Bazel sharding sequencer can produce empty shards even when test_file_count >= shard_count

[dmiller-figma]

Summary

rules_vitest's Bazel test sequencer currently computes shard ranges using:

const shardSize = Math.ceil(files.length / shardCount);
const shardStart = shardSize * shardIndex;
const shardEnd = shardSize * (shardIndex + 1);

This can produce an empty trailing shard even when there are at least as many test files as Bazel shards.

For example, with 10 test files and 6 shards:

shardSize = ceil(10 / 6) = 2

shard 0 => files[0:2]
shard 1 => files[2:4]
shard 2 => files[4:6]
shard 3 => files[6:8]
shard 4 => files[8:10]
shard 5 => files[10:12]  // empty

That last shard exits with Vitest's "No test files found" failure.

Expected Behavior

When files.length >= shardCount, every shard should receive at least one test file.

Actual Behavior

The final shard gets zero files and the Bazel test fails even though there are more test files than shards.

Repro

git clone https://github.com/fremtind/rules_vitest.git
cd rules_vitest
git checkout v0.2.3

Replace example/sharding/BUILD.bazel with:

load("@fremtind_rules_vitest//vitest:defs.bzl", "vitest_test")

vitest_test(
    name = "test",
    config = "vitest.config.js",
    data = [
        "index.js",
        "index_01.test.js",
        "index_02.test.js",
        "index_03.test.js",
        "index_04.test.js",
        "index_05.test.js",
        "index_06.test.js",
        "index_07.test.js",
        "index_08.test.js",
        "index_09.test.js",
        "index_10.test.js",
    ],
    node_modules = "//:node_modules",
    shard_count = 6,
)

Generate 10 trivial test files:

for i in $(seq -w 1 10); do
  cat > "example/sharding/index_${i}.test.js" <<'EOF'
const { test, expect } = await import("vitest");

test("runs", () => {
  expect(1).toBe(1);
});
EOF
done

Run:

bazel test //example/sharding:test

Result

One of the shards receives no test files and fails with output like:

No test files found, exiting with code 1

The target fails even though there are 10 test files and only 6 shards.

Environment

Observed with:

rules_vitest v0.2.3
Vitest 4.x
Bazel test sharding enabled via shard_count