[RivTian]

Description

Fixes #799.

When an outer graph input payload decodes successfully but contains the wrong number of tuple elements, GraphCodec::decode currently panics. The same input also makes GraphNodeService::call panic synchronously through NodeInput::Single.

This change returns the existing GraphFlowError::InputCountMismatch { expected, actual } before decoding elements. Invalid input never reaches the handler. Codec failures retain their original source, and successful encoded bytes and element order are unchanged. Node registration accepts graph-input errors independently of backend codec errors.

Compatibility: for 2–12 element tuples, both GraphCodec::encode and decode now return GraphFlowError, and codec errors must implement Into<BoxDynError>. Callers that previously downcast node errors directly to a codec error must follow the source chain. These changes are documented in rustdoc and the changelog.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Testing

• I have added tests that prove my fix is effective or that my feature works
• I have run the existing tests and they pass
• I have run cargo fmt and cargo clippy

The regression tests failed at runtime against the original implementation at their no-panic assertions. The retained suite now passes all 24 tests, covering 2- and 12-element tuples, synchronous and asynchronous node execution, handler call counts, exact wire bytes, codec error sources, and all three node registration APIs. A NodeInput::FanIn control exercises its separate serde decoding path.

Local validation passed:

cargo test --locked -p apalis-workflow --test graph_tuple_decode_repro --test graph_tuple_node_repro
cargo check --locked --workspace --all-features
cargo test --locked --all
cargo test --locked -p apalis-workflow --no-default-features
cargo test --locked -p apalis-workflow --all-features
cargo clippy --locked --workspace --all-features --all -- -D warnings
cargo fmt --all -- --check
RUSTDOCFLAGS="-D warnings" cargo doc --locked -p apalis-workflow --no-deps --all-features

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional Notes

Validation used macOS arm64 and Rust 1.96.0. No dependency, lockfile, or CI workflow changes are included.

[geofmureithi]

@RivTian thanks for your contribution as always.