fix(server-rs): derive info.json scaleFactors/sizes from one tile-grid pyramid

[subotic]

Fixes DEV-6981

Motivation

SIPI's IIIF info.json advertised scaleFactors as consecutive ordinals
([1,2,3,4]) instead of powers of two, and a sizes array that described a
different pyramid than scaleFactors. OpenLayers derives
maxZoom = round(log2(max(scaleFactors))) and keeps every resolution, so any
level beyond maxZoom gets no tile URL — assets black out at deep zoom (root
cause of DEV-6933). OpenSeadragon masks the defect by discarding our sizes and
rebuilding its own power-of-two pyramid, so OSD rendering correctly was never
evidence the descriptor was sound.

This is a descriptor-only fix. The server already serves every level (all
HTTP 200; SipiSize::REDUCE resamples on the fly), so correcting the advertised
arrays repairs the entire existing JP2 corpus with no re-encoding. It also
lands the pyramid contract the pyramidal-TIFF writer will later produce by
construction (project T1).

Summary

• scaleFactors are now contiguous powers of two [2^0..2^n] where the deepest
  factor puts the whole image inside a single tile on both axes.
• sizes and scaleFactors are derived from one function, so they describe the
  same pyramid (sizes.length == scaleFactors.length); both emitted ascending.
• Untiled images (tile_width == 0) derive sizes from a 512 reference tile and
  keep tiles omitted.

Key Changes

src/server-rs/src/info.rs

• New pyramid_scale_factors(w, h, tile_w, tile_h) — the single source of truth,
  n = max(levels_w, levels_h), u64 shift so a deep pyramid can't overflow.
• Replaced size_pyramid() with sizes_for(w, h, &scale_factors) — a pure
  transform of the factor list (native-inclusive, ascending), so the two arrays
  cannot drift apart.
• image_info_json computes the factor list once and feeds both sizes and the
  tiles block from it; the clevels read and the ordinal (1..cnt).collect()
  are gone (clevels was the source of the bug and is no longer consulted for the
  descriptor).
• tiles[].width/height still pass through the stored tile size verbatim (no
  regression of report finding 1).

Tests

• Updated the two defect-encoding fixtures to the corrected pyramid:
  inline info_json_matches_lena512_golden (now scaleFactors [1], sizes [{512,512}]) and the e2e golden snapshot
  iiif_compliance__info-json-lena512.snap. Reviewer: confirm these are the
  corrected pyramid, not a re-blessed bug.
• Updated untiled_image_omits_tiles for the 512-reference ladder.
• New descriptor_conformance test pins acceptance criteria 1–5 plus §5.4 tile
  uniqueness across a deep pyramid (3505×5156/1024 → [1,2,4,8]), a single-tile
  image (512×512/512 → [1]), and a non-square-tile case (4096×4096, 1024×256 →
  [1,2,4,8,16]).

Challenges and Decisions

Local Bazel gates could not run

Problem: the local Bazel graph failed to fetch Kakadu — gh release download kakadu-v8.7 from the private dasch-swiss/dsp-ci-assets repo hit a stable REST
404 on GitHub's release-by-tag endpoint, even though GraphQL (gh release list) saw the release and repo access was fine (a concurrent gh api user
returned 503, so GitHub's release service looked degraded). Every gate
(bazel-test-unit, bazel-test-e2e, bazel-rustfmt-check, bazel-clippy-check)
configures //src/server-rs:lib, which transitively depends on Kakadu, so all
four were unrunnable.
Solution: the change is pure Rust arithmetic. Verified the pyramid math
standalone with rustc against hand-computed expected values (lena512, the 0803
deep-pyramid worked example, the untiled ladder, and the non-square-tile case all
match). just commit-lint passed. CI runs the full gate set on this PR.

Coupling sizes to scaleFactors is convention, not spec

Problem: the IIIF spec (§5.9) deliberately ships two different ladders, so
coherence is not a MUST.
Solution: we couple them anyway because it is what makes OpenSeadragon use
our sizes (OSD keeps sizes only when sizes.length ∈ {maxLevel, maxLevel+1},
verified against OSD v5.0.1). Including the native size makes our count
n+1 = maxLevel+1, satisfying the check. §5.3 already requires the server to
serve any advertised size — verified HTTP 200 — so coupling adds no capability the
server lacks.

Gotchas

• The two updated fixtures encoded the defect. Do not "fix" the snapshot back
  to [1,2,3,4,5,6,7] — that is the bug. descriptor_conformance pins 2^i to
  prevent regression.
• clevels stays in the FFI struct (SipiImageDims, ABI-asserted in
  ffi.rs); this change only stops info.rs from consulting it. No FFI change.
• OpenLayers is the conformance canary, not OpenSeadragon — OSD masks the bug.
  Verify a previously-blacking-out asset against OL locally (just run + OL
  viewer) before relying on the exact orphaned-level count.

Test Plan

• Pyramid arithmetic verified standalone with rustc (all cases match the
  plan's worked examples)
• just commit-lint — green
• CI: bazel-test-unit, bazel-test-e2e, bazel-rustfmt-check,
  bazel-clippy-check (deferred to CI; blocked locally by the Kakadu fetch)
• Post-merge: OpenLayers reaches native zoom on an 0803-class asset with no
  orphaned levels (needs running SIPI + real asset)
• Post-merge: stage corpus sweep — no asset advertises non-power-of-two
  scaleFactors or a sizes/scaleFactors length mismatch (operator, read-only)

[linear-code]

DEV-6981

[github-actions]

Overview

Image reference	daschswiss/sipi:latest-arm64	daschswiss/sipi:latest
- digest	39e05acccf2b	75cb7501feb8
- tag	latest-arm64	latest
- environment	production
- provenance	7ee93b4	e8c6323
- vulnerabilities
- platform	linux/arm64	linux/amd64
- size	192 MB	201 MB (+8.3 MB)
- packages	63	63
Base Image	distroless/static-debian12:latest	distroless/static-debian12:latest
- vulnerabilities

Labels (1 changes)

• ± 1 changed
• 5 unchanged

 org.opencontainers.image.description=IIIF-compatible media server.
 org.opencontainers.image.licenses=AGPL-3.0-only
-org.opencontainers.image.revision=7ee93b4b22e93e60e49663a4b4c13e68bec49df0
+org.opencontainers.image.revision=e8c63231e7d6ceae7324d4ad4dca5a9db285c27e
 org.opencontainers.image.source=https://github.com/dasch-swiss/sipi
 org.opencontainers.image.title=Sipi
 org.opencontainers.image.version=v6.4.0

Policies (0 improved, 0 worsened, 2 missing data)

Policy Name	daschswiss/sipi:latest-arm64	daschswiss/sipi:latest	Change	Standing
Default non-root user	❓ No data	⚠️
No AGPL v3 licenses	❓ No data	✅
No fixable critical or high vulnerabilities	❓ No data	✅
No high-profile vulnerabilities	❓ No data	✅
No outdated base images	❓ No data	❓ No data
No unapproved base images	❓ No data	❓ No data
Supply chain attestations	❓ No data	⚠️ 2