));
+    QString dir = filePath.mid(0, filePath.lastIndexOf(QLatin1Char('/')));
     QString fileName = filePath.mid(dir.length() + 1);
     fetchExtendedInformation(dir, QStringList(fileName));
 }
@@ -267,19 +267,19 @@ QExtendedInformation QFileInfoGatherer::getInfo(const QFileInfo &fileInfo) const
     info.icon = m_iconProvider->icon(fileInfo);
     info.displayType = m_iconProvider->type(fileInfo);
 #ifndef QT_NO_FILESYSTEMWATCHER
-    // ### Not ready to listen all modifications
-    #if 0
-        // Enable the next two commented out lines to get updates when the file sizes change...
+    // ### Not ready to listen all modifications by default
+    static const bool watchFiles = qEnvironmentVariableIsSet("QT_FILESYSTEMMODEL_WATCH_FILES");
+    if (watchFiles) {
         if (!fileInfo.exists() && !fileInfo.isSymLink()) {
-            info.size = -1;
-            //watcher->removePath(fileInfo.absoluteFilePath());
+            watcher->removePath(fileInfo.absoluteFilePath());
         } else {
-            if (!fileInfo.absoluteFilePath().isEmpty() && fileInfo.exists() && fileInfo.isReadable()
-                && !watcher->files().contains(fileInfo.absoluteFilePath())) {
-                //watcher->addPath(fileInfo.absoluteFilePath());
+            const QString path = fileInfo.absoluteFilePath();
+            if (!path.isEmpty() && fileInfo.exists() && fileInfo.isFile() && fileInfo.isReadable()
+                && !watcher->files().contains(path)) {
+                watcher->addPath(path);
             }
         }
-    #endif
+    }
 #endif
 
 #ifdef Q_OS_WIN
@@ -329,14 +329,15 @@ void QFileInfoGatherer::getFileInfos(const QString &path, const QStringList &fil
     QVector<QPair<QString, QFileInfo> > updatedFiles;
     QStringList filesToCheck = files;
 
-    QString itPath = QDir::fromNativeSeparators(files.isEmpty() ? path : QLatin1String(""));
-    QDirIterator dirIt(itPath, QDir::AllEntries | QDir::System | QDir::Hidden);
     QStringList allFiles;
-    while (!abort.load() && dirIt.hasNext()) {
-        dirIt.next();
-        fileInfo = dirIt.fileInfo();
-        allFiles.append(fileInfo.fileName());
-        fetch(fileInfo, base, firstTime, updatedFiles, path);
+    if (files.isEmpty()) {
+        QDirIterator dirIt(path, QDir::AllEntries | QDir::System | QDir::Hidden);
+        while (!abort.load() && dirIt.hasNext()) {
+            dirIt.next();
+            fileInfo = dirIt.fileInfo();
+            allFiles.append(fileInfo.fileName());
+            fetch(fileInfo, base, firstTime, updatedFiles, path);
+        }
     }
     if (!allFiles.isEmpty())
         emit newListOfFiles(path, allFiles);

src/widgets/dialogs/qfileinfogatherer_p.h

@@ -84,7 +84,8 @@ public:
     bool operator ==(const QExtendedInformation &fileInfo) const {
        return mFileInfo == fileInfo.mFileInfo
        && displayType == fileInfo.displayType
-       && permissions() == fileInfo.permissions();
+       && permissions() == fileInfo.permissions()
+       && lastModified() == fileInfo.lastModified();
     }
 
 #ifndef QT_NO_FSFILEENGINE