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fix: Isolate thread reactor workers to prevent head-of-line blocking #1945

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fix: Isolate thread reactor workers to prevent head-of-line blocking #1945

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145 changes: 125 additions & 20 deletions apps/server/src/orchestration/Layers/ProviderCommandReactor.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -66,6 +66,16 @@ async function waitFor(
}

describe("ProviderCommandReactor", () => {
type HarnessOptions = {
readonly baseDir?: string;
readonly threadModelSelection?: ModelSelection;
readonly sessionModelSwitch?: "unsupported" | "in-session";
readonly startSessionImplementation?: (
input: unknown,
session: ProviderSession,
) => Effect.Effect<ProviderSession>;
};

let runtime: ManagedRuntime.ManagedRuntime<
OrchestrationEngineService | ProviderCommandReactor,
unknown
Expand Down Expand Up @@ -93,11 +103,7 @@ describe("ProviderCommandReactor", () => {
createdBaseDirs.clear();
});

async function createHarness(input?: {
readonly baseDir?: string;
readonly threadModelSelection?: ModelSelection;
readonly sessionModelSwitch?: "unsupported" | "in-session";
}) {
async function createHarness(input?: HarnessOptions) {
const now = new Date().toISOString();
const baseDir = input?.baseDir ?? fs.mkdtempSync(path.join(os.tmpdir(), "t3code-reactor-"));
createdBaseDirs.add(baseDir);
Expand All @@ -110,37 +116,46 @@ describe("ProviderCommandReactor", () => {
provider: "codex",
model: "gpt-5-codex",
};
const startSession = vi.fn((_: unknown, input: unknown) => {
const startSessionImplementation: NonNullable<HarnessOptions["startSessionImplementation"]> =
input?.startSessionImplementation ??
((_: unknown, session: ProviderSession) => Effect.succeed(session));
const startSession = vi.fn((_: unknown, startInput: unknown) => {
const sessionIndex = nextSessionIndex++;
const resumeCursor =
typeof input === "object" && input !== null && "resumeCursor" in input
? input.resumeCursor
typeof startInput === "object" && startInput !== null && "resumeCursor" in startInput
? startInput.resumeCursor
: undefined;
const threadId =
typeof input === "object" &&
input !== null &&
"threadId" in input &&
typeof input.threadId === "string"
? ThreadId.make(input.threadId)
typeof startInput === "object" &&
startInput !== null &&
"threadId" in startInput &&
typeof startInput.threadId === "string"
? ThreadId.make(startInput.threadId)
: ThreadId.make(`thread-${sessionIndex}`);
const session: ProviderSession = {
provider: modelSelection.provider,
status: "ready" as const,
runtimeMode:
typeof input === "object" &&
input !== null &&
"runtimeMode" in input &&
(input.runtimeMode === "approval-required" || input.runtimeMode === "full-access")
? input.runtimeMode
typeof startInput === "object" &&
startInput !== null &&
"runtimeMode" in startInput &&
(startInput.runtimeMode === "approval-required" ||
startInput.runtimeMode === "full-access")
? startInput.runtimeMode
: "full-access",
...(modelSelection.model !== undefined ? { model: modelSelection.model } : {}),
threadId,
resumeCursor: resumeCursor ?? { opaque: `resume-${sessionIndex}` },
createdAt: now,
updatedAt: now,
};
runtimeSessions.push(session);
return Effect.succeed(session);
return startSessionImplementation(startInput, session).pipe(
Effect.tap((resolvedSession) =>
Effect.sync(() => {
runtimeSessions.push(resolvedSession);
}),
),
);
});
const sendTurn = vi.fn((_: unknown) =>
Effect.succeed({
Expand Down Expand Up @@ -325,6 +340,96 @@ describe("ProviderCommandReactor", () => {
expect(thread?.session?.runtimeMode).toBe("approval-required");
});

it("does not let one hung thread start block another thread", async () => {
const harness = await createHarness({
startSessionImplementation: (input, session) => {
const threadId =
typeof input === "object" &&
input !== null &&
"threadId" in input &&
typeof input.threadId === "string"
? input.threadId
: null;
return threadId === "thread-1" ? Effect.never : Effect.succeed(session);
},
});
const now = new Date().toISOString();

await Effect.runPromise(
harness.engine.dispatch({
type: "thread.create",
commandId: CommandId.make("cmd-thread-create-2"),
threadId: ThreadId.make("thread-2"),
projectId: asProjectId("project-1"),
title: "Thread 2",
modelSelection: {
provider: "codex",
model: "gpt-5-codex",
},
interactionMode: DEFAULT_PROVIDER_INTERACTION_MODE,
runtimeMode: "approval-required",
branch: null,
worktreePath: null,
createdAt: now,
}),
);

await Effect.runPromise(
harness.engine.dispatch({
type: "thread.turn.start",
commandId: CommandId.make("cmd-turn-start-hung-thread-1"),
threadId: ThreadId.make("thread-1"),
message: {
messageId: asMessageId("user-message-hung-thread-1"),
role: "user",
text: "thread one hangs on start",
attachments: [],
},
interactionMode: DEFAULT_PROVIDER_INTERACTION_MODE,
runtimeMode: "approval-required",
createdAt: now,
}),
);

await Effect.runPromise(
harness.engine.dispatch({
type: "thread.turn.start",
commandId: CommandId.make("cmd-turn-start-hung-thread-2"),
threadId: ThreadId.make("thread-2"),
message: {
messageId: asMessageId("user-message-hung-thread-2"),
role: "user",
text: "thread two should still run",
attachments: [],
},
interactionMode: DEFAULT_PROVIDER_INTERACTION_MODE,
runtimeMode: "approval-required",
createdAt: now,
}),
);

await waitFor(() => harness.startSession.mock.calls.length >= 2);
await waitFor(() =>
harness.sendTurn.mock.calls.some(
([payload]) =>
typeof payload === "object" &&
payload !== null &&
"threadId" in payload &&
payload.threadId === ThreadId.make("thread-2"),
),
);

expect(
harness.sendTurn.mock.calls.some(
([payload]) =>
typeof payload === "object" &&
payload !== null &&
"threadId" in payload &&
payload.threadId === ThreadId.make("thread-1"),
),
).toBe(false);
});

it("generates a thread title on the first turn", async () => {
const harness = await createHarness();
const now = new Date().toISOString();
Expand Down
25 changes: 22 additions & 3 deletions apps/server/src/orchestration/Layers/ProviderCommandReactor.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ import {
type TurnId,
} from "@t3tools/contracts";
import { Cache, Cause, Duration, Effect, Equal, Layer, Option, Schema, Stream } from "effect";
import { makeDrainableWorker } from "@t3tools/shared/DrainableWorker";
import { type DrainableWorker, makeDrainableWorker } from "@t3tools/shared/DrainableWorker";

import { resolveThreadWorkspaceCwd } from "../../checkpointing/Utils.ts";
import { GitCore } from "../../git/Services/GitCore.ts";
Expand Down Expand Up @@ -787,7 +787,19 @@ const make = Effect.gen(function* () {
}),
);

const worker = yield* makeDrainableWorker(processDomainEventSafely);
const workersByThreadId = new Map<ThreadId, DrainableWorker<ProviderIntentEvent>>();
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@juliusmarminge juliusmarminge Apr 12, 2026

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this map is ever growing, workers are never torn down?

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@ashvinnihalani ashvinnihalani Apr 12, 2026 •
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Yeah good point, I have never reached a point where the workers threads have grown that much since I regulalry rebuild and clear the thread history. Let me make sure when either we send a session_close id or something similar we remove delete the thread form the map

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@ashvinnihalani ashvinnihalani Apr 12, 2026 •
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Maybe this isn't the right abstraction here. I made this because I ran into a very specific use case where I wanted to start a new thread on the existing thread was still starting/hanging. Given that we have a DurableWorker for the ProviderCommandReactor, ProviderRuntimeIngestion, CheckpointReactor it may make more sense to instead make DurableWorker non-blocking for seperate thread events

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@juliusmarminge juliusmarminge Apr 12, 2026

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Honestly not sure how much punch the workers are pulling anymore. They did a good job guaranteeing order before when stuff were more async. Now we're more bought into the effect runtime so I think some of the issues the worker aimed to solve went away so I don't think we need these workers in every place we currently have them anymore.

I have some branches with some performance work that I got sidetracked from finishing. Will see if I can dig those up


const workerForThread = (threadId: ThreadId) =>
Effect.gen(function* () {
const existing = workersByThreadId.get(threadId);
if (existing) {
return existing;
}

const created = yield* makeDrainableWorker(processDomainEventSafely);
workersByThreadId.set(threadId, created);
return created;
});

const start: ProviderCommandReactorShape["start"] = Effect.fn("start")(function* () {
const processEvent = Effect.fn("processEvent")(function* (event: OrchestrationEvent) {
Expand All @@ -799,6 +811,7 @@ const make = Effect.gen(function* () {
event.type === "thread.user-input-response-requested" ||
event.type === "thread.session-stop-requested"
) {
const worker = yield* workerForThread(event.payload.threadId);
return yield* worker.enqueue(event);
}
});
Expand All @@ -810,7 +823,13 @@ const make = Effect.gen(function* () {

return {
start,
drain: worker.drain,
drain: Effect.gen(function* () {
const workers = Array.from(workersByThreadId.values());
yield* Effect.forEach(workers, (worker) => worker.drain, {
concurrency: "unbounded",
discard: true,
});
}),
} satisfies ProviderCommandReactorShape;
});

Expand Down
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