Multi-instance <TimeToInitialDisplay> / <TimeToFullDisplay> coordination; a multi-signal TTID/TTFD system

CHANGELOG.md

@@ -6,6 +6,14 @@
 > make sure you follow our [migration guide](https://docs.sentry.io/platforms/react-native/migration/) first.
 <!-- prettier-ignore-end -->
 
+## Unreleased
+
+### Features
+
+- Multi-instance `<TimeToInitialDisplay>` / `<TimeToFullDisplay>` coordination ([#6090](https://github.com/getsentry/sentry-react-native/pull/6090))
+  - New `ready` prop. When a screen has multiple async data sources, mount one `<TimeToFullDisplay ready={...} />` per source — TTID/TTFD is recorded only when every instance reports `ready === true`.
+  - The existing `record` prop is unchanged BUT it is now deprecated in favor of `ready`.
+
 ## 8.11.0
 
 ### Features

packages/core/src/js/tracing/integrations/timeToDisplayIntegration.ts

@@ -4,6 +4,7 @@ import { debug } from '@sentry/core';
 
 import { NATIVE } from '../../wrapper';
 import { UI_LOAD_FULL_DISPLAY, UI_LOAD_INITIAL_DISPLAY } from '../ops';
+import { clearSpan as clearTimeToDisplayCoordinatorSpan } from '../timeToDisplayCoordinator';
 import { SPAN_ORIGIN_AUTO_UI_TIME_TO_DISPLAY, SPAN_ORIGIN_MANUAL_UI_TIME_TO_DISPLAY } from '../origin';
 import { getReactNavigationIntegration } from '../reactnavigation';
 import { SEMANTIC_ATTRIBUTE_ROUTE_HAS_BEEN_SEEN } from '../semanticAttributes';
@@ -86,6 +87,8 @@ export const timeToDisplayIntegration = (): Integration => {
         event.timestamp = newTransactionEndTimestampSeconds;
       }
 
+      clearTimeToDisplayCoordinatorSpan(rootSpanId);
+
       return event;
     },
   };

packages/core/src/js/tracing/timeToDisplayCoordinator.ts

@@ -0,0 +1,248 @@
+/**
+ * Coordinator for multi-instance `<TimeToInitialDisplay>` / `<TimeToFullDisplay>`
+ * components on a single screen (active span).
+ */
+
+type Checkpoint = { ready: boolean };
+type Listener = () => void;
+
+interface SpanRegistry {
+  checkpoints: Map<string, Checkpoint>;
+  listeners: Set<Listener>;
+  // this value answers the question "are all checkpoints on this span ready?"
+  // when the raw value goes from false to true, aggregateReady does NOT flip immediately, it gets
+  // scheduled with setTimeout(0) in `reevaluate` function
+  //
+  aggregateReady: boolean;
+  // when non-null, an up-flip is scheduled but has not yet been applied to `aggregateReady`
+  pendingUpFlip: ReturnType<typeof setTimeout> | null;
+  // `sticky` is used indicate checkpints that gets cleared when the screen fully unmounts
+  // it's useful
+  sticky: Set<string>;
+}
+
+const TTID = 'ttid';
+const TTFD = 'ttfd';
+
+export type DisplayKind = typeof TTID | typeof TTFD;
+
+const registries: Record<DisplayKind, Map<string, SpanRegistry>> = {
+  ttid: new Map(),
+  ttfd: new Map(),
+};
+
+function getOrCreate(kind: DisplayKind, parentSpanId: string): SpanRegistry {
+  const map = registries[kind];
+  let entry = map.get(parentSpanId);
+  if (!entry) {
+    entry = {
+      checkpoints: new Map(),
+      listeners: new Set(),
+      aggregateReady: false,
+      pendingUpFlip: null,
+      sticky: new Set(),
+    };
+    map.set(parentSpanId, entry);
+  }
+  return entry;
+}
+
+function cancelPendingUpFlip(entry: SpanRegistry): void {
+  if (entry.pendingUpFlip !== null) {
+    clearTimeout(entry.pendingUpFlip);
+    entry.pendingUpFlip = null;
+  }
+}
+
+function computeAggregate(entry: SpanRegistry): boolean {
+  if (entry.checkpoints.size === 0) {
+    return false;
+  }
+  for (const cp of entry.checkpoints.values()) {
+    if (!cp.ready) {
+      return false;
+    }
+  }
+  return true;
+}
+
+// Recompute the raw aggregate and reconcile it with the cached `aggregateReady`
+function reevaluate(entry: SpanRegistry): void {
+  const raw = computeAggregate(entry);
+
+  if (raw === entry.aggregateReady) {
+    cancelPendingUpFlip(entry);
+    return;
+  }
+
+  if (!raw) {
+    cancelPendingUpFlip(entry);
+    entry.aggregateReady = false;
+    notifyListeners(entry);
+    return;
+  }
+
+  if (entry.pendingUpFlip !== null) {
+    return;
+  }
+  // the delay here is set to 0 because in React 18 that
+  // will schedule the callback to be run asynchronously after the shortest possible delay
+  entry.pendingUpFlip = setTimeout(() => {
+    entry.pendingUpFlip = null;
+    // Re-check on fire — a peer may have un-readied between schedule and now.
+    if (!computeAggregate(entry) || entry.aggregateReady) {
+      return;
+    }
+    entry.aggregateReady = true;
+    notifyListeners(entry);
+  }, 0);
+}
+
+function notifyListeners(entry: SpanRegistry): void {
+  for (const listener of entry.listeners) {
+    listener();
+  }
+}
+
+function performCleanup(kind: DisplayKind, parentSpanId: string, entry: SpanRegistry): void {
+  if (entry.sticky.size > 0) {
+    return;
+  }
+  if (entry.checkpoints.size === 0 && entry.listeners.size === 0) {
+    cancelPendingUpFlip(entry);
+    registries[kind].delete(parentSpanId);
+  }
+}
+
+// A bit of a hack but this is used to detect the premature-fire scenario
+// where a not-ready checkpoint unmounts while every other checkpoint is ready:
+// deleting it would let the aggregate flip to true and immediately record TTFD/TTID,
+// even though the unmounting source never actually became ready.
+function isSoleBlocker(entry: SpanRegistry, checkpointId: string): boolean {
+  if (entry.aggregateReady) {
+    return false;
+  }
+  if (entry.checkpoints.size <= 1) {
+    // because removing the only checkpoint leaves the registry empty
+    return false;
+  }
+  const cp = entry.checkpoints.get(checkpointId);
+  if (!cp || cp.ready) {
+    return false;
+  }
+  for (const [id, other] of entry.checkpoints) {
+    if (id === checkpointId) {
+      continue;
+    }
+    if (!other.ready) {
+      return false;
+    }
+  }
+  return true;
+}
+
+export function registerCheckpoint(
+  kind: DisplayKind,
+  parentSpanId: string,
+  checkpointId: string,
+  ready: boolean,
+): () => void {
+  const entry = getOrCreate(kind, parentSpanId);
+
+  // Any new registration means the screen's component graph is changing.
+  // Drop leftover sticky entries from previous unmount cycles -- otherwise
+  // a remounted checkpoint would be permanently blockedю
+  if (entry.sticky.size > 0) {
+    for (const id of entry.sticky) {
+      entry.checkpoints.delete(id);
+    }
+    entry.sticky.clear();
+  }
+
+  entry.checkpoints.set(checkpointId, { ready });
+  reevaluate(entry);
+
+  return () => {
+    const e = registries[kind].get(parentSpanId);
+    if (!e) {
+      return;
+    }
+    // if the checkpoint is the only blocker then removing it would flip the
+    // aggregate to true and fire TTFD/TTID even though the unmounting source never became ready.
+    // that's why we use `sticky` here to indicate that it gets cleared when the screen fully unmounts
+    if (isSoleBlocker(e, checkpointId)) {
+      e.sticky.add(checkpointId);
+      performCleanup(kind, parentSpanId, e);
+      return;
+    }
+    if (e.checkpoints.delete(checkpointId)) {
+      e.sticky.delete(checkpointId);
+      reevaluate(e);
+    }
+    performCleanup(kind, parentSpanId, e);
+  };
+}
+
+export function updateCheckpoint(
+  kind: DisplayKind,
+  parentSpanId: string,
+  checkpointId: string,
+  ready: boolean,
+): void {
+  const entry = registries[kind].get(parentSpanId);
+  const cp = entry?.checkpoints.get(checkpointId);
+  if (!entry || !cp || cp.ready === ready) {
+    return;
+  }
+  cp.ready = ready;
+  reevaluate(entry);
+}
+
+// Returns true if at least one checkpoint is registered AND all checkpoints are ready
+export function isAllReady(kind: DisplayKind, parentSpanId: string): boolean {
+  const entry = registries[kind].get(parentSpanId);
+  return !!entry && entry.aggregateReady;
+}
+
+// Returns true if there is at least one registered checkpoint on this span
+export function hasAnyCheckpoints(kind: DisplayKind, parentSpanId: string): boolean {
+  const entry = registries[kind].get(parentSpanId);
+  return !!entry && entry.checkpoints.size > 0;
+}
+
+// Subscribe to aggregate-ready transitions for a given span
+export function subscribe(kind: DisplayKind, parentSpanId: string, listener: Listener): () => void {
+  const entry = getOrCreate(kind, parentSpanId);
+  entry.listeners.add(listener);
+  return () => {
+    const e = registries[kind].get(parentSpanId);
+    if (!e) {
+      return;
+    }
+    e.listeners.delete(listener);
+    performCleanup(kind, parentSpanId, e);
+  };
+}
+
+// Drop coordinator state for `parentSpanId` across both kinds.
+// Called by the time-to-display integration once a transaction has been
+// processed, since the per-span coordinator state is no longer relevant
+// after the native draw timestamps have been read.
+export function clearSpan(parentSpanId: string): void {
+  for (const kind of [TTID, TTFD] as const) {
+    const entry = registries[kind].get(parentSpanId);
+    if (entry) {
+      cancelPendingUpFlip(entry);
+      registries[kind].delete(parentSpanId);
+    }
+  }
+}
+
+export function _resetTimeToDisplayCoordinator(): void {
+  for (const kind of [TTID, TTFD] as const) {
+    for (const entry of registries[kind].values()) {
+      cancelPendingUpFlip(entry);
+    }
+    registries[kind].clear();
+  }
+}