What causes "Failed to retrieve alternate data stream size." in duplicati/duplicati?

Inspect Win32Exception.NativeErrorCode to determine the kernel-level cause.

duplicati/duplicati · error · Win32Exception

Failed to retrieve alternate data stream size.

Error message

Failed to retrieve alternate data stream size.

What it means

Thrown by SystemIOWindows.FileLength after a valid handle to an alternate data stream was opened but Win32API.GetFileSizeEx returned false. This is rarer than [420]: the stream was found and opened, yet the kernel could not report its size. The native error code in the Win32Exception explains why.

Source

Thrown at Duplicati/Library/Common/IO/SystemIOWindows.cs:719

        {
            if (!IsAlternateDataStream(path))
                return new FileInfo(AddExtendedDevicePathPrefix(path)).Length;

            using SafeFileHandle handle = Win32API.CreateFileW(
                        AddExtendedDevicePathPrefix(path),
                        Win32API.FILE_READ_ATTRIBUTES,
                        Win32API.FILE_SHARE_READ | Win32API.FILE_SHARE_WRITE | Win32API.FILE_SHARE_DELETE, // Broad sharing flags avoid collisions
                        IntPtr.Zero,
                        Win32API.OPEN_EXISTING,
                        0,
                        IntPtr.Zero);

            if (handle.IsInvalid)
                throw new Win32Exception(Marshal.GetLastWin32Error(), "Failed to open handle to alternative data stream ");

            // Retrieve the size directly from the stream handle
            if (!Win32API.GetFileSizeEx(handle, out long streamSize))
                throw new Win32Exception(Marshal.GetLastWin32Error(), "Failed to retrieve alternate data stream size.");

            return streamSize;
        }

        public void CreateSymlink(string symlinkfile, string target, bool asDir)
        {
            if (FileExists(symlinkfile) || DirectoryExists(symlinkfile))
                throw new IOException(string.Format("File already exists: {0}", symlinkfile));


            if (asDir)
            {
                Directory.CreateSymbolicLink(AddExtendedDevicePathPrefix(symlinkfile), target);
            }
            else
            {
                File.CreateSymbolicLink(AddExtendedDevicePathPrefix(symlinkfile), target);
            }

View on GitHub (pinned to 3f348be3e3)

Solutions

Inspect Win32Exception.NativeErrorCode to determine the kernel-level cause.
Retry once for transient network errors (e.g. ERROR_NETNAME_DELETED).
Fall back to reading the stream to compute length if size cannot be queried.
Skip the stream and log a warning if it is a non-critical metadata stream.

Example fix

null

Defensive patterns

Strategy: try-catch

Validate before calling

null

Type guard

null

Try / catch

try { len = systemIO.FileLength(adsPath); }
catch (System.ComponentModel.Win32Exception ex)
{
    logger.LogError(ex, "GetFileSizeEx failed for {Path} (code {Code})", adsPath, ex.NativeErrorCode);
    // fall back to reading the stream to compute length, or skip
}

Prevention

For network-hosted ADS, prefer local staging so size queries are reliable.
Log NativeErrorCode to distinguish transient network faults from permanent failures.
Fall back to streaming the stream content to compute size when the kernel cannot report it.

When it happens

Trigger: CreateFileW succeeded but GetFileSizeEx failed on the resulting handle, which can happen when the handle is to a device-backed reparse point, a remote stream whose size is not reported by the redirector, or the stream was truncated mid-call.

Common situations: ADS on files hosted on SMB/Samba shares with protocol limitations; ADS on files backed by special devices; transient I/O during concurrent writers.

Related errors

AI-assisted analysis of duplicati/duplicati@3f348be3e3 (2026-08-13). Data as JSON: /api/errors/9bb15029621ceb33. Report an issue: GitHub.