fix(js): use shared runtime and concurrency limit for tool callbacks (#1047)

## Summary

- Replace unbounded `std::thread::spawn` + per-call `Runtime::new()`
with a shared lazily-initialized tokio runtime (2 worker threads)
- Add semaphore with 10 permits to cap concurrent in-flight tool
callbacks
- Prevents OS thread exhaustion from scripts invoking tools in loops

## Test plan

- [ ] Tool callbacks still execute correctly
- [ ] Concurrent tool invocations bounded by semaphore
- [ ] `cargo check -p bashkit-js` passes
- [ ] No thread exhaustion under heavy tool callback load

Closes #982

Author: chaliy

crates/bashkit-js/src/lib.rs

@@ -27,6 +27,33 @@ use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, Ordering};
 use tokio::sync::Mutex;
 
+// ---------------------------------------------------------------------------
+// Shared tokio runtime + concurrency limiter for JS tool callbacks (issue #982).
+// A single multi-thread runtime is created lazily and reused for every callback
+// invocation, replacing the previous pattern of spawning an unbounded number of
+// OS threads each with its own single-threaded runtime. A semaphore caps the
+// maximum number of concurrent in-flight callbacks to prevent DoS.
+// ---------------------------------------------------------------------------
+const MAX_CONCURRENT_TOOL_CALLBACKS: usize = 10;
+
+fn callback_runtime() -> &'static tokio::runtime::Runtime {
+    use std::sync::OnceLock;
+    static RT: OnceLock<tokio::runtime::Runtime> = OnceLock::new();
+    RT.get_or_init(|| {
+        tokio::runtime::Builder::new_multi_thread()
+            .worker_threads(2)
+            .enable_all()
+            .build()
+            .expect("failed to create shared callback runtime")
+    })
+}
+
+fn callback_semaphore() -> &'static tokio::sync::Semaphore {
+    use std::sync::OnceLock;
+    static SEM: OnceLock<tokio::sync::Semaphore> = OnceLock::new();
+    SEM.get_or_init(|| tokio::sync::Semaphore::new(MAX_CONCURRENT_TOOL_CALLBACKS))
+}
+
 // ============================================================================
 // MontyObject <-> JSON conversion
 // ============================================================================
@@ -1064,20 +1091,21 @@ impl ScriptedTool {
                 });
                 let request_str = serde_json::to_string(&request).map_err(|e| e.to_string())?;
 
-                // Use a dedicated thread so the TSFN can dispatch to the JS event loop.
-                // The main thread must NOT be blocked (use async `execute`, not `executeSync`).
+                // Dispatch the TSFN call on the shared callback runtime with a
+                // concurrency semaphore to prevent unbounded thread/task creation
+                // (see issue #982).
                 let tsfn_clone = tsfn.clone();
                 let tool_name_clone = tool_name.clone();
+                let rt = callback_runtime();
+                let sem = callback_semaphore();
                 let (tx, rx) = std::sync::mpsc::channel();
-                std::thread::spawn(move || {
-                    let rt = tokio::runtime::Builder::new_current_thread()
-                        .enable_all()
-                        .build();
-                    let result = match rt {
-                        Ok(rt) => rt
-                            .block_on(tsfn_clone.call_async((request_str,)))
+                rt.spawn(async move {
+                    let result = match sem.acquire().await {
+                        Ok(_permit) => tsfn_clone
+                            .call_async((request_str,))
+                            .await
                             .map_err(|e| format!("{}: {}", tool_name_clone, e)),
-                        Err(e) => Err(format!("{}: runtime error: {}", tool_name_clone, e)),
+                        Err(e) => Err(format!("{}: semaphore error: {}", tool_name_clone, e)),
                     };
                     let _ = tx.send(result);
                 });