[react-cache] Remove cache as argument to read

Updated is API is `Resource.read(key)` instead of
`Resource.read(cache, key)`.

The cache is read from context using `readContext`.

This also removes cache invalidation entirely (other than the default
LRU mechanism), as well as the ability to have multiple caches. We'll
add it back once `Context.write` lands and we can implement it the
right way.

Since there's now only a single cache (the global one), we don't
actually need to use context yet, but I've added a dummy context
anyway so the user gets an error if they attempt to read outside the
render phase.

packages/jest-react/src/JestReact.js

@@ -35,19 +35,17 @@ function assertYieldsWereCleared(root) {
 }
 
 export function toFlushAndYield(root, expectedYields) {
+  assertYieldsWereCleared(root);
+  const actualYields = root.unstable_flushAll();
   return captureAssertion(() => {
-    assertYieldsWereCleared(root);
-    const actualYields = root.unstable_flushAll();
     expect(actualYields).toEqual(expectedYields);
   });
 }
 
 export function toFlushAndYieldThrough(root, expectedYields) {
+  assertYieldsWereCleared(root);
+  const actualYields = root.unstable_flushNumberOfYields(expectedYields.length);
   return captureAssertion(() => {
-    assertYieldsWereCleared(root);
-    const actualYields = root.unstable_flushNumberOfYields(
-      expectedYields.length,
-    );
     expect(actualYields).toEqual(expectedYields);
   });
 }
@@ -76,8 +74,8 @@ export function toHaveYielded(ReactTestRenderer, expectedYields) {
 }
 
 export function toFlushAndThrow(root, ...rest) {
+  assertYieldsWereCleared(root);
   return captureAssertion(() => {
-    assertYieldsWereCleared(root);
     expect(() => {
       root.unstable_flushAll();
     }).toThrow(...rest);

packages/react-cache/src/LRU.js

@@ -0,0 +1,142 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import {unstable_scheduleCallback as scheduleCallback} from 'scheduler';
+
+type Entry<T> = {|
+  value: T,
+  onDelete: () => mixed,
+  previous: Entry<T>,
+  next: Entry<T>,
+|};
+
+export function createLRU<T>(limit: number) {
+  let LIMIT = limit;
+
+  // Circular, doubly-linked list
+  let first: Entry<T> | null = null;
+  let size: number = 0;
+
+  let cleanUpIsScheduled: boolean = false;
+
+  function scheduleCleanUp() {
+    if (cleanUpIsScheduled === false && size > LIMIT) {
+      // The cache size exceeds the limit. Schedule a callback to delete the
+      // least recently used entries.
+      cleanUpIsScheduled = true;
+      scheduleCallback(cleanUp);
+    }
+  }
+
+  function cleanUp() {
+    cleanUpIsScheduled = false;
+    deleteLeastRecentlyUsedEntries(LIMIT);
+  }
+
+  function deleteLeastRecentlyUsedEntries(targetSize: number) {
+    // Delete entries from the cache, starting from the end of the list.
+    if (first !== null) {
+      const resolvedFirst: Entry<T> = (first: any);
+      let last = resolvedFirst.previous;
+      while (size > targetSize && last !== null) {
+        const onDelete = last.onDelete;
+        const previous = last.previous;
+        last.onDelete = (null: any);
+
+        // Remove from the list
+        last.previous = last.next = (null: any);
+        if (last === first) {
+          // Reached the head of the list.
+          first = last = null;
+        } else {
+          (first: any).previous = previous;
+          previous.next = (first: any);
+          last = previous;
+        }
+
+        size -= 1;
+
+        // Call the destroy method after removing the entry from the list. If it
+        // throws, the rest of cache will not be deleted, but it will be in a
+        // valid state.
+        onDelete();
+      }
+    }
+  }
+
+  function add(value: T, onDelete: () => mixed): Entry<T> {
+    const entry = {
+      value,
+      onDelete,
+      next: (null: any),
+      previous: (null: any),
+    };
+    if (first === null) {
+      entry.previous = entry.next = entry;
+      first = entry;
+    } else {
+      // Append to head
+      const last = first.previous;
+      last.next = entry;
+      entry.previous = last;
+
+      first.previous = entry;
+      entry.next = first;
+
+      first = entry;
+    }
+    size += 1;
+    return entry;
+  }
+
+  function update(entry: Entry<T>, newValue: T): void {
+    entry.value = newValue;
+  }
+
+  function access(entry: Entry<T>): T {
+    const next = entry.next;
+    if (next !== null) {
+      // Entry already cached
+      const resolvedFirst: Entry<T> = (first: any);
+      if (first !== entry) {
+        // Remove from current position
+        const previous = entry.previous;
+        previous.next = next;
+        next.previous = previous;
+
+        // Append to head
+        const last = resolvedFirst.previous;
+        last.next = entry;
+        entry.previous = last;
+
+        resolvedFirst.previous = entry;
+        entry.next = resolvedFirst;
+
+        first = entry;
+      }
+    } else {
+      // Cannot access a deleted entry
+      // TODO: Error? Warning?
+    }
+    scheduleCleanUp();
+    return entry.value;
+  }
+
+  function setLimit(newLimit: number) {
+    LIMIT = newLimit;
+    scheduleCleanUp();
+  }
+
+  return {
+    add,
+    update,
+    access,
+    setLimit,
+  };
+}