Initial implementation of cache cleanup (facebook#22510)

This is an initial, partial implementation of a cleanup mechanism for the experimental Cache API. The idea is that consumers of the Cache API can register to be informed when a given Cache instance is no longer needed so that they can perform associated cleanup tasks to free resources stored in the cache. A canonical example would be cancelling pending network requests.

An overview of the high-level changes:

* Changes the `Cache` type from a Map of cache instances to be an object with the original Map of instances, a reference count (to count roughly "active references" to the cache instances - more below), and an AbortController.
* Adds a new public API, `unstable_getCacheSignal(): AbortSignal`, which is callable during render. It returns an AbortSignal tied to the lifetime of the cache - developers can listen for the 'abort' event on the signal, which React now triggers when a given cache instance is no longer referenced. 
  * Note that `AbortSignal` is a web standard that is supported by other platform APIs; for example a signal can be passed to `fetch()` to trigger cancellation of an HTTP request.
* Implements the above - triggering the 'abort' event - by handling passive mount/unmount for HostRoot and CacheComponent fiber nodes.

Cases handled:
* Aborted transitions: we clean up a new cache created for an aborted transition
* Suspense: we retain a fresh cache instance until a suspended tree resolves

For follow-ups:
* When a subsequent cache refresh is issued before a previous refresh completes, the refreshes are queued. Fresh cache instances for previous refreshes in the queue should be cleared, retaining only the most recent cache. I plan to address this in a follow-up PR.
* If a refresh is cancelled, the fresh cache should be cleaned up.

packages/react-devtools-shared/src/__tests__/__snapshots__/profilingCache-test.js.snap

@@ -73,7 +73,7 @@ Object {
     },
   },
   "duration": 15,
-  "effectDuration": null,
+  "effectDuration": 0,
   "fiberActualDurations": Map {
     1 => 15,
     2 => 15,
@@ -86,7 +86,7 @@ Object {
     3 => 3,
     4 => 2,
   },
-  "passiveEffectDuration": null,
+  "passiveEffectDuration": 0,
   "priorityLevel": "Immediate",
   "timestamp": 15,
   "updaters": Array [

packages/react-dom/src/server/ReactPartialRendererHooks.js

@@ -216,6 +216,10 @@ export function resetHooksState(): void {
   workInProgressHook = null;
 }
 
+function getCacheSignal() {
+  throw new Error('Not implemented.');
+}
+
 function getCacheForType<T>(resourceType: () => T): T {
   throw new Error('Not implemented.');
 }
@@ -551,6 +555,7 @@ export const Dispatcher: DispatcherType = {
 };
 
 if (enableCache) {
+  Dispatcher.getCacheSignal = getCacheSignal;
   Dispatcher.getCacheForType = getCacheForType;
   Dispatcher.useCacheRefresh = useCacheRefresh;
 }

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

@@ -18,8 +18,13 @@ import {REACT_CONTEXT_TYPE} from 'shared/ReactSymbols';
 import {isPrimaryRenderer} from './ReactFiberHostConfig';
 import {createCursor, push, pop} from './ReactFiberStack.new';
 import {pushProvider, popProvider} from './ReactFiberNewContext.new';
+import * as Scheduler from 'scheduler';
 
-export type Cache = Map<() => mixed, mixed>;
+export type Cache = {|
+  controller: AbortController,
+  data: Map<() => mixed, mixed>,
+  refCount: number,
+|};
 
 export type CacheComponentState = {|
   +parent: Cache,
@@ -31,6 +36,13 @@ export type SpawnedCachePool = {|
   +pool: Cache,
 |};
 
+// Intentionally not named imports because Rollup would
+// use dynamic dispatch for CommonJS interop named imports.
+const {
+  unstable_scheduleCallback: scheduleCallback,
+  unstable_NormalPriority: NormalPriority,
+} = Scheduler;
+
 export const CacheContext: ReactContext<Cache> = enableCache
   ? {
       $$typeof: REACT_CONTEXT_TYPE,
@@ -57,6 +69,58 @@ let pooledCache: Cache | null = null;
 // cache from the render that suspended.
 const prevFreshCacheOnStack: StackCursor<Cache | null> = createCursor(null);
 
+// Creates a new empty Cache instance with a ref-count of 0. The caller is responsible
+// for retaining the cache once it is in use (retainCache), and releasing the cache
+// once it is no longer needed (releaseCache).
+export function createCache(): Cache {
+  if (!enableCache) {
+    return (null: any);
+  }
+  const cache: Cache = {
+    controller: new AbortController(),
+    data: new Map(),
+    refCount: 0,
+  };
+
+  return cache;
+}
+
+export function retainCache(cache: Cache) {
+  if (!enableCache) {
+    return;
+  }
+  if (__DEV__) {
+    if (cache.controller.signal.aborted) {
+      console.warn(
+        'A cache instance was retained after it was already freed. ' +
+          'This likely indicates a bug in React.',
+      );
+    }
+  }
+  cache.refCount++;
+}
+
+// Cleanup a cache instance, potentially freeing it if there are no more references
+export function releaseCache(cache: Cache) {
+  if (!enableCache) {
+    return;
+  }
+  cache.refCount--;
+  if (__DEV__) {
+    if (cache.refCount < 0) {
+      console.warn(
+        'A cache instance was released after it was already freed. ' +
+          'This likely indicates a bug in React.',
+      );
+    }
+  }
+  if (cache.refCount === 0) {
+    scheduleCallback(NormalPriority, () => {
+      cache.controller.abort();
+    });
+  }
+}
+
 export function pushCacheProvider(workInProgress: Fiber, cache: Cache) {
   if (!enableCache) {
     return;
@@ -78,8 +142,14 @@ export function requestCacheFromPool(renderLanes: Lanes): Cache {
   if (pooledCache !== null) {
     return pooledCache;
   }
-  // Create a fresh cache.
-  pooledCache = new Map();
+  // Create a fresh cache. The pooled cache must be owned - it is freed
+  // in releaseRootPooledCache() - but the cache instance handed out
+  // is retained/released in the commit phase of the component that
+  // references is (ie the host root, cache boundary, suspense component)
+  // Ie, pooledCache is conceptually an Option<Arc<Cache>> (owned),
+  // whereas the return value of this function is a &Arc<Cache> (borrowed).
+  pooledCache = createCache();
+  retainCache(pooledCache);
   return pooledCache;
 }
 
@@ -91,7 +161,13 @@ export function pushRootCachePool(root: FiberRoot) {
   // from `root.pooledCache`. If it's currently `null`, we will lazily
   // initialize it the first type it's requested. However, we only mutate
   // the root itself during the complete/unwind phase of the HostRoot.
-  pooledCache = root.pooledCache;
+  const rootCache = root.pooledCache;
+  if (rootCache != null) {
+    pooledCache = rootCache;
+    root.pooledCache = null;
+  } else {
+    pooledCache = null;
+  }
 }
 
 export function popRootCachePool(root: FiberRoot, renderLanes: Lanes) {
@@ -157,7 +233,6 @@ export function getSuspendedCachePool(): SpawnedCachePool | null {
   if (!enableCache) {
     return null;
   }
-
   // We check the cache on the stack first, since that's the one any new Caches
   // would have accessed.
   let pool = pooledCache;

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

@@ -18,8 +18,13 @@ import {REACT_CONTEXT_TYPE} from 'shared/ReactSymbols';
 import {isPrimaryRenderer} from './ReactFiberHostConfig';
 import {createCursor, push, pop} from './ReactFiberStack.old';
 import {pushProvider, popProvider} from './ReactFiberNewContext.old';
+import * as Scheduler from 'scheduler';
 
-export type Cache = Map<() => mixed, mixed>;
+export type Cache = {|
+  controller: AbortController,
+  data: Map<() => mixed, mixed>,
+  refCount: number,
+|};
 
 export type CacheComponentState = {|
   +parent: Cache,
@@ -31,6 +36,13 @@ export type SpawnedCachePool = {|
   +pool: Cache,
 |};
 
+// Intentionally not named imports because Rollup would
+// use dynamic dispatch for CommonJS interop named imports.
+const {
+  unstable_scheduleCallback: scheduleCallback,
+  unstable_NormalPriority: NormalPriority,
+} = Scheduler;
+
 export const CacheContext: ReactContext<Cache> = enableCache
   ? {
       $$typeof: REACT_CONTEXT_TYPE,
@@ -57,6 +69,58 @@ let pooledCache: Cache | null = null;
 // cache from the render that suspended.
 const prevFreshCacheOnStack: StackCursor<Cache | null> = createCursor(null);
 
+// Creates a new empty Cache instance with a ref-count of 0. The caller is responsible
+// for retaining the cache once it is in use (retainCache), and releasing the cache
+// once it is no longer needed (releaseCache).
+export function createCache(): Cache {
+  if (!enableCache) {
+    return (null: any);
+  }
+  const cache: Cache = {
+    controller: new AbortController(),
+    data: new Map(),
+    refCount: 0,
+  };
+
+  return cache;
+}
+
+export function retainCache(cache: Cache) {
+  if (!enableCache) {
+    return;
+  }
+  if (__DEV__) {
+    if (cache.controller.signal.aborted) {
+      console.warn(
+        'A cache instance was retained after it was already freed. ' +
+          'This likely indicates a bug in React.',
+      );
+    }
+  }
+  cache.refCount++;
+}
+
+// Cleanup a cache instance, potentially freeing it if there are no more references
+export function releaseCache(cache: Cache) {
+  if (!enableCache) {
+    return;
+  }
+  cache.refCount--;
+  if (__DEV__) {
+    if (cache.refCount < 0) {
+      console.warn(
+        'A cache instance was released after it was already freed. ' +
+          'This likely indicates a bug in React.',
+      );
+    }
+  }
+  if (cache.refCount === 0) {
+    scheduleCallback(NormalPriority, () => {
+      cache.controller.abort();
+    });
+  }
+}
+
 export function pushCacheProvider(workInProgress: Fiber, cache: Cache) {
   if (!enableCache) {
     return;
@@ -78,8 +142,14 @@ export function requestCacheFromPool(renderLanes: Lanes): Cache {
   if (pooledCache !== null) {
     return pooledCache;
   }
-  // Create a fresh cache.
-  pooledCache = new Map();
+  // Create a fresh cache. The pooled cache must be owned - it is freed
+  // in releaseRootPooledCache() - but the cache instance handed out
+  // is retained/released in the commit phase of the component that
+  // references is (ie the host root, cache boundary, suspense component)
+  // Ie, pooledCache is conceptually an Option<Arc<Cache>> (owned),
+  // whereas the return value of this function is a &Arc<Cache> (borrowed).
+  pooledCache = createCache();
+  retainCache(pooledCache);
   return pooledCache;
 }
 
@@ -91,7 +161,13 @@ export function pushRootCachePool(root: FiberRoot) {
   // from `root.pooledCache`. If it's currently `null`, we will lazily
   // initialize it the first type it's requested. However, we only mutate
   // the root itself during the complete/unwind phase of the HostRoot.
-  pooledCache = root.pooledCache;
+  const rootCache = root.pooledCache;
+  if (rootCache != null) {
+    pooledCache = rootCache;
+    root.pooledCache = null;
+  } else {
+    pooledCache = null;
+  }
 }
 
 export function popRootCachePool(root: FiberRoot, renderLanes: Lanes) {
@@ -157,7 +233,6 @@ export function getSuspendedCachePool(): SpawnedCachePool | null {
   if (!enableCache) {
     return null;
   }
-
   // We check the cache on the stack first, since that's the one any new Caches
   // would have accessed.
   let pool = pooledCache;