The document magic as an item magic is a nested document, not a defect

[thedavidmeister]

Closes #204.

The adjudication

#204 asks, neutrally, whether the codec should reject KnownMagic::RainMetaDocumentV1 as an item's own key-1 magic, on the premise that such an item is "representable-but-meaningless" because unpack_into and KnownMeta::try_from both answer UnsupportedMeta.

It must not reject, and the premise does not hold. The document magic as an item's own magic already has a defined meaning in this crate: the payload is itself a complete rain meta document. OrderBuilderStateV1::extract_from_meta (crates/cli/src/meta/types/dotrain/order_builder_state_v1.rs:99) branches on exactly that and recurses:

if item.magic == KnownMagic::RainMetaDocumentV1 {
    if let Some(instance) = OrderBuilderStateV1::extract_from_meta(item.payload.as_ref())? {
        return Ok(Some(instance));
    }
}

That branch is human-authored and shipped -- introduced in ddaf63d (2025-07-11, feat: implement DotrainInstance1::extract_from_meta), carried through the rename in a712031, and pinned by the pre-existing test_extract_from_meta_nested_rain_document. Rejecting the magic at encode or decode deletes nesting from the crate. Verified, not assumed: applying the rejection makes that pre-existing test fail (evidence under QA).

The reading is the spec's own. Key 1 is defined as "a signal of intent for the payload"; the payload of a nesting item is a rain meta document, so the document magic is the correct signal for it. The magic-number table's "Prefixes every rain meta document" says where the number is required, not that it may appear nowhere else, and the design goals ask for "a graph structure between meta such that meta can be about other meta".

The unpack layer is right to keep rejecting it, and that is the answer to the issue's second clause: nesting is not a leaf meta type, so unpack_into must never hand a whole document to a payload conversion. Representability here is not "harmless", it is load-bearing, and the UnsupportedMeta at the unpack layer is the correct boundary rather than evidence of meaninglessness.

The issue's scanner-confusion note is real but external to this repo. Every prefix check here is at offset 0 -- cbor_decode and Store::store_content use starts_with, and Solidity's LibMeta.isRainMetaV1 reads the leading 8 bytes. Nothing scans mid-stream, and cbor's length-prefixed framing disambiguates for any correct decoder, as the issue itself notes.

What changed

No behaviour change. The decision is stated where the wrong inference was drawn, and enforced where a future change would otherwise make it silently.

• crates/cli/src/meta/magic.rs -- the RainMetaDocumentV1 doc comment read only "Prefixes every rain meta document", which is the text that invites RainMetaDocumentV1 document magic is encodable as an item's own magic, producing representable-but-unusable metas #204's reading. It now names both roles.
• crates/cli/src/meta/mod.rs -- three tests pin the semantic at the codec level, where it previously existed only inside one consumer in another module:
  • test_cbor_decode_handwritten_document_magic_item -- a handwritten {0: h'01', 1: 0xff0a89c674ee7874} map decodes, so acceptance is the decoder's own behaviour and not an artefact of this crate's encoder.
  • test_document_magic_item_carries_a_nested_document -- an item whose magic is the document magic and whose payload is a complete prefixed document survives cbor_encode_seq -> cbor_decode with its payload byte for byte intact, and that payload decodes to the inner item.
  • test_document_magic_item_is_not_unpackable -- the boundary: KnownMeta::try_from and unpack_into still answer UnsupportedMeta.

Relation to the other cbor_decode / KnownMagic work

All read before writing this.

• fix: drop unknown-magic items in cbor_decode instead of failing the document #274 (issue cbor_decode rejects entire documents containing unknown-magic items; spec expects unsupported metas to be droppable/ignorable #186, drop unknown-magic items) -- orthogonal and compatible. RainMetaDocumentV1 is a known magic, so it is never on the drop path; the tests here would pass unchanged on top of it.
• Drop cbor items missing mandatory keys 0/1 instead of failing the whole document #267 (issue Items missing mandatory cbor keys 0/1 abort the whole decode; spec says they MUST be treated as unexpected and dropped/ignored #188, drop items missing keys 0/1) -- orthogonal and compatible, and its boundary test test_cbor_decode_mandatory_keys_present_still_type_checked states the same rule this PR relies on: an item carrying both mandatory keys is not "unexpected" and is not dropped. A nesting item carries both.
• reject duplicate cbor map keys on meta item decode #276 (issue RainMetaDocumentV1Item decode accepts duplicate cbor map keys with last-wins instead of treating the map as invalid #191, reject duplicate map keys) -- orthogonal; the items here have no repeated keys.
• Add KnownMagic::WebDataV1 for the spec's 0xff5dcce9b571ba42 #268 (issue KnownMagic is missing Web data v1 (0xff5dcce9b571ba42) listed in the metadata-v1 spec magic number table #187, add WebDataV1) -- different concern, but it also edits magic.rs. The edits are on different lines (its variant sits after the rain-specific numbers; this changes the doc comment of the first variant), so they rebase cleanly.
• fix: drop unreachable sub-conditions from the cbor_decode corrupt-meta guard #269 (issue cbor_decode corrupt-meta guard carries unreachable sub-conditions (eof offset equality, track/metas length) — equivalent-mutant evidence #190, drop unreachable guard sub-conditions) -- orthogonal; this PR does not touch cbor_decode's corrupt-meta guard.
• cbor_decode accepts input without the rain meta document prefix; spec says tooling MUST discard data not beginning with the magic number #203 (prefix optional on decode) -- adjacent and consistent. cbor_decode accepts input without the rain meta document prefix; spec says tooling MUST discard data not beginning with the magic number #203 asks whether cbor_decode is document-level or item/sequence-level. The nesting semantic pinned here is evidence for the item/sequence reading: extract_from_meta hands a prefixed inner document straight back to cbor_decode, so the same entry point has to accept both prefixed and bare input. Whatever cbor_decode accepts input without the rain meta document prefix; spec says tooling MUST discard data not beginning with the magic number #203 decides must not break that recursion.

No open PR in the group decides anything that contradicts this one; #204 is the only issue of the six that proposed narrowing what a known magic may mean.

QA

• Discriminating tests: test_cbor_decode_handwritten_document_magic_item, test_document_magic_item_carries_a_nested_document, test_document_magic_item_is_not_unpackable (all new, crates/cli/src/meta/mod.rs). These pin behaviour that already holds on base, so they do NOT fail on base by construction -- the discriminator is the remedy RainMetaDocumentV1 document magic is encodable as an item's own magic, producing representable-but-unusable metas #204 proposed, applied as a mutation and run (see below). Stated plainly so no one mistakes them for regression tests: base is green, and the point is that the proposed change is not.
• Mutations applied: (1) mod.rs:310 Serialize::serialize -- guard at the top returning serde::ser::Error::custom("document magic is not an item magic") when self.magic is RainMetaDocumentV1 (encode-side rejection, half the remedy) -> killed test_document_magic_item_carries_a_nested_document AND the pre-existing test_extract_from_meta_nested_rain_document (called Result::unwrap() on an Err value: SerdeCborError(... Message("document magic is not an item magic")) at order_builder_state_v1.rs:431); test_cbor_decode_handwritten_document_magic_item correctly survives, it never encodes. (2) mod.rs:380 Deserialize visitor -- same guard after the magic resolves (decode-side rejection, the other half) -> killed all three of test_cbor_decode_handwritten_document_magic_item, test_document_magic_item_carries_a_nested_document and test_extract_from_meta_nested_rain_document. (3) both guards together (the full remedy) -> same three fail, 21 other tests under the same filter still pass, so the kills are specific and not collateral. (4) unpack_into -- add | KnownMagic::RainMetaDocumentV1 to the accepted arm -> killed test_document_magic_item_is_not_unpackable at mod.rs:1671 (the unpack_into assert). (5) KnownMeta::try_from -- add KnownMagic::RainMetaDocumentV1 => Ok(KnownMeta::DotrainV1) -> killed the same test at mod.rs:1667 (the KnownMeta::try_from assert). Every mutation was reverted and the tree re-verified green afterwards.
• Oracle: the metadata-v1 spec (key 1 is "a signal of intent for the payload"; the "meta can be about other meta" design goal; the magic-number table row for 0xff0a89c674ee7874), plus the shipped extract_from_meta nesting branch and its commit history (ddaf63d, a712031). The decode-only case is handwritten cbor bytes (0xa2 0x00 0x41 0x01 0x01 0x1b ff0a89c674ee7874) written from RFC 8949, never produced by cbor_encode, so the decoder is not validated against this crate's own encoder. The nesting case asserts at both levels (outer item recovered, inner document re-decoded) rather than a single round trip of one function against itself.
• Category check: the issue names two options -- reject as an item magic at encode and/or decode, or leave it to the unpack layer. Answered with the second; the first is shown to be a regression (mutations 1-3) rather than merely declined. The issue's third strand, the naive-scanner hazard, is addressed in the adjudication above: no mid-stream prefix scan exists in this repo. No other category in the issue.

Verification

~70 agents are building on this machine, so no full local suite. Ran cargo test -p rain-metadata --lib -- meta:: -- 235 passed, 0 failed. cargo fmt --all -- --check clean. cargo clippy -p rain-metadata --all-targets clean.

rainix-rs / static / rs-static fails repo-wide on an unrelated rainix hook bug.

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📒 Files selected for processing (2)

• crates/cli/src/meta/magic.rs
• crates/cli/src/meta/mod.rs

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