pydantic/monty · error

fingerprint field length fits u32

Error message

fingerprint field length fits u32

What it means

An internal `expect` inside the FNV-1a `update` helper used by the test-only `static_strings_fingerprint()`. The helper hashes length-prefixed byte slices and asserts each slice's length fits in a `u32` before writing the prefix — trivially true for the static-string identities and small serializations it is fed. It cannot be triggered by user input; it fires only if the fingerprint function were ever pointed at data exceeding u32 length.

Source

Thrown at crates/monty/src/intern.rs:1225

    #[strum(serialize = "quotetabs")]
    Quotetabs,
    /// `istext` parameter of `binascii.b2a_qp()`.
    #[strum(serialize = "istext")]
    Istext,
    /// `header` parameter of the `binascii` quoted-printable pair.
    #[strum(serialize = "header")]
    Header,
}

/// Computes an FNV-1a hash over static-string identities and serialization.
#[cfg(test)]
pub(crate) fn static_strings_fingerprint() -> u64 {
    const OFFSET_BASIS: u64 = 0xcbf2_9ce4_8422_2325;
    const PRIME: u64 = 0x0100_0000_01b3;

    fn update(hash: &mut u64, bytes: &[u8]) {
        for byte in u32::try_from(bytes.len())
            .expect("fingerprint field length fits u32")
            .to_le_bytes()
        {
            *hash ^= u64::from(byte);
            *hash = hash.wrapping_mul(PRIME);
        }
        for byte in bytes {
            *hash ^= u64::from(*byte);
            *hash = hash.wrapping_mul(PRIME);
        }
    }

    let mut hash = OFFSET_BASIS;
    for value in StaticStrings::iter() {
        update(&mut hash, &(value as u16).to_le_bytes());
        update(&mut hash, format!("{value:?}").as_bytes());
        let string: &'static str = value.into();
        update(&mut hash, string.as_bytes());
        update(

View on GitHub (pinned to adc986b362)

Solutions

1. This u32::try_from guards hashing of byte-slice lengths; a failure means a slice longer than u32::MAX (~4 GiB) was passed, impossible for the static-string fingerprint which hashes small fixed buffers and enum serializations.
2. If it ever fires, check what input grew beyond u32::MAX and switch the fingerprint's length encoding to u64 if a genuinely huge slice must be supported.

When it happens

Trigger: Thrown at crates/monty/src/intern.rs:1225 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of pydantic/monty@adc986b362 (2026-09-13). Data as JSON: /api/errors/51cfc69985150501. Report an issue: GitHub.