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glm-5.3-flash e76f91f6d7 docs(architecture): resolve OQ-04 — receive-pack state machine (ADR-013)

- ADR-013: V0-framed push machine grounded in real git 2.43.0 captures
  (file://, git://, smart-http mock, raw stdio into real receive-pack):
  V0-shaped ref advertisement (caps on first ref line, capabilities^{}
  sentinel only for empty repos), served capability set, shallow requests
  rejected for v1, thin packs accepted with server-odb bases (no
  capability involved; push.thin default), ingestion bound to
  Bundle::write_to_directory_eagerly + gix-fsck + one gix-ref transaction
  per push (.keep-guarded), unpack-first CAS timing with observed
  upstream order, band-1 pkt-line-framed status report, http framing
  (probe/Content-Length/chunked), v1 update policy (CAS only; deletes
  and force-push allowed)
- docs/research/push-captures.md: the normative push wire record
- transport.md/backend.md/doors.md: receive-pack sections rewritten to
  the decided shapes; backend.md ingestion composition bound; stale
  OQ-04 references resolved
- ADR-003 amended: V2-only governs fetch; push is V0-framed by upstream
  design (fixes the V2-only contradiction found in review)
- ADR-009 amended: haves default reconciled with the client's stateless
  ceiling (16384); blocking-pipeline budget covers generation+ingestion
- OQ-04 resolved; tracker task closed; CAS-fail-fast optimization
  tracked (tasks/architecture/oq-13-cas-failfast.md)
- research index: poc findings + capture docs listed

Verification: cargo test / clippy -D warnings / fmt --check / doc pass

2026-09-25 04:05:07 +00:00

3.8 KiB

Raw Blame History

ADR-004: Pack pipeline — gitoxide `data::output` generation, `data::input` streaming ingestion

Status

Accepted

Context

Pack generation and ingestion are the two halves of fetch and receive-pack. POC-2 resolved the generation question empirically; the ingestion tool turned out to be mislabeled in the original research plan (the plan called it bundle::write — the correct tool is data::input streaming) and was corrected there.

Generation options were:

a. gix-pack::bundle::write into a temp dir, read back — wrong tool: it is the index-from-stream machinery (consumes an existing pack stream, mmaps to resolve deltas, always writes files). Not a generation path. b. gix-pack::data::output::bytes fed by an odb walk — validated live: streams to any io::Write with O(counts) memory (60k-object pack → 5.3 MB out, ~11 MB extra RSS), real git 2.43 clones fsck-clean.

Ingestion (receive-pack): client sends a possibly-thin pack stream; server must index it, fsck it, and apply ref updates via CAS.

Decision

Fetch-side generation is gitoxide's own pipeline, composed exactly as gitoxide-core/src/pack/create.rs does (POC-2's validated composition):

peel wants to commit tips
  → commit-ancestry walk (gix_traverse::commit::Simple, Parents::All)
      [+ non-commit tips as-is]
  → count::objects(_unthreaded, TreeContents)   [per-commit tree expansion]
  → entry::iter_from_counts                      [chunked deflate + pack-delta copy]
  → InOrderIter → bytes::FromEntriesIter (V2, sha1)
  → io::Write sink (sideband on the wire, or http response body)

Two-stage closure is mandatory: TreeContents does not follow commit parents; feeding tip commits alone produces incomplete packs (POC-2's course correction). The odb handle needs prevent_pack_unload() + ignore_replacements = true. gix_odb::Cache is not Sync: share Arc<Store>, build a handle per session, generate on blocking threads.

Missing objects mid-generation must abort the fetch (sideband error band), never emit a broken pack — check entry statistics (missing_objects > 0).

Receive-side ingestion parses the client pack stream (gix-pack::data::input with streaming-input), fscks it (gix-fsck connectivity), and applies ref updates via gix-ref transaction CAS. The push state machine (capability advertisement set, CAS timing, status report, http framing) is decided in ADR-013; the ingestion-tool choice above is decided.

Delta synthesis for loose objects is an optimization backlog item, not a v1 commitment: existing pack deltas copy through for free; loose objects ship as compressed bases. No upstream delta-encode API exists (gix-delta is apply-only).

Consequences

(2026-09-21 amendment, ADR-010: the crate layout changed — the type below is now the GitPackGen backend trait in the single crate, gix-free by signature (repo, wants, haves, limits); backend.md is authoritative for the trait shape. The pipeline itself is unchanged.)

Fresh clones of loose-ish repos ship uncompressed bases (fine for v1; clients re-pack at rest); repos kept packed get pack-copy efficiency.
Memory stays O(counts); streaming under back pressure is proven on the http path (POC-3: flat RSS under a 650 KB/s reader).
Determinism: objects_unthreaded gives deterministic order; threaded count + InOrderIter is the scale path later.
The generation seam is negotiation-agnostic: boundary sets in → pack out (POC-2 follow-up 1).

References

docs/research/poc2-findings.md (the whole basis), docs/research/poc3-findings.md (streaming proof)
docs/research/gitoxide.md §"Storage" (generation-pipeline notes)
docs/research/git-protocol.md §"Server-side pack generation"
ADR-005 (how the sink reaches the wire), ADR-009 (memory/time budgets)
transport.md §fetch, backend.md §"The trait family" (GitPackGen)

3.8 KiB Raw Blame History

ADR-004: Pack pipeline — gitoxide data::output generation, data::input streaming ingestion