prometeu-studio/docs/packer/specs/4. Build Artifacts and Deterministic Packing Specification.md

Build Artifacts and Deterministic Packing Specification

Status: Draft Scope: Runtime-facing artifacts and deterministic build behavior Purpose: Define how the packer emits assets.pa and companion artifacts.

Authority and Precedence

This specification consolidates the initial packer agenda and decision wave into normative form.

Runtime-side reading semantics remain upstream in:

• ../runtime/docs/runtime/specs/13-cartridge.md
• ../runtime/docs/runtime/specs/15-asset-management.md

Core Rules

1. assets.pa is the authoritative runtime-facing artifact.
2. Companion JSON files do not replace the internal header as the runtime contract.
3. The global asset table order is deterministic by increasing asset_id.
4. The runtime-facing asset_id is the same stable asset_id allocated by the packer registry.

assets.pa Structure

Baseline structure:

[fixed binary prelude]
[canonical JSON header]
[binary payload region]

Prelude Fields

The baseline prelude includes:

• magic
• schema_version
• header_len
• payload_offset
• flags
• reserved

Rules:

• flags and reserved exist from day 1;
• flags = 0 unless later specified otherwise;
• reserved = 0 unless later specified otherwise;
• header_checksum is not part of the baseline envelope contract.

Canonical JSON Header

The header is serialized canonically.

Rules:

• UTF-8 encoding;
• no extra whitespace;
• object keys sorted lexicographically;
• arrays preserve declared order;
• canonicalization applies recursively;
• runtime-facing header values in v1 must avoid floating-point numbers.

Header Contents

The header carries:

• asset_table
• preload

Asset Table

asset_table is emitted as a deterministically ordered list of asset entries.

Rules:

• one entry per included asset in the current build set;
• no secondary runtime-only identity layer;
• no synthetic dense reindexing layer;
• asset_name remains present as logical/API-facing metadata.

Preload

Preload is emitted deterministically from per-asset declaration.

Rules:

• assets with preload.enabled = false do not appear in emitted preload data;
• assets with preload.enabled = true do appear;
• preload ordering is deterministic by increasing asset_id.

Companion Artifacts

The baseline companion artifacts are:

• build/asset_table.json
• build/preload.json
• build/asset_table_metadata.json

Rules:

• build/asset_table.json mirrors header.asset_table 1:1;
• build/preload.json mirrors header.preload 1:1;
• build/asset_table_metadata.json is tooling-only;
• richer tooling data must not be added to the 1:1 mirror files.

Alignment and Offsets

Alignment exists only when explicitly required by spec.

Rules:

• there is no implicit baseline alignment beyond envelope and format requirements;
• any required alignment must be normative and visible;
• emitted offsets are always relative to the payload region, never the start of the full file.

Determinism

Equivalent build inputs must produce equivalent outputs.

Rules:

• no filesystem iteration dependence;
• no hidden defaults that affect output;
• no usage-based hot-first packing in the baseline contract;
• determinism is preferred over speculative physical-layout optimization.

Non-Goals

• per-format payload internals for every output family
• future locality optimization policy
• cartridge-level integrity/signature strategy

Exit Criteria

This specification is complete enough when:

• runtime-facing artifact authority is explicit;
• companion artifacts are bounded;
• emitted ordering and header behavior are deterministic.