# Plan: packages — hierarchical builds for libraries and binaries

**Status: proposed, awaiting sign-off. No implementation yet.**

## Why

`build.zig` is ~1,250 lines and grows by three hand-written stanzas per binary;
at a driver per device family that does not scale. More fundamentally: in one
monolithic build every binary compiles against library *source*, so a library
interface break is silently absorbed by whoever edits everything in one commit —
the interface never has to be honest. danos is about isolation; the build should
mirror it.

A **package** here is a build-time unit only — a directory owning a `build.zig`
(recipe: what it exports, how to test it) and a `build.zig.zon` (manifest: name
+ dependencies). Binaries remain fully static freestanding ELFs; packages change
who declares what, not what links to what. Source code is untouched: `@import`
uses module names (`"pci"`, `"service"`) exactly as today — only build files
know where anything lives.

## Target shape

```
build-support/                 package: the danos build API (userBinary(), targets, default imports)
library/kernel/                package "kernel":   modules ipc, service, memory, process, logging, time, ...
library/device/                package "device":   modules driver, pci, usb-abi, model, ...   (depends on kernel)
library/protocol/              package "protocol": the wire protocols
library/client/                package "client"    (depends on kernel, protocol)
library/csv/                   package "csv"
library/xkeyboard-config/      package "xkeyboard-config"
system/services/<name>/        one package per binary: ~10-line build.zig + zon
system/drivers/<name>/         one package per binary
build.zig (root)               orchestrator: dependency() per binary, image assembly, QEMU, test steps
```

Rules:

- **Dependencies are declared at domain level** (a binary's zon names `kernel`,
  `device`), **imports stay module-level** (`@import("pci")`). `build-support`
  pre-wires the core set every binary uses (service, ipc, memory, process,
  logging); per-binary build files name only extras.
- **Modules export source, not artifacts** — each consumer compiles libraries
  with its own flags, so per-binary optimization choices keep working; Zig's
  cache deduplicates.
- **Zon paths are relative and that is accepted.** Binaries sit exactly three
  levels deep, so the `../../../` prefix is a constant idiom; a library-domain
  move is a rare, already-breaking event fixed by one sed across manifests, and
  a stale path fails loudly before anything compiles.
- **Cross-cutting build changes live in `build-support` only** — that is the
  contract that keeps per-binary build files declarative.

## What this buys

1. Library interfaces become machine-checked: a consumer can only import what a
   domain exports, and each domain's zon declares what it needs (claim-before-
   touch, applied to source).
2. Each library domain gets a standalone `zig build test` — runtime-library
   stability testing in isolation.
3. Adding a binary = adding a directory (source + two small files), not editing
   three places in a 1,250-line file.
4. Later: `lazyDependency` lets an image target build only what it ships.

## Phases

Each phase ends green: `zig build test` passes (88/88 QEMU) and the boot
image's file list is unchanged. Byte-identical binaries are expected but not
required (module reorganization can perturb symbol order); file list is the
hard gate.

**Phase 0 — `build-support`.** Extract `addUserBinary`/`addThreadedUserBinary`,
the freestanding target setup, and the default-import wiring into the
`build-support` package. Root build consumes it; nothing else moves. This is
the cross-cutting-change home, so it lands first.

**Phase 1 — library domains become packages.** In dependency order: `kernel`
(no deps) → `csv`, `protocol` → `device`, `client` → `xkeyboard-config`. Each
gets build.zig + zon + a standalone test step. The root build swaps its
`createModule` calls for `b.dependency("<domain>").module("<name>")`. **No
binary moves in this phase** — the root build is the pilot consumer, which
proves the packages without touching 30 binaries.

**Phase 2 — binaries become packages, in waves.** Wave A: two small services
(e.g. logger, display-demo) to shake out the template. Wave B: remaining
services. Wave C: drivers. Wave D: test fixtures. Root build shrinks to
orchestration per wave.

**Phase 3 — root cleanup.** Split what remains of the root build into
`build/images.zig`, `build/qemu.zig`, imported by a short root `build.zig`.

**Afterwards** (outside this plan): the intel-uhd-graphics-750 driver is
(re)created as a greenfield package — the new-driver checklist's step 2 gets
rewritten against the package template at that point.

## Execution notes (for whichever session runs this)

Anchors in today's root `build.zig` (~1,250 lines):

- `addUserBinary` / `addThreadedUserBinary` / `addUserBinaryImpl` and the
  `default_imports` plumbing start around line 63 — this is what phase 0
  extracts into `build-support`. `addUserBinaryImpl` also wires the `start`
  root shim from `default_imports`; that trick must survive the move.
- `programModule(<exe>).addImport(...)` calls (search `programModule`) are the
  per-binary extra imports — the data for each binary's future build.zig.
- The boot-tree array (search `"etc/init.csv"` or `.getEmittedBin()`) is the
  image file list — the authoritative before/after comparison target.
- Protocol/module definitions (search `createModule`) map module names to
  `library/...` source paths — the data for each domain package's exports.
- The QEMU size-check test hardcodes source paths (search
  `virtio-gpu-protocol.zig` near line 1149) — moves with phase 2 wave C.

Verification per phase:

- Unit tests: `zig build test`.
- QEMU integration suite: `python3 test/qemu_test.py` (docs/testing.md; the
  full suite, all cases must pass).
- Image file list: the boot-tree array is the source of truth — snapshot it
  (paths only) before phase 0 and diff after each phase; `zig build
  check-fat-image` must also stay green.

Context a fresh session should read first: this doc, docs/testing.md,
docs/coding-standards.md (kebab-case names, no abbreviations), and the
`addUserBinaryImpl` body. Commit style: no Co-Authored-By trailers.

## Risks / notes

- Zig version churn: the package API (`b.dependency`, zon schema) has moved
  between releases; the work pins against the repo's current Zig and any
  upgrade lands separately, never mid-phase.
- The QEMU size-check tests hardcode source paths (e.g. virtio-gpu protocol
  struct sizes in the root build) — phase 2 wave C must carry those along.
- Doc updates ride each phase: drivers.md, new-driver-checklist.md, and
  docs/README.md reference build steps that will change shape.