Quickpack Rust backend

A compiled alternative to the Python quickpack that reduces packing overhead and startup latency.

Background

Quickpack is Cantrip's fast, local-only alternative to charmcraft pack. It skips LXD builds, linting, and analysis, producing a valid .charm file suitable for development deploys and upgrade testing. The original implementation is written in Python and lives in src/quickpack/.

Because the agent packs charms frequently during its build–test–debug loop, even small latency reductions compound. The Rust implementation (src/quickpack-rs/) eliminates the Python interpreter startup cost and handles file I/O and ZIP compression natively, while producing byte-compatible output.

Architecture

The Rust binary mirrors the Python package module-for-module:

Both implementations share the same command-line interface:

quickpack [charm_dir] [--output-dir DIR] [--quiet]

The Rust version does not expose a Python importable API — it is a standalone binary only. Both versions shell out to uv venv and uv sync to build the charm's virtual environment; that subprocess call is identical in both cases and dominates the total wall-clock time.

How the backend is selected

When the agent's quick_pack tool is invoked, it checks for the Rust binary in two locations, in order:

1. quickpack-rs on $PATH — this is the installed name, useful when the binary has been placed in /usr/local/bin or similar.
2. The in-tree build artefact at src/quickpack-rs/target/release/quickpack, relative to the cantrip package directory.

If neither is found, the tool falls back to the Python quickpack library transparently. The backend field in the tool result data indicates which implementation was used ("rust" or "python").

Performance

Benchmarks on an Ubuntu 24.04 machine (x86_64), packing a minimal charm with one ops dependency:

The full-pack speedup is bounded by uv sync, which accounts for the majority of wall-clock time in both implementations. The Rust advantage is most visible in the non-uv overhead: file copying, YAML serialisation, ZIP compression, and process startup.

For charms with large dependency trees or slow networks, the relative speedup shrinks because uv sync dominates further. For charms with few or no dependencies, the ~5× startup advantage is more visible.

Building from source

The Rust toolchain (1.70+) is required. From the repository root:

cd src/quickpack-rs
cargo build --release

The binary is written to src/quickpack-rs/target/release/quickpack. Cantrip will find it automatically at this location. To install system-wide:

sudo cp src/quickpack-rs/target/release/quickpack /usr/local/bin/quickpack-rs

Running the tests

The Rust implementation has its own spread test suite at tests/spread/quickpack-rs/task.yaml. It runs the same 24 functional tests as the Python version (the python -m quickpack test is excluded as Python-specific). To run locally:

spread tests/spread/quickpack-rs

The Python test suite remains the canonical source of truth. The Rust suite verifies output compatibility: both implementations must produce structurally identical .charm archives.

See also:

• How Cantrip works
• Agent tools