Implement xattr probing behaviour as sugggested by copilot

Author: TheOneRing

src/openvfsfuse.cpp

@@ -703,19 +703,27 @@ static int openVFSfuse_getxattr(const char *orig_path, const char *name, char *v
             return -errno;
         }
         const auto realSize = std::to_string(statbuf.st_size);
-        if (realSize.size() < size) {
+        if (size == 0) {
+            return static_cast<int>(realSize.size());
+        } else if (realSize.size() <= size) {
             std::ranges::copy(realSize, value);
             return static_cast<int>(realSize.size());
         }
         return -ERANGE;
     }
     const auto ret = Xattr::getxattr(path, name, value, size);
     if (ret == -ENODATA && name == OpenVfsConstants::XAttributeNames::Data) {
+        errno = 0;
         // return the default value for the attributes
         // message pack is null terminated
-        static const auto defaultData = OpenVfsAttributes::PlaceHolderAttributes({}).toData();
-        if (size > defaultData.size()) {
-            errno = 0;
+        static const auto defaultData = [] {
+            auto data = OpenVfsAttributes::PlaceHolderAttributes({}).toData();
+            data.push_back('\0');
+            return data;
+        }();
+        if (size == 0) {
+            return static_cast<int>(defaultData.size());
+        } else if (defaultData.size() <= size) {
             std::ranges::copy(defaultData, value);
             openvfsfuse_log(path, "getxattr", 0, "Returning default data for %s: %s", name, value);
             return static_cast<int>(defaultData.size());