Allow suspending outside a Suspense boundary

(If the update is wrapped in startTransition)

Currently you're not allowed to suspend outside of a Suspense boundary.
We throw an error:

> A React component suspended while rendering, but no fallback UI
was specified

We treat this case like an error because discrete renders are expected
to finish synchronously to maintain consistency with external state.
However, during a concurrent transition (startTransition), what we can
do instead is treat this case like a refresh transition: suspend the
commit without showing a fallback.

The behavior is roughly as if there were a built-in Suspense boundary
at the root of the app with unstable_avoidThisFallback enabled.
Conceptually it's very similar because during hydration you're already
showing server-rendered UI; there's no need to replace that with
a fallback when something suspends.

Author: acdlite

packages/react-reconciler/src/ReactFiberThrow.new.js

@@ -63,6 +63,7 @@ import {
 import {
   renderDidError,
   renderDidErrorUncaught,
+  renderDidSuspendDelayIfPossible,
   queueConcurrentError,
   onUncaughtError,
   markLegacyErrorBoundaryAsFailed,
@@ -80,6 +81,7 @@ import {
   includesSomeLane,
   mergeLanes,
   pickArbitraryLane,
+  includesOnlyTransitions,
 } from './ReactFiberLane.new';
 import {
   getIsHydrating,
@@ -167,12 +169,7 @@ function createClassErrorUpdate(
   return update;
 }
 
-function attachWakeableListeners(
-  suspenseBoundary: Fiber,
-  root: FiberRoot,
-  wakeable: Wakeable,
-  lanes: Lanes,
-) {
+function attachPingListener(root: FiberRoot, wakeable: Wakeable, lanes: Lanes) {
   // Attach a ping listener
   //
   // The data might resolve before we have a chance to commit the fallback. Or,
@@ -185,34 +182,39 @@ function attachWakeableListeners(
   //
   // We only need to do this in concurrent mode. Legacy Suspense always
   // commits fallbacks synchronously, so there are no pings.
-  if (suspenseBoundary.mode & ConcurrentMode) {
-    let pingCache = root.pingCache;
-    let threadIDs;
-    if (pingCache === null) {
-      pingCache = root.pingCache = new PossiblyWeakMap();
+  let pingCache = root.pingCache;
+  let threadIDs;
+  if (pingCache === null) {
+    pingCache = root.pingCache = new PossiblyWeakMap();
+    threadIDs = new Set();
+    pingCache.set(wakeable, threadIDs);
+  } else {
+    threadIDs = pingCache.get(wakeable);
+    if (threadIDs === undefined) {
       threadIDs = new Set();
       pingCache.set(wakeable, threadIDs);
-    } else {
-      threadIDs = pingCache.get(wakeable);
-      if (threadIDs === undefined) {
-        threadIDs = new Set();
-        pingCache.set(wakeable, threadIDs);
-      }
     }
-    if (!threadIDs.has(lanes)) {
-      // Memoize using the thread ID to prevent redundant listeners.
-      threadIDs.add(lanes);
-      const ping = pingSuspendedRoot.bind(null, root, wakeable, lanes);
-      if (enableUpdaterTracking) {
-        if (isDevToolsPresent) {
-          // If we have pending work still, restore the original updaters
-          restorePendingUpdaters(root, lanes);
-        }
+  }
+  if (!threadIDs.has(lanes)) {
+    // Memoize using the thread ID to prevent redundant listeners.
+    threadIDs.add(lanes);
+    const ping = pingSuspendedRoot.bind(null, root, wakeable, lanes);
+    if (enableUpdaterTracking) {
+      if (isDevToolsPresent) {
+        // If we have pending work still, restore the original updaters
+        restorePendingUpdaters(root, lanes);
       }
-      wakeable.then(ping, ping);
     }
+    wakeable.then(ping, ping);
   }
+}
 
+function attachRetryListener(
+  suspenseBoundary: Fiber,
+  root: FiberRoot,
+  wakeable: Wakeable,
+  lanes: Lanes,
+) {
   // Retry listener
   //
   // If the fallback does commit, we need to attach a different type of
@@ -472,24 +474,47 @@ function throwException(
         root,
         rootRenderLanes,
       );
-      attachWakeableListeners(
-        suspenseBoundary,
-        root,
-        wakeable,
-        rootRenderLanes,
-      );
+      // We only attach ping listeners in concurrent mode. Legacy Suspense always
+      // commits fallbacks synchronously, so there are no pings.
+      if (suspenseBoundary.mode & ConcurrentMode) {
+        attachPingListener(root, wakeable, rootRenderLanes);
+      }
+      attachRetryListener(suspenseBoundary, root, wakeable, rootRenderLanes);
       return;
     } else {
-      // No boundary was found. Fallthrough to error mode.
+      // No boundary was found. If we're inside startTransition, this is OK.
+      // We can suspend and wait for more data to arrive.
+
+      if (includesOnlyTransitions(rootRenderLanes)) {
+        // This is a transition. Suspend. Since we're not activating a Suspense
+        // boundary, this will unwind all the way to the root without performing
+        // a second pass to render a fallback. (This is arguably how refresh
+        // transitions should work, too, since we're not going to commit the
+        // fallbacks anyway.)
+        attachPingListener(root, wakeable, rootRenderLanes);
+        renderDidSuspendDelayIfPossible();
+        return;
+      }
+
+      // We're not in a transition. We treat this case like an error because
+      // discrete renders are expected to finish synchronously to maintain
+      // consistency with external state.
+      // TODO: This will error during non-transition concurrent renders, too.
+      // But maybe it shouldn't?
+
       // TODO: We should never call getComponentNameFromFiber in production.
       // Log a warning or something to prevent us from accidentally bundling it.
-      value = new Error(
+      const uncaughtSuspenseError = new Error(
         (getComponentNameFromFiber(sourceFiber) || 'A React component') +
           ' suspended while rendering, but no fallback UI was specified.\n' +
           '\n' +
           'Add a <Suspense fallback=...> component higher in the tree to ' +
           'provide a loading indicator or placeholder to display.',
       );
+
+      // If we're outside a transition, fall through to the regular error path.
+      // The error will be caught by the nearest suspense boundary.
+      value = uncaughtSuspenseError;
     }
   } else {
     // This is a regular error, not a Suspense wakeable.

packages/react-reconciler/src/ReactFiberThrow.old.js

@@ -63,6 +63,7 @@ import {
 import {
   renderDidError,
   renderDidErrorUncaught,
+  renderDidSuspendDelayIfPossible,
   queueConcurrentError,
   onUncaughtError,
   markLegacyErrorBoundaryAsFailed,
@@ -80,6 +81,7 @@ import {
   includesSomeLane,
   mergeLanes,
   pickArbitraryLane,
+  includesOnlyTransitions,
 } from './ReactFiberLane.old';
 import {
   getIsHydrating,
@@ -167,12 +169,7 @@ function createClassErrorUpdate(
   return update;
 }
 
-function attachWakeableListeners(
-  suspenseBoundary: Fiber,
-  root: FiberRoot,
-  wakeable: Wakeable,
-  lanes: Lanes,
-) {
+function attachPingListener(root: FiberRoot, wakeable: Wakeable, lanes: Lanes) {
   // Attach a ping listener
   //
   // The data might resolve before we have a chance to commit the fallback. Or,
@@ -185,34 +182,39 @@ function attachWakeableListeners(
   //
   // We only need to do this in concurrent mode. Legacy Suspense always
   // commits fallbacks synchronously, so there are no pings.
-  if (suspenseBoundary.mode & ConcurrentMode) {
-    let pingCache = root.pingCache;
-    let threadIDs;
-    if (pingCache === null) {
-      pingCache = root.pingCache = new PossiblyWeakMap();
+  let pingCache = root.pingCache;
+  let threadIDs;
+  if (pingCache === null) {
+    pingCache = root.pingCache = new PossiblyWeakMap();
+    threadIDs = new Set();
+    pingCache.set(wakeable, threadIDs);
+  } else {
+    threadIDs = pingCache.get(wakeable);
+    if (threadIDs === undefined) {
       threadIDs = new Set();
       pingCache.set(wakeable, threadIDs);
-    } else {
-      threadIDs = pingCache.get(wakeable);
-      if (threadIDs === undefined) {
-        threadIDs = new Set();
-        pingCache.set(wakeable, threadIDs);
-      }
     }
-    if (!threadIDs.has(lanes)) {
-      // Memoize using the thread ID to prevent redundant listeners.
-      threadIDs.add(lanes);
-      const ping = pingSuspendedRoot.bind(null, root, wakeable, lanes);
-      if (enableUpdaterTracking) {
-        if (isDevToolsPresent) {
-          // If we have pending work still, restore the original updaters
-          restorePendingUpdaters(root, lanes);
-        }
+  }
+  if (!threadIDs.has(lanes)) {
+    // Memoize using the thread ID to prevent redundant listeners.
+    threadIDs.add(lanes);
+    const ping = pingSuspendedRoot.bind(null, root, wakeable, lanes);
+    if (enableUpdaterTracking) {
+      if (isDevToolsPresent) {
+        // If we have pending work still, restore the original updaters
+        restorePendingUpdaters(root, lanes);
       }
-      wakeable.then(ping, ping);
     }
+    wakeable.then(ping, ping);
   }
+}
 
+function attachRetryListener(
+  suspenseBoundary: Fiber,
+  root: FiberRoot,
+  wakeable: Wakeable,
+  lanes: Lanes,
+) {
   // Retry listener
   //
   // If the fallback does commit, we need to attach a different type of
@@ -472,24 +474,47 @@ function throwException(
         root,
         rootRenderLanes,
       );
-      attachWakeableListeners(
-        suspenseBoundary,
-        root,
-        wakeable,
-        rootRenderLanes,
-      );
+      // We only attach ping listeners in concurrent mode. Legacy Suspense always
+      // commits fallbacks synchronously, so there are no pings.
+      if (suspenseBoundary.mode & ConcurrentMode) {
+        attachPingListener(root, wakeable, rootRenderLanes);
+      }
+      attachRetryListener(suspenseBoundary, root, wakeable, rootRenderLanes);
       return;
     } else {
-      // No boundary was found. Fallthrough to error mode.
+      // No boundary was found. If we're inside startTransition, this is OK.
+      // We can suspend and wait for more data to arrive.
+
+      if (includesOnlyTransitions(rootRenderLanes)) {
+        // This is a transition. Suspend. Since we're not activating a Suspense
+        // boundary, this will unwind all the way to the root without performing
+        // a second pass to render a fallback. (This is arguably how refresh
+        // transitions should work, too, since we're not going to commit the
+        // fallbacks anyway.)
+        attachPingListener(root, wakeable, rootRenderLanes);
+        renderDidSuspendDelayIfPossible();
+        return;
+      }
+
+      // We're not in a transition. We treat this case like an error because
+      // discrete renders are expected to finish synchronously to maintain
+      // consistency with external state.
+      // TODO: This will error during non-transition concurrent renders, too.
+      // But maybe it shouldn't?
+
       // TODO: We should never call getComponentNameFromFiber in production.
       // Log a warning or something to prevent us from accidentally bundling it.
-      value = new Error(
+      const uncaughtSuspenseError = new Error(
         (getComponentNameFromFiber(sourceFiber) || 'A React component') +
           ' suspended while rendering, but no fallback UI was specified.\n' +
           '\n' +
           'Add a <Suspense fallback=...> component higher in the tree to ' +
           'provide a loading indicator or placeholder to display.',
       );
+
+      // If we're outside a transition, fall through to the regular error path.
+      // The error will be caught by the nearest suspense boundary.
+      value = uncaughtSuspenseError;
     }
   } else {
     // This is a regular error, not a Suspense wakeable.

packages/react-reconciler/src/ReactFiberUnwindWork.new.js

@@ -89,16 +89,17 @@ function unwindWork(workInProgress: Fiber, renderLanes: Lanes) {
       popTopLevelLegacyContextObject(workInProgress);
       resetMutableSourceWorkInProgressVersions();
       const flags = workInProgress.flags;
-
-      if ((flags & DidCapture) !== NoFlags) {
-        throw new Error(
-          'The root failed to unmount after an error. This is likely a bug in ' +
-            'React. Please file an issue.',
-        );
+      if (
+        (flags & ShouldCapture) !== NoFlags &&
+        (flags & DidCapture) === NoFlags
+      ) {
+        // There was an error during render that wasn't captured by a suspense
+        // boundary. Do a second pass on the root to unmount the children.
+        workInProgress.flags = (flags & ~ShouldCapture) | DidCapture;
+        return workInProgress;
       }
-
-      workInProgress.flags = (flags & ~ShouldCapture) | DidCapture;
-      return workInProgress;
+      // We unwound to the root without completing it. Exit.
+      return null;
     }
     case HostComponent: {
       // TODO: popHydrationState

packages/react-reconciler/src/ReactFiberUnwindWork.old.js

@@ -89,16 +89,17 @@ function unwindWork(workInProgress: Fiber, renderLanes: Lanes) {
       popTopLevelLegacyContextObject(workInProgress);
       resetMutableSourceWorkInProgressVersions();
       const flags = workInProgress.flags;
-
-      if ((flags & DidCapture) !== NoFlags) {
-        throw new Error(
-          'The root failed to unmount after an error. This is likely a bug in ' +
-            'React. Please file an issue.',
-        );
+      if (
+        (flags & ShouldCapture) !== NoFlags &&
+        (flags & DidCapture) === NoFlags
+      ) {
+        // There was an error during render that wasn't captured by a suspense
+        // boundary. Do a second pass on the root to unmount the children.
+        workInProgress.flags = (flags & ~ShouldCapture) | DidCapture;
+        return workInProgress;
       }
-
-      workInProgress.flags = (flags & ~ShouldCapture) | DidCapture;
-      return workInProgress;
+      // We unwound to the root without completing it. Exit.
+      return null;
     }
     case HostComponent: {
       // TODO: popHydrationState