feat(client): pin the weight download to the snapshot's commit

[scydas]

Summary

Part of #569 — first of two client changes for pinned revisions (this one needs no user-facing configuration).

The weight phase (install_weight_files) asked the server for whatever revision it resolves by default. Two problems follow:

• A moved default fails the phase. When the server's default revision has advanced past the snapshot the engine resolved — a pod restart after the server's cache was rebuilt is enough — the server downloads the newer revision, the client's mismatch check trips, and the phase fails even though the server could have fetched the snapshot's own commit. PR feat(client): load models from the server cache when workers have no shared storage #592's hardware validation hit exactly this (DGD snapshot at a338b55, server holding 7ae5576).
• The warm path leaves the mismatch check unarmed. A server that already holds an unpinned model reports no revision, so the check in front of the has_files shortcut compares nothing.

The fix: pin the download request to snapshot_path.name — the directory name is the commit. The server resolves a pinned revision before claiming its download lease (#598), so the claim is scoped to that exact commit and the reply names it on warm and cold paths alike.

One deliberate degradation path: resolving a pin reaches the Hub, and the server's cache fallback during a Hub outage exists only for unpinned requests (resolve_target_revision returns NOT_FOUND for pinned ones). A pinned request would therefore turn an outage the current behavior survives into a failure. On grpc.RpcError the client logs and retries unpinned — exactly the previous behavior, with the mismatch check and the stream's first-chunk commit validation still in force.

Behavior matrix:

Scenario	Before	After
Server default moved past the snapshot	wrong revision downloaded → mismatch → phase fails	server fetches the snapshot's commit → works
Warm server (#598)	reports nothing → mismatch check unarmed	reports the pinned commit → check always armed
Pre-revision server	—	ignores the field; byte-identical to before
Hub outage, warm cache	unpinned request falls back to cache → works	pinned request errors → degrades to unpinned → works

Validation

• pytest tests/ (non-GPU): 1215 passed, 29 skipped.
• New tests: the download request carries the snapshot's commit; a failing pinned request falls back to the unpinned one and completes; the existing "server reports a different commit → refuse" case is unchanged.

Summary by CodeRabbit

• Bug Fixes

  • Improved model downloads to use the snapshot’s specific revision when retrieving weights.
  • Added a fallback to unpinned downloads when the requested revision cannot be resolved.
  • Prevented weights from being installed when their revision does not match the local snapshot.

• Documentation

  • Clarified revision-aware model caching, deployment behavior, mismatch handling, and fallback scenarios.

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[coderabbitai]

Walkthrough

The client now supports revision-pinned downloads. Weight installation uses the snapshot commit, retries without a revision after gRPC failure, validates commits, and documents the behavior with tests.

Changes

Revision-aware model downloads

Layer / File(s)	Summary
Download revision contract modelexpress_client/python/modelexpress/model_client.py	ensure_downloaded accepts an optional revision and sends it with download requests.
Weight installation fallback and validation modelexpress_client/python/modelexpress/model_client.py, modelexpress_client/python/tests/test_model_client.py, docs/ARCHITECTURE.md, docs/DEPLOYMENT.md	Weight installation pins requests to the snapshot commit, retries unpinned requests after grpc.RpcError, validates commits, and documents the revision flow. Tests cover pinning and fallback.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🔵 Low · up to 91e05

The client now pins weight downloads to the snapshot commit and falls back to an unpinned download only for transport errors; the accompanying documentation currently describes that fallback too broadly, so the PR is mergeable with a small documentation correction to avoid misleading operators.

Poem

I’m a rabbit with a snapshot key,
Pinning weights carefully.
If the server says “not today,”
I hop back without the pin away.
Commit checks guard the final byte!
Revision paths now work just right.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 71.43% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: pinning the client weight download to the snapshot commit.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@docs/ARCHITECTURE.md`:
- Line 809: Document the precise fallback behavior: the pinned
EnsureModelDownloaded request retries unpinned only when it raises
grpc.RpcError; server status errors converted to ModelCacheError must not retry.
Update docs/ARCHITECTURE.md line 809 by replacing the pin-resolution wording,
and apply the same clarification to docs/DEPLOYMENT.md line 851 by replacing the
wording that says any pinned request failure triggers fallback.

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Fix all unresolved CodeRabbit comments on this PR:

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between d7319a0 and 91e05cb.

📒 Files selected for processing (4)

• docs/ARCHITECTURE.md
• docs/DEPLOYMENT.md
• modelexpress_client/python/modelexpress/model_client.py
• modelexpress_client/python/tests/test_model_client.py

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[zhengluo-nv]

/ok to test 9809032

[zhengluo-nv]

@scydas Merged. Thank you!