feat(telemetry): nest event loop stalls under the blocked span and sample their stacks

.changeset/loop-stall-stacks.md

@@ -0,0 +1,5 @@
+---
+'@livekit/agents': patch
+---
+
+Event loop stalls now nest under the span that was running when the loop blocked (`function_tool`, `rpc_handler`, `on_user_turn_completed`, ...) and carry `lk.blocking.stack`, the loop thread's call stack sampled by the monitor's watchdog thread through an inspector session (at the warn threshold and again at ten times it). Sampling starts after a process's first stall, or from the start with `LIVEKIT_AGENTS_LOOP_BLOCK_STACKS=1`, never with `0`; it costs a one-time enumeration of the loaded scripts on the loop when it starts and about 1% of throughput afterwards. Not enabled while an inspector is attached to the process.

agents/api-extractor.json

@@ -16,5 +16,17 @@
    * DEFAULT VALUE: ""
    */
   "extends": "../api-extractor-shared.json",
-  "mainEntryPointFilePath": "./dist/index.d.ts"
+  "mainEntryPointFilePath": "./dist/index.d.ts",
+  "messages": {
+    "extractorMessageReporting": {
+      /**
+       * Exports are not release-tagged in this package; the warning for every untagged symbol
+       * only buries the real changes in the API report.
+       */
+      "ae-missing-release-tag": {
+        "logLevel": "none",
+        "addToApiReportFile": false
+      }
+    }
+  }
 }

agents/src/telemetry/blocked_span_tracker.ts

@@ -0,0 +1,159 @@
+// SPDX-FileCopyrightText: 2026 LiveKit, Inc.
+//
+// SPDX-License-Identifier: Apache-2.0
+import { type Context, type Span, trace } from '@opentelemetry/api';
+import type { ReadableSpan, SpanProcessor } from '@opentelemetry/sdk-trace-base';
+
+/**
+ * Which span was running when the event loop stalled, without sampling a stack.
+ *
+ * Every framework span is created and ended on the main thread, so a span created before a
+ * stall and ended after it (or at its end, when the blocking call returned) was current while
+ * the loop was blocked. The innermost such span, the one created last, is where the stall hurt:
+ * `function_tool` for a slow tool, `rpc_handler` for a slow RPC, `on_user_turn_completed` for a
+ * slow hook. The Python monitor reads the same span off the blocked task's context; Node has no
+ * cross-thread view of a task's context, so the span processor keeps the bookkeeping instead.
+ *
+ * Creation and end are timed by the wall clock at the processor call, not by the span's own
+ * timestamps: a span back-dated at creation (`eou_wait` starts at the user's last speech) must
+ * not claim a stall that predates it.
+ */
+export class BlockedSpanTracker implements SpanProcessor {
+  /** Spans not yet ended, by span id. */
+  readonly #open = new Map<string, { span: Span; createdAt: number }>();
+  /** Spans ended recently: a stall's report runs a heartbeat after the blocking call returned. */
+  readonly #ended: { span: Span; createdAt: number; endedAt: number }[] = [];
+  readonly #retention: number;
+  readonly #maxEnded: number;
+
+  constructor(options: { retention?: number; maxEnded?: number } = {}) {
+    this.#retention = options.retention ?? 5_000;
+    this.#maxEnded = options.maxEnded ?? 256;
+  }
+
+  onStart(span: Span): void {
+    this.#open.set(span.spanContext().spanId, { span, createdAt: Date.now() });
+  }
+
+  onEnd(span: ReadableSpan): void {
+    const id = span.spanContext().spanId;
+    const entry = this.#open.get(id);
+    if (!entry) return;
+    this.#open.delete(id);
+    const now = Date.now();
+    this.#ended.push({ span: entry.span, createdAt: entry.createdAt, endedAt: now });
+    this.#prune(now);
+  }
+
+  async forceFlush(): Promise<void> {}
+
+  async shutdown(): Promise<void> {
+    this.#open.clear();
+    this.#ended.length = 0;
+  }
+
+  /**
+   * The span that was current across `[startedAt, endedAt]` (epoch ms): created before the
+   * window opened and still open, or ended no earlier than the window closed. Spans of the given
+   * names are skipped (the stall span itself). `slack` widens both ends: the block's start is
+   * known to within a heartbeat, and its end is the late heartbeat's run, a little after the
+   * blocking call returned and the span it was in ended.
+   *
+   * Among the spans that qualify, the innermost of one ancestry is the answer. When two
+   * operations of the same kind were both in flight (two tools, two RPC handlers), timing alone
+   * cannot say which one blocked: the answer is then their nearest common ancestor, or nothing,
+   * rather than a guess at one of them. Operations of different kinds overlap all the time (a
+   * user turn is open while an RPC handler runs) and the newest of them is the one that blocked.
+   */
+  blockedSpan(
+    startedAt: number,
+    endedAt: number,
+    exclude: ReadonlySet<string>,
+    slack = 2,
+  ): Span | undefined {
+    const opened = startedAt + slack;
+    const closed = endedAt - slack;
+    const candidates = new Map<string, { span: Span; createdAt: number }>();
+    const consider = (entry: { span: Span; createdAt: number }) => {
+      if (entry.createdAt > opened) return;
+      if (exclude.has(spanName(entry.span))) return;
+      candidates.set(entry.span.spanContext().spanId, entry);
+    };
+    for (const entry of this.#open.values()) consider(entry);
+    for (const entry of this.#ended) {
+      if (entry.endedAt >= closed) consider(entry);
+    }
+    if (!candidates.size) return undefined;
+
+    // the leaves: candidates no other candidate descends from
+    const hasChild = new Set<string>();
+    for (const entry of candidates.values()) {
+      let parent = parentSpanId(entry.span);
+      while (parent !== undefined && candidates.has(parent) && !hasChild.has(parent)) {
+        hasChild.add(parent);
+        parent = parentSpanId(candidates.get(parent)!.span);
+      }
+    }
+    const leaves = [...candidates.entries()].filter(([id]) => !hasChild.has(id));
+    // ties (same millisecond) go to the later-created span, which the maps yield last
+    let newest = leaves[0]!;
+    for (const leaf of leaves) if (leaf[1].createdAt >= newest[1].createdAt) newest = leaf;
+    const sameKind = leaves.filter(
+      ([, entry]) => spanName(entry.span) === spanName(newest[1].span),
+    );
+    if (sameKind.length <= 1) return newest[1].span;
+
+    // indistinguishable: the nearest ancestor (among the candidates) common to all of them
+    const chains = sameKind.map(([id]) => {
+      const chain: string[] = [];
+      let current: string | undefined = id;
+      while (current !== undefined && candidates.has(current)) {
+        chain.push(current);
+        current = parentSpanId(candidates.get(current)!.span);
+      }
+      return chain;
+    });
+    const shared = chains[0]!.find((id) => chains.every((chain) => chain.includes(id)));
+    if (shared === undefined) return undefined;
+    // the common ancestor is the deepest one of that kind that is not itself ambiguous
+    return candidates.get(shared)!.span;
+  }
+
+  /** The context carrying {@link blockedSpan}, or undefined. */
+  blockedContext(
+    startedAt: number,
+    endedAt: number,
+    exclude: ReadonlySet<string>,
+    base: Context,
+    slack = 2,
+  ): Context | undefined {
+    const span = this.blockedSpan(startedAt, endedAt, exclude, slack);
+    return span ? trace.setSpan(base, span) : undefined;
+  }
+
+  /** @internal test hook */
+  get openCount(): number {
+    return this.#open.size;
+  }
+
+  #prune(now: number): void {
+    const cutoff = now - this.#retention;
+    while (
+      this.#ended.length &&
+      (this.#ended[0]!.endedAt < cutoff || this.#ended.length > this.#maxEnded)
+    ) {
+      this.#ended.shift();
+    }
+  }
+}
+
+function spanName(span: Span): string {
+  return (span as { name?: string }).name ?? '';
+}
+
+function parentSpanId(span: Span): string | undefined {
+  return (span as { parentSpanContext?: { spanId: string } }).parentSpanContext?.spanId;
+}
+
+/** The one tracker the loop monitor consults; installed on every provider the framework owns. */
+export const blockedSpanTracker = new BlockedSpanTracker();

agents/src/telemetry/loop_monitor.test.ts

@@ -99,6 +99,7 @@ describe.sequential('event loop monitor', () => {
       warnThreshold: WARN,
       errorThreshold: ERROR,
       tickInterval: TICK,
+      stacks: 'never', // sampled stacks have their own tests; reports stay synchronous here
     });
     monitor.onReport = (report) => reports.push(report);
     sessionRoot = tracer.startSpan({ name: 'agent_session' });
@@ -303,6 +304,7 @@ describe.sequential('event loop monitor', () => {
       errorThreshold: ERROR,
       tickInterval: TICK,
       emitSpans: false,
+      stacks: 'never',
     });
     const workerReports: BlockedReport[] = [];
     workerMonitor.onReport = (report) => workerReports.push(report);
@@ -416,6 +418,7 @@ describe.sequential('event loop monitor', () => {
         errorThreshold: ERROR,
         tickInterval: TICK,
         watchdog: false,
+        stacks: 'never',
       });
       bare.start();
       expect(bare.watchdogActive).toBe(false);