add integration test

Author: milaGGL

packages/firestore/test/integration/api/query.test.ts

@@ -47,6 +47,7 @@ import {
   Query,
   query,
   QuerySnapshot,
+  runTransaction,
   setDoc,
   startAfter,
   startAt,
@@ -1614,6 +1615,30 @@ apiDescribe('Queries', (persistence: boolean) => {
       });
     });
   });
+
+  it('can handle existence filter', () => {
+    const testDocs = {};
+    for (let i = 1; i <= 100; i++) {
+      Object.assign(testDocs, { ['doc' + i]: { key: i } });
+    }
+    return withTestCollection(persistence, testDocs, async (coll, db) => {
+      const snapshot1 = await getDocs(coll);
+      expect(snapshot1.size).to.equal(100);
+      // Delete 50 docs in transaction so that it doesn't affect local cache.
+      await runTransaction(db, async txn => {
+        for (let i = 1; i <= 50; i++) {
+          txn.delete(doc(coll, 'doc' + i));
+        }
+      });
+      // Wait 10 seconds, during which the watch will stop tracking the query
+      // and will send an existence filter rather than "delete" events.
+      await (function () {
+        return new Promise(resolve => setTimeout(resolve, 10000));
+      })();
+      const snapshot2 = await getDocs(coll);
+      expect(snapshot2.size).to.equal(50);
+    });
+  }).timeout('15s');
 });
 
 function verifyDocumentChange<T>(