Preserve parent tiles in the source cache

src/source/source_cache.js

@@ -262,7 +262,7 @@ class SourceCache extends Evented {
             }
             if (this._cache.has(coord.id)) {
                 retain[coord.id] = true;
-                return this._cache.get(coord.id);
+                return this._cache.getWithoutRemoving(coord.id);
             }
         }
     }

src/util/lru_cache.js

@@ -110,6 +110,21 @@ class LRUCache<T> {
         return data;
     }
 
+    /**
+     * Get the value attached to a specific key without removing data
+     * from the cache. If the key is not found, returns `null`
+     *
+     * @param {string} key the key to look up
+     * @returns {*} the data, or null if it isn't found
+     * @private
+     */
+    getWithoutRemoving(key: string): ?T {
+        if (!this.has(key)) { return null; }
+
+        const data = this.data[key];
+        return data;
+    }
+
     /**
      * Remove a key/value combination from the cache.
      *

test/unit/source/source_cache.test.js

@@ -871,7 +871,7 @@ test('SourceCache#findLoadedParent', (t) => {
         t.equal(sourceCache.findLoadedParent(new TileCoord(2, 3, 3), 0, retain), undefined);
         t.equal(sourceCache.findLoadedParent(new TileCoord(2, 0, 0), 0, retain), tile);
         t.deepEqual(retain, expectedRetain);
-        t.equal(sourceCache._cache.order.length, 0);
+        t.equal(sourceCache._cache.order.length, 1);
 
         t.end();
     });

test/unit/util/lru_cache.test.js

@@ -18,6 +18,16 @@ test('LRUCache', (t) => {
     t.end();
 });
 
+test('LRUCache - getWithoutRemoving', (t) => {
+    const cache = new LRUCache(10, () => {
+        t.fail();
+    });
+    t.equal(cache.add('washington', 'dc'), cache, '.add()');
+    t.equal(cache.getWithoutRemoving('washington'), 'dc', '.getWithoutRemoving()');
+    t.deepEqual(cache.keys(), ['washington'], '.keys()');
+    t.end();
+});
+
 test('LRUCache - duplicate add', (t) => {
     const cache = new LRUCache(10, () => {
         t.fail();