fix: avoid panic while hitting non-null empty multi-vector

[Xuanwo]

This PR will avoid panic while hitting non-null empty multi-vector

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Parts of this PR were drafted with assistance from Codex (with gpt-5.2) and fully reviewed and edited by me. I take full responsibility for all changes.

[github-actions]

Code Review

P0: Bug in flat.rs - Panic replaced with silently propagated panic

The change in flat.rs from .await.unwrap() to .await? addresses the wrong layer of the problem:

// Before
.await.unwrap()

// After
.await?

The .unwrap() here is for the JoinHandle result from spawn_blocking, which panics if the spawned task panicked. Replacing it with ? changes the behavior: now if the spawned task panics, the panic is converted to a JoinError and propagated as an error rather than panicking the current task. This is actually a behavior change that may hide panics.

Recommendation: If the intent is to properly handle task panics, this change is fine but should be documented. However, the original issue ("panic while hitting non-null empty multi-vector") should be fixed by the error handling in multivec_distance, not by changing how join errors are handled.

P1: Error message could include more context

The error in multivec_distance is clear but consider including which field/column this is from if available, to help users identify which data is malformed in large datasets.

Test Coverage

The test adequately covers the new error path. 👍

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Overall the core fix in multivec_distance converting a panic into a proper error for empty multi-vectors is correct and well-tested.

[BubbleCal]

Where does it panic before this change?

[BubbleCal]

can we just assume the distance to a empty multivector is NaN and then exclude it from the results?

[Xuanwo]

Let me know if 0a61738 is what you want

[Xuanwo]

Where does it panic before this change?

Our tokio runtime worker panic at

lance/rust/lance-index/src/vector/flat.rs

Line 147 in 543eb86

.unwrap()

Resulted by

lance/rust/lance-linalg/src/distance.rs

Lines 142 to 149 in 543eb86

	multivector
	.values()
	.as_primitive::<UInt8Type>()
	.values()
	.chunks_exact(dim)
	.map(|v| hamming::hamming(q, v))
	.min_by(|a, b| a.partial_cmp(b).unwrap())
	.unwrap()

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/io/exec/knn.rs	75.00%	0 Missing and 2 partials ⚠️
rust/lance-linalg/src/distance.rs	96.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!