# Building qroissant wheels

This project ships as a [maturin](https://www.maturin.rs/) / PyO3 mixed
Rust+Python package. The native extension uses `#![feature(portable_simd)]` so
**nightly Rust is required**, pinned via `rust-toolchain.toml`.

All builds run inside a Docker image (`scripts/Dockerfile.build`) so the host
needs only Docker and Python 3 — no Rust, no MSVC SDK, no mingw to install.

## Prerequisites

- Docker (tested with 29.x)
- Python 3.11+ on the host, **only** if you want to install/test the produced
  Linux wheel locally via `scripts/build.sh check`.

## One-time setup

```sh
scripts/build.sh image
```

Builds `qroissant-build:latest`. Pulls `ghcr.io/rust-cross/cargo-xwin`,
installs the nightly toolchain + `x86_64-pc-windows-msvc` target + maturin.
~5 min on a cold cache, then it's a no-op.

## Build commands

```sh
scripts/build.sh linux      # -> dist-linux/qroissant-*-manylinux_*_x86_64.whl
scripts/build.sh windows    # -> dist-windows/qroissant-*-win_amd64.whl
scripts/build.sh all        # image + linux + windows
scripts/build.sh check      # install latest linux wheel into .venv, import-smoke,
                            # then `zipfile -l` the windows wheel
scripts/build.sh clean      # rm dist-linux dist-windows
scripts/build.sh clean-cache  # also drop the cargo/target/xwin Docker volumes
```

The wheels target **abi3-py311**, so a single artifact per platform covers
CPython 3.11, 3.12, 3.13, and onward.

## What lives where

| Artifact | Path |
| --- | --- |
| Build image definition | `scripts/Dockerfile.build` |
| Wrapper script | `scripts/build.sh` |
| Toolchain pin | `rust-toolchain.toml` |
| Linux wheels | `dist-linux/` (gitignored) |
| Windows wheels | `dist-windows/` (gitignored) |
| Cargo registry cache | Docker volume `qroissant-cargo-registry` |
| Cargo target dir | Docker volume `qroissant-target` |
| cargo-xwin MSVC cache | Docker volume `qroissant-xwin` |

The three Docker volumes persist between runs so reruns only rebuild changed
crates; nuke them with `scripts/build.sh clean-cache` if anything looks stale.

## Debug symbols on Windows

The release profile sets `debug = "line-tables-only"`, so a `_native.pdb` is
produced alongside the wheel. `scripts/build.sh windows` copies it into
`dist-windows/`. To get resolved frames in a Rust panic backtrace on a Windows
machine:

1. `pip install qroissant-*-win_amd64.whl`
2. Locate the installed extension, e.g.
   `Lib\site-packages\qroissant\_native.pyd`
3. Drop `_native.pdb` next to it (same directory, same basename).

Symbol info is line-tables-only, so frames resolve to `file:line` but local
variables and types aren't included. That's enough for backtraces and is
~30 MB; full debug info would be much larger.

## How Windows cross-compile works

The Windows path uses [cargo-xwin](https://github.com/rust-cross/cargo-xwin),
which downloads the Microsoft CRT + Windows SDK headers (cached in the xwin
volume; license auto-accepted via `XWIN_ACCEPT_LICENSE=1`). PyO3's
`generate-import-lib` feature — enabled in
`crates/qroissant-python/Cargo.toml` — lets us produce the `python3.dll`
import library at build time, so no Windows Python install is required on the
host.

## Quick wheel sanity checks

```sh
# Listing
python3 -m zipfile -l dist-windows/qroissant-*-win_amd64.whl

# Validate the dist-info
pipx run twine check dist-linux/*.whl dist-windows/*.whl
```

## Iterating on Python code

For Python-side changes (no Rust touched), the fastest loop is still
`maturin develop` against a local rustup install. The Docker flow above is
optimized for producing wheels, not for inner-loop iteration. If you have a
nightly rustup toolchain on the host, you can do:

```sh
python3 -m venv .venv && .venv/bin/pip install 'maturin>=1.8,<2.0'
.venv/bin/maturin develop --release
.venv/bin/python -c 'import qroissant'
```

## Troubleshooting

- **"manylinux_2_34" wheel won't install on old distros** — the image base is
  Debian 13 (glibc 2.38). For broader compatibility, build inside a
  manylinux2014 image or run `auditwheel repair` against the produced wheel.
- **"failed to generate python3.dll import library"** — means `llvm-dlltool`
  is missing inside the image. Rebuild it with `scripts/build.sh image`.
- **xwin download is slow / fails** — Microsoft's CDN occasionally rate-limits.
  Drop the cache (`docker volume rm qroissant-xwin`) and retry.
