HNSW loses vector dimension state after a rejected mixed-dimension batch

How to reproduce this bug?

import requests

BASE = "http://localhost:8080"

requests.post(
    f"{BASE}/v1/schema",
    json={
        "class": "DimensionRetry",
        "vectorizer": "none",
        "vectorIndexType": "hnsw",
        "properties": [{"name": "text", "dataType": ["text"]}],
    },
).raise_for_status()

def batch(vectors):
    return requests.post(
        f"{BASE}/v1/batch/objects",
        json={"objects": [
            {
                "class": "DimensionRetry",
                "properties": {"text": str(i)},
                "vector": vector,
            }
            for i, vector in enumerate(vectors)
        ]},
    )

print("mixed batch:", batch([[1, 0, 0, 0], [0, 1, 0]]).text)
print("valid retry:", batch([[1, 0, 0, 0], [0, 1, 0, 0]]).text)

r = requests.post(
    f"{BASE}/v1/objects",
    json={
        "class": "DimensionRetry",
        "properties": {"text": "wrong dimension"},
        "vector": [1, 2, 3],
    },
)
print("wrong dimension:", r.status_code, r.text)

What is the expected behavior?

The successful four-dimensional retry should initialize the HNSW dimension to 4. Any later vector with a different dimension should be rejected.

What is the actual behavior?

After the rejected mixed-dimension batch, the valid four-dimensional retry succeeds, but the HNSW dimension remains unset or zero. Consequently, a three-dimensional vector may bypass dimension validation.

Supporting information

The issue is caused by the sync.Once dimension initializer being consumed when the first mixed-dimension batch returns an error. The callback returns before storing h.dims, so later calls cannot initialize the dimension.

Relevant files:

• adapters/repos/db/vector/hnsw/insert.go
• adapters/repos/db/vector/hnsw/index.go

This is different from a normal wrong-dimension insertion error because the trigger is a rejected first batch followed by a valid retry.

Server Version

1.40.0-dev

Weaviate Setup

Single Node

Nodes count

No response

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