What causes "WinFsp mountpoint is not valid UTF-16" in astrid-runtime/astrid?

Check the mountpoint value passed to the WinFsp launch for embedded NUL bytes and remove them.

WinFsp mountpoint is not valid UTF-16

Error message

WinFsp mountpoint is not valid UTF-16

What it means

Raised in `run_private_service` (crates/astrid-storage-provider-winfsp/src/win.rs:210) when `U16CString::from_os_str` fails to convert the configured mountpoint path into a NUL-terminated UTF-16 string, which WinFsp requires. The error is thrown before `FileSystem::start` is invoked. It means the mountpoint `Path` contains characters that cannot be represented as UTF-16 (interior NUL bytes on Windows paths), so a valid Windows mountpoint string could not be built.

Source

Thrown at crates/astrid-storage-provider-winfsp/src/win.rs:210

    launch: StorageProviderServiceLaunchV1,
    challenge: String,
    runtime: Arc<tokio::runtime::Runtime>,
) -> Result<()> {
    if !parent_is_alive(&launch.parent) {
        bail!("WinFsp service parent process is not alive");
    }
    probe_callback(&launch).await?;
    let listener = local_transport::bind(&launch.control_path).with_context(|| {
        format!(
            "bind WinFsp service control {}",
            launch.control_path.display()
        )
    })?;
    let callback = CallbackFs::new(launch.lease.clone(), runtime)
        .map_err(|failure| anyhow::anyhow!("build WinFsp callback filesystem: {failure:?}"))?;
    initialize_winfsp()?;
    let mountpoint = U16CString::from_os_str(launch.mountpoint.as_os_str())
        .map_err(|_| anyhow::anyhow!("WinFsp mountpoint is not valid UTF-16"))?;
    let filesystem = FileSystem::start(
        volume_params(launch.lease.access),
        Some(&mountpoint),
        callback,
    )
    .map_err(|status| anyhow::anyhow!("WinFsp failed to start private mount: {status:#x}"))?;
    let ready = StorageProviderServiceReadyV1 {
        schema: STORAGE_FILESYSTEM_SERVICE_READY_SCHEMA_V1,
        provider: crate::PROVIDER_NAME.to_owned(),
        mount_id: launch.lease.mount_id.as_uuid(),
        control_path: launch.control_path.clone(),
        challenge,
    };
    let mut stdout = std::io::stdout().lock();
    serde_json::to_writer(&mut stdout, &ready).context("encode WinFsp readiness")?;
    stdout
        .write_all(b"\n")
        .context("terminate WinFsp readiness response")?;

View on GitHub (pinned to affd8760f4)

Solutions

Check the mountpoint value passed to the WinFsp launch for embedded NUL bytes and remove them.
Validate the mountpoint path in the daemon/config layer before spawning the WinFsp service (e.g. reject paths whose OsStr encoding contains NUL).
Use a conventional mountpoint such as a drive designator (`X:`) or a normal directory path and retry.
Re-encode the path from its original text source rather than from raw bytes.

Example fix

// before
let mountpoint = U16CString::from_os_str(launch.mountpoint.as_os_str())
    .map_err(|_| anyhow::anyhow!("WinFsp mountpoint is not valid UTF-16"))?;
// after (validate earlier)
if launch.mountpoint.as_os_str().to_str().map_or(true, |s| s.contains('\0')) {
    bail!("mountpoint contains NUL bytes: {:?}", launch.mountpoint);
}
let mountpoint = U16CString::from_os_str(launch.mountpoint.as_os_str())?;

Defensive patterns

Strategy: validation

Validate before calling

fn mountpoint_is_utf16_safe(p: &Path) -> bool {
    p.as_os_str().to_str().map_or(false, |s| !s.contains('\0'))
}
assert!(mountpoint_is_utf16_safe(&PathBuf::from("Q:\\")));

Type guard

fn valid_mountpoint(p: &std::path::Path) -> Option<&str> {
    p.to_str().filter(|s| !s.contains('\0'))
}

Try / catch

let mountpoint = U16CString::from_os_str(launch.mountpoint.as_os_str())
    .map_err(|_| anyhow::anyhow!(
        "WinFsp mountpoint {:?} is not valid UTF-16 (contains NUL?)",
        launch.mountpoint
    ))?;

Prevention

Never build mountpoint paths from raw byte buffers; construct them from validated text.
Validate mountpoint strings for NUL bytes at config-parse time.
Prefer drive designators (`X:`) or normal directories as mountpoints.

When it happens

Trigger: Specifically: `U16CString::from_os_str(launch.mountpoint.as_os_str())` returns `Err`, i.e. `launch.mountpoint` contains an interior NUL byte or is otherwise not convertible to a UTF-16 C string. All normal Windows paths convert fine; only paths with embedded NULs fail.

Common situations: A mountpoint was built programmatically from raw bytes that include a NUL; a corrupted config file or IPC payload produced a mangled path; a caller passed a path constructed with `CString`-style data instead of normal OS path text.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of astrid-runtime/astrid@affd8760f4 (2026-09-09). Data as JSON: /api/errors/97bfa079f4c02429. Report an issue: GitHub.