Check path validity on Windows will validate NTFS permissions for network drives

src/common/utility.h

@@ -275,6 +275,8 @@ namespace Utility {
 
     OCSYNC_EXPORT QString formatWinError(long error);
 
+    OCSYNC_EXPORT bool canCreateFileInPath(const QString &path);
+
     class OCSYNC_EXPORT NtfsPermissionLookupRAII
     {
     public:

src/common/utility_win.cpp

@@ -1,4 +1,4 @@
 /*
  * Copyright (C) by Daniel Molkentin <[email protected]>
  *
  * This library is free software; you can redistribute it and/or
@@ -33,6 +33,7 @@
 #include <QFile>
 #include <QLibrary>
 #include <QSettings>
+#include <QTemporaryFile>
 
 extern Q_CORE_EXPORT int qt_ntfs_permission_lookup;
 
@@ -430,6 +431,15 @@
     hundredNSecs->HighPart = ll >>32;
 }
 
+bool Utility::canCreateFileInPath(const QString &path)
+{
+    Q_ASSERT(!path.isEmpty());
+    const auto pathWithSlash = !path.endsWith(QLatin1Char('/'))
+        ? path + QLatin1Char('/')
+        : path;
+    QTemporaryFile testFile(pathWithSlash + QStringLiteral("~$write-test-file-XXXXXX"));
+    return testFile.open();
+}
 
 QString Utility::formatWinError(long errorCode)
 {

src/gui/folderman.cpp

@@ -1,4 +1,4 @@
 /*
  * Copyright (C) by Klaas Freitag <[email protected]>
  *
  * This program is free software; you can redistribute it and/or modify
@@ -1701,10 +1701,20 @@
         return FolderMan::tr("The selected path is not a folder!");
     }
 
+    #ifdef Q_OS_WIN
+    if (!selFile.isWritable()) {
+        // isWritable() doesn't cover all NTFS permissions
+        // try creating and removing a test file, and make sure it is excluded from sync
+        if (!Utility::canCreateFileInPath(path)) {
+            return FolderMan::tr("You have no permission to write to the selected folder!");
+        }
+    }
+    #else
     if (!selFile.isWritable()) {
         return FolderMan::tr("You have no permission to write to the selected folder!");
     }
-    return QString();
+    #endif
+    return {};
 }
 
 // QFileInfo::canonicalPath returns an empty string if the file does not exist.