FuelLabs/fuel-core · error · Error::Serialization

Could not convert subsections_number to u16: {}

Error message

Could not convert subsections_number to u16: {}

What it means

subsections_number counts how many pieces the uploaded bytecode was split into, stored as u16 in UploadBody. When the proto value does not fit u16 (> 65535 or negative), u16::try_from fails and this error aborts the Upload conversion.

Source

Thrown at crates/services/block_aggregator_api/src/blocks/old_block_source/convertor_adapter/proto_to_fuel_conversions.rs:784

                ))
            })?;
            let witness_index =
                u16::try_from(proto_upload.witness_index).map_err(|e| {
                    Error::Serialization(anyhow!(
                        "Could not convert witness_index to u16: {}",
                        e
                    ))
                })?;
            let subsection_index =
                u16::try_from(proto_upload.subsection_index).map_err(|e| {
                    Error::Serialization(anyhow!(
                        "Could not convert subsection_index to u16: {}",
                        e
                    ))
                })?;
            let subsections_number = u16::try_from(proto_upload.subsections_number)
                .map_err(|e| {
                    Error::Serialization(anyhow!(
                        "Could not convert subsections_number to u16: {}",
                        e
                    ))
                })?;
            let proof_set = proto_upload
                .proof_set
                .iter()
                .map(|entry| {
                    Bytes32::try_from(entry.as_slice()).map_err(|e| {
                        Error::Serialization(anyhow!(
                            "Could not convert proof_set entry to Bytes32: {}",
                            e
                        ))
                    })
                })
                .collect::<crate::result::Result<Vec<_>>>()?;

            let body = UploadBody {

View on GitHub (pinned to b9d4d170da)

Solutions

1. Validate subsections_number <= 65535 (and >= 1) before converting; reject with the actual value in the message
2. Fix the uploading client to use the chain-mandated subsection size so the count fits u16
3. Cross-check subsection_index < subsections_number as part of the same pre-validation
4. Round-trip test upload protos (encode→decode) in CI for both producer and consumer versions

Example fix

// before
let subsections_number = u16::try_from(proto_upload.subsections_number).map_err(...)?;

// after
let sn = proto_upload.subsections_number;
if !(1..=u16::MAX as u32).contains(&sn) {
    return Err(anyhow!("subsections_number {sn} out of u16 range"));
}
let subsections_number = sn as u16;

fn upload_subsections_number_ok(u: &ProtoUpload) -> bool {
    (1..=u16::MAX as u32).contains(&u.subsections_number)
}

fn valid_subsections_count(n: u32) -> bool { (1..=u16::MAX as u32).contains(&n) }

let n = u16::try_from(u.subsections_number)
    .with_context(|| format!("subsections_number {} invalid", u.subsections_number))?;

When it happens

Trigger: Producer encoded a subsections_number above 65535 — meaning the blob was chunked too finely (chunk size too small) — or garbage data reached the decoder.

Common situations: Chunk-size mismatch between uploader and chain rules; a client uploading a huge blob with 1-byte subsections as a bug; corrupted transport payloads.

Related errors

• Could not convert witness_index to u16: {}
• Could not convert subsection_index to u16: {}
• Could not convert upload root to Bytes32: {}
• Could not convert proof_set entry to Bytes32: {}
• Could not convert blob witness_index to u16: {}

AI-assisted analysis of FuelLabs/fuel-core@b9d4d170da (2026-08-16). Data as JSON: /api/errors/f6b555b85fafb4d0. Report an issue: GitHub.