astrid-runtime/astrid · error

mount callback response is too large

Error message

mount callback response is too large

What it means

After the MAX_CALLBACK_FRAME_BYTES check, write_response converts the serialized length to u32 for the big-endian length prefix. If the length does not fit in u32 (practically: > 4 GiB, i.e. the frame-bytes check did not catch it), this second InvalidData error fires. It is a defensive guard ensuring the wire format's u32 length field is always representable.

Solutions

1. Restore/keep MAX_CALLBACK_FRAME_BYTES below u32::MAX so the length always fits
2. Fix the earlier guard rather than the conversion: the frame-bytes check should reject the response first
3. If huge responses are required, redesign the protocol to chunk responses instead of enlarging the length prefix

Example fix

// before
const MAX_CALLBACK_FRAME_BYTES: usize = usize::MAX;
// after
const MAX_CALLBACK_FRAME_BYTES: usize = u32::MAX as usize;

fn fits_u32_length_prefix(bytes_len: usize) -> bool {
    bytes_len <= u32::MAX as usize
}
assert!(fits_u32_length_prefix(ser_len), "length must fit u32 frame prefix");

match result {
    Err(e) if e.to_string() == "mount callback response is too large" => {
        eprintln!("protocol bug: response exceeded u32 length prefix; enforce frame cap");
    }
    other => other?,
}

When it happens

Trigger: bytes.len() > u32::MAX after passing the MAX_CALLBACK_FRAME_BYTES check — only reachable if MAX_CALLBACK_FRAME_BYTES is configured above 4 GiB or the check is bypassed by code changes.

Common situations: Someone raised MAX_CALLBACK_FRAME_BYTES to u64/usize max to 'remove limits'; a refactor removed the earlier size check so giant responses reach the u32 conversion.

Understand the failure class

Background: payload too large / request exceeds maximum size: why libraries cap bytes and how to fix oversize payloads — this error's family across 50 libraries.

Related errors

• mount callback response exceeds limit
• environment value for
• filesystem payload exceeds limit
• FSKit callback response exceeds the bounded frame size
• FSKit service control request exceeds limit

AI-assisted analysis of astrid-runtime/astrid@affd8760f4 (2026-09-09). Data as JSON: /api/errors/55b28825309f0371. Report an issue: GitHub.

Appendix: source

Thrown at crates/astrid-kernel/src/storage_mount.rs:687

#[cfg(any(unix, windows))]
async fn write_response(
    stream: &mut LocalStream,
    response: CallbackResponse,
) -> Result<(), io::Error> {
    let bytes = match response {
        CallbackResponse::V1(response) => serde_json::to_vec(&response),
        CallbackResponse::V2(response) => serde_json::to_vec(&response),
    }
    .map_err(io::Error::other)?;
    if bytes.len() > MAX_CALLBACK_FRAME_BYTES {
        return Err(io::Error::new(
            io::ErrorKind::InvalidData,
            "mount callback response exceeds limit",
        ));
    }
    let length = u32::try_from(bytes.len()).map_err(|_| {
        io::Error::new(
            io::ErrorKind::InvalidData,
            "mount callback response is too large",
        )
    })?;
    stream.write_all(&length.to_be_bytes()).await?;
    stream.write_all(&bytes).await?;
    stream.flush().await
}

async fn dispatch_request(
    kernel: &Kernel,
    state: &StorageMountLeaseState,
    callback: CallbackRequest,
) -> CallbackResponse {
    let request = callback.request;
    let request_id = request.request_id.clone();
    let outcome = if !state.is_live() {
        failure("stale-lease", "storage mount lease is expired or revoked")

View on GitHub (pinned to affd8760f4)