handle size calculation of background fetches, add test

Re: #257

index.js

@@ -379,6 +379,11 @@ class LRUCache {
       this.sizes[index] = 0
     }
     this.requireSize = (k, v, size, sizeCalculation) => {
+      // provisionally accept background fetches.
+      // actual value size will be checked when they return.
+      if (this.isBackgroundFetch(v)) {
+        return 0
+      }
       if (!isPosInt(size)) {
         if (sizeCalculation) {
           if (typeof sizeCalculation !== 'function') {
@@ -588,6 +593,9 @@ class LRUCache {
     // if the item doesn't fit, don't do anything
     // NB: maxEntrySize set to maxSize by default
     if (this.maxEntrySize && size > this.maxEntrySize) {
+      // have to delete, in case a background fetch is there already.
+      // in non-async cases, this is a no-op
+      this.delete(k)
       return this
     }
     let index = this.size === 0 ? undefined : this.keyMap.get(k)

test/size-calculation.ts

@@ -242,3 +242,19 @@ t.test('large item falls out of cache because maxEntrySize', t => {
 
   t.end()
 })
+
+t.test('maxEntrySize, no maxSize', async t => {
+  const c = new LRU<number, string>({
+    max: 10,
+    maxEntrySize: 10,
+    sizeCalculation: s => s.length,
+    fetchMethod: async n => 'x'.repeat(n),
+  })
+  t.equal(await c.fetch(2), 'xx')
+  t.equal(c.size, 1)
+  t.equal(await c.fetch(3), 'xxx')
+  t.equal(c.size, 2)
+  t.equal(await c.fetch(11), 'x'.repeat(11))
+  t.equal(c.size, 2)
+  t.equal(c.has(11), false)
+})