facebook/flow · error

failed to deserialize FileSig

Error message

failed to deserialize FileSig

What it means

Panics when bincode fails to decode a FileSig from decompressed bytes. FileSig caches per-file signatures (content hash, dependencies) used to decide rechecking; the file_key thread-local is set before decoding because locations are resolved against it. Corruption or writer/reader version mismatch triggers the panic.

Source

Thrown at rust_port/crates/flow_heap_serialization/src/lib.rs:115

    let (module, _): (Module<Loc>, _) =
        bincode::serde::decode_from_slice(&decompressed, bincode::config::legacy())
            .expect("failed to deserialize type sig Module");
    clear_file_key();
    Arc::new(module)
}

pub fn serialize_file_sig(file_sig: &FileSig) -> Vec<u8> {
    let bytes = bincode::serde::encode_to_vec(file_sig, bincode::config::legacy())
        .expect("failed to serialize FileSig");
    compress(&bytes)
}

pub fn deserialize_file_sig(file_key: &FileKey, bytes: &[u8]) -> Arc<FileSig> {
    let decompressed = decompress(bytes);
    set_file_key(file_key);
    let (file_sig, _): (FileSig, _) =
        bincode::serde::decode_from_slice(&decompressed, bincode::config::legacy())
            .expect("failed to deserialize FileSig");
    clear_file_key();
    Arc::new(file_sig)
}

pub fn serialize_exports(exports: &Exports) -> Vec<u8> {
    let bytes = bincode::serde::encode_to_vec(exports, bincode::config::legacy())
        .expect("failed to serialize Exports");
    compress(&bytes)
}

pub fn deserialize_exports(bytes: &[u8]) -> Arc<Exports> {
    let decompressed = decompress(bytes);
    let (exports, _): (Exports, _) =
        bincode::serde::decode_from_slice(&decompressed, bincode::config::legacy())
            .expect("failed to deserialize Exports");
    Arc::new(exports)
}

View on GitHub (pinned to f88ac94bcf)

Solutions

1. Delete the file-sig cache/saved state so signatures are recomputed
2. Version-stamp cache paths so old entries are never read by new binaries
3. Write cache entries atomically (temp + rename)

match std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
    flow_heap_serialization::deserialize_file_sig(&file_key, &bytes)
})) {
    Ok(s) => s,
    Err(_) => { std::fs::remove_file(&cache_path).ok(); recompute_file_sig(&file_key)? }
}

When it happens

Trigger: deserialize_file_sig on a file-sig cache entry that is truncated, from another flow version, or decoded under the wrong file_key; cache files partially overwritten.

Common situations: Saved state reused across flow upgrades; a killed daemon leaving half-written sig files; cache dirs copied between machines with different builds.

Related errors

• failed to deserialize FileSig with errors
• failed to deserialize AST
• failed to deserialize Docblock
• failed to deserialize PackedALocTable
• failed to deserialize type sig Module

AI-assisted analysis of facebook/flow@f88ac94bcf (2026-08-20). Data as JSON: /api/errors/2d1b7d40509a1efd. Report an issue: GitHub.