`UNION ALL` panics on `wasm32-unknown-unknown`

[kentkwu]

Describe the bug

DataFusion compiled to wasm32-unknown-unknown panics on UNION ALL and any other multi-partition query whose collect funnels through CoalescePartitionsExec (aggregations over partitioned scans, sorts across partitions, etc.).

CoalescePartitionsExec::execute spawns one task per input partition via JoinSet::spawn on the _ => (multi-input) branch. JoinSet::spawn needs a tokio reactor that isn't installed under wasm-bindgen-futures. The panic:

there is no reactor running, must be called from the context of a Tokio 1.x runtime

The single-input branch of CoalescePartitionsExec::execute works — it passes the child stream through directly. Only the multi-input branch spawns.

Follow-up to #24275, which fixed the same class of panic at a different call site (collect_partitioned).

To Reproduce

Add this to datafusion/wasmtest/src/lib.rs:

#[wasm_bindgen_test(unsupported = tokio::test)]
async fn test_union_all() {
    let ctx = get_ctx();
    ctx.sql("SELECT 1 AS n UNION ALL SELECT 2 AS n")
        .await.unwrap()
        .collect().await.unwrap();
}

Run from the repo root:

RUSTFLAGS='--cfg getrandom_backend="wasm_js"' \
wasm-pack test --headless --chrome datafusion/wasmtest

The test panics:

panicked at datafusion/common-runtime/src/join_set.rs:69:20:
there is no reactor running, must be called from the context of a Tokio 1.x runtime

Expected behavior

Multi-partition collect should succeed under wasm-bindgen-futures when the session has asked for single-threaded execution (target_partitions == 1). Sequential drain of the input partitions is a valid interleaving — CoalescePartitionsExec's own docstring states "No guarantees are made about the order of the resulting partition."

Additional context

Related: #24274, #24275, #13815

[yinli-systems]

take

[kentkwu]

@yinli-systems sorry I didn't see your comment before I put up the pr

[yinli-systems]

untake

[kentkwu]

take