fix(wren-core-wasm): drive query() through a tokio runtime (UNION ALL trap)

[goldmedal]

Summary

@wrenai/wren-core-wasm@0.4.0 (and 0.3.0 before it) traps with
RuntimeError: unreachable on any SQL whose physical plan contains
CoalescePartitionsExec over more than one partition — UNION ALL,
UNION, INTERSECT, EXCEPT, and some parallel hash joins. The
trivial form (no MDL, no parquet, no tables) reproduces:

await engine.query(\`SELECT a FROM (SELECT 1 AS a UNION ALL SELECT 2) t\`);
// → RuntimeError: unreachable

Reported in BUG_REPORT_wren_core_wasm_union_all.md.

Root cause

CoalescePartitionsExec::execute (DataFusion 53) spawns one task per input
partition via JoinSet::spawn → tokio::task::spawn. That spawn panics
with

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

because `wasm-bindgen-futures` drives Rust futures directly off JS
microtasks and never enters a tokio runtime. Single-partition queries hit
the `1 =>` short-circuit in `CoalescePartitionsExec` and never spawn,
which is why every `registerJson` + raw `query` example, every cube
query, and every dashboard query worked — and the bug stayed hidden until
the first `UNION ALL` in the wild.

The bug is wasm32-only: the same SQL + same DataFusion 53.0.0 + same
crate feature set succeeds when compiled natively (verified). It's not a
0.4.0 regression — 0.3.0 traps identically.

Fix

• `WrenEngine` now owns a `current_thread` `tokio::runtime::Runtime`.
• `query()` wraps its body in `runtime.block_on(async { … })`; `cube_query()` delegates to `query()` so it inherits the fix.
• `WrenEngine::new` installs `console_error_panic_hook::set_once()`. Future panics surface in the JS console with a Rust message instead of the opaque `RuntimeError: unreachable` that hid this bug for two releases.

The public JS surface is unchanged — both methods are still `async` and
still return a Promise. The block_on drives the inner future synchronously
on the JS thread (which is what wasm-single-thread already does anyway).

Coverage

• 6 Node SDK tests under `describe("set operators", …)`: `UNION ALL` subquery + top-level, `UNION` dedup, `INTERSECT`, `EXCEPT`, and `UNION ALL` across two registered tables (sdk/tests/index.test.mjs — 34 total tests now pass).
• 1 Rust `wasm_bindgen_test` (`test_union_all_does_not_trap`).
• 1 native `wren-core` test (`test_union_all_local_runtime`) confirms the wren-core LocalRuntime path itself is unaffected — the bug really is wasm32-only.

Verification

$ node verify-union.mjs   # bug-report minimal repro + sibling set ops
✓ UNION ALL in subquery (bug repro): 2 row(s)
✓ top-level UNION ALL: 2 row(s)
✓ UNION (dedup): 2 row(s)
✓ INTERSECT: 1 row(s)
✓ EXCEPT: 1 row(s)
✓ simple aggregate (regression): 1 row(s)
6 passed, 0 failed

$ just test    # full SDK suite
ℹ pass 34  ℹ fail 0

Release

This should ship as a `@wrenai/wren-core-wasm` patch (`0.4.1`). The fix
is purely additive on the Rust side (new field, new dep `console_error_panic_hook`) — the JS API is unchanged.

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🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Fixed set operators (UNION ALL, UNION, INTERSECT, EXCEPT) that were causing failures in WASM environments
  • Improved error visibility with enhanced console error reporting

• Tests

  • Added comprehensive test coverage for set operators and regression tests to prevent future issues

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: c4df2311-4c58-4385-b3c1-9b046dca0243

📥 Commits

Reviewing files that changed from the base of the PR and between 6ebed77 and 3247f1b.

⛔ Files ignored due to path filters (1)

• core/wren-core-wasm/Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (4)

• core/wren-core-wasm/Cargo.toml
• core/wren-core-wasm/sdk/tests/index.test.mjs
• core/wren-core-wasm/src/lib.rs
• core/wren-core/core/src/mdl/mod.rs

---

Walkthrough

This PR fixes WASM runtime traps when executing set-operator queries by introducing an owned tokio current-thread runtime to WrenEngine. The constructor now installs a panic hook and builds the runtime; query() executes within block_on() to provide a live reactor for DataFusion's internal task spawning. Regression tests validate the fix across WASM and core platforms.

Changes

Set Operator Runtime Fix

Layer / File(s)	Summary
WASM Runtime Field and Panic Hook Setup core/wren-core-wasm/Cargo.toml, core/wren-core-wasm/src/lib.rs	WrenEngine now owns a tokio::runtime::Runtime field. Constructor installs console_error_panic_hook for JS console panic visibility and builds a current-thread runtime via tokio::runtime::Builder. The console_error_panic_hook dependency is added to Cargo.toml.
Query Execution Within Runtime core/wren-core-wasm/src/lib.rs	WrenEngine::query() wraps all async DataFusion operations and JSON serialization inside self.runtime.block_on(...) to ensure spawned tasks execute within the engine's tokio scheduler. cube_query() explicitly documents that it delegates to query() for runtime entry.
Regression Test Coverage core/wren-core-wasm/src/lib.rs, core/wren-core/core/src/mdl/mod.rs, core/wren-core-wasm/sdk/tests/index.test.mjs	WASM regression test validates UNION ALL does not trap. Core MDL regression test reproduces the trap scenario under Mode::LocalRuntime. SDK test suite covers UNION ALL (subquery and top-level), UNION deduplication, INTERSECT, and EXCEPT across registered tables.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• Canner/WrenAI#2278: Both PRs modify core/wren-core-wasm/src/lib.rs's WrenEngine and its execution path; #2278 adds cubeQuery()/listCubes() delegating into query(), while this PR changes query() runtime wiring to prevent WASM runtime traps, directly affecting those cube APIs.

Suggested labels

rust, core, wasm

---

🐰 A tokio thread joins the dance,
Set operators get their chance,
No more traps in WASM land,
Block_on's magic, isn't grand?
The compiler nods, "It's planned!" ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically describes the main fix: addressing a UNION ALL trap by driving query() through a tokio runtime in wren-core-wasm.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
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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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/wasm-union-all-v2

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