facebook/flow · error

failed to deserialize type sig Module

Error message

failed to deserialize type sig Module

What it means

Panics when bincode fails to decode a type-sig Module from decompressed bytes. The decoder sets the file_key thread-local first because Module<Loc> deserialization resolves source locations against it. A mismatch between the bytes' provenance (version/layout/file) and the current decoder is the failure mode.

Source

Thrown at rust_port/crates/flow_heap_serialization/src/lib.rs:99

    let decompressed = decompress(bytes);
    let (table, _): (PackedALocTable, _) =
        bincode::serde::decode_from_slice(&decompressed, bincode::config::legacy())
            .expect("failed to deserialize PackedALocTable");
    Arc::new(table)
}

pub fn serialize_type_sig(module: &Module<Loc>) -> Vec<u8> {
    let bytes = bincode::serde::encode_to_vec(module, bincode::config::legacy())
        .expect("failed to serialize type sig Module");
    compress(&bytes)
}

pub fn deserialize_type_sig(file_key: &FileKey, bytes: &[u8]) -> Arc<Module<Loc>> {
    let decompressed = decompress(bytes);
    set_file_key(file_key);
    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)

View on GitHub (pinned to f88ac94bcf)

Solutions

1. Invalidate the signature cache so type sigs are recomputed
2. Tie cache directories to the flow version (or hash of serialized type layouts)
3. Regenerate any serialized test fixtures after editing Module or Loc serialization

match std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
    flow_heap_serialization::deserialize_type_sig(&file_key, &bytes)
})) {
    Ok(m) => m,
    Err(_) => { invalidate_sig_cache(); recompute_sig(&file_key)? }
}

When it happens

Trigger: deserialize_type_sig on signature-cache bytes written by a different flow build; type signature schema changes without cache invalidation; bytes decoded with a file_key that doesn't match the serialized Loc offsets.

Common situations: Reusing a sig cache after upgrading flow; multi-checkout workspaces sharing one cache dir; CI caches persisted across toolchain bumps.

Related errors

• failed to deserialize AST
• failed to deserialize Docblock
• failed to deserialize PackedALocTable
• failed to deserialize FileSig
• failed to deserialize Exports

AI-assisted analysis of facebook/flow@f88ac94bcf (2026-08-20). Data as JSON: /api/errors/2adb0ab4bcf195eb. Report an issue: GitHub.