fix(session): type busy errors (#27410)

packages/opencode/src/effect/runner.ts

@@ -4,11 +4,12 @@ export interface Runner<A, E = never> {
   readonly state: State<A, E>
   readonly busy: boolean
   readonly ensureRunning: (work: Effect.Effect<A, E>) => Effect.Effect<A, E>
-  readonly startShell: (work: Effect.Effect<A, E>, ready?: Latch.Latch) => Effect.Effect<A, E>
+  readonly startShell: (work: Effect.Effect<A, E>, ready?: Latch.Latch) => Effect.Effect<A, E | Busy>
   readonly cancel: Effect.Effect<void>
 }
 
 export class Cancelled extends Schema.TaggedErrorClass<Cancelled>()("RunnerCancelled", {}) {}
+export class Busy extends Schema.TaggedErrorClass<Busy>()("RunnerBusy", {}) {}
 
 interface RunHandle<A, E> {
   id: number
@@ -41,12 +42,11 @@ export const make = <A, E = never>(
     onIdle?: Effect.Effect<void>
     onBusy?: Effect.Effect<void>
     onInterrupt?: Effect.Effect<A, E>
-    busy?: () => never
   },
 ): Runner<A, E> => {
   const ref = SynchronizedRef.makeUnsafe<State<A, E>>({ _tag: "Idle" })
   const idle = opts?.onIdle ?? Effect.void
-  const busy = opts?.onBusy ?? Effect.void
+  const onBusy = opts?.onBusy ?? Effect.void
   const onInterrupt = opts?.onInterrupt
   let ids = 0
 
@@ -137,20 +137,15 @@ export const make = <A, E = never>(
       }),
     ).pipe(Effect.flatten)
 
-  const startShell = (work: Effect.Effect<A, E>, ready?: Latch.Latch) =>
+  const startShell = (work: Effect.Effect<A, E>, ready?: Latch.Latch): Effect.Effect<A, E | Busy> =>
     SynchronizedRef.modifyEffect(
       ref,
       Effect.fnUntraced(function* (st) {
         if (st._tag !== "Idle") {
-          return [
-            Effect.sync(() => {
-              if (opts?.busy) opts.busy()
-              throw new Error("Runner is busy")
-            }),
-            st,
-          ] as const
+          const reject: Effect.Effect<A, E | Busy> = Effect.fail(new Busy())
+          return [reject, st] as const
         }
-        yield* busy
+        yield* onBusy
         const id = next()
         const cancelled = yield* Deferred.make<void>()
         const fiber = yield* work.pipe(Effect.ensuring(finishShell(id)), Effect.forkChild)