Fix snapshots when values are added and removed.

src/main/generic/Snapshot.js

@@ -55,10 +55,10 @@ class Snapshot extends Transaction {
         }
         for (const [key, value] of tx._modified) {
             // Continue if we already have the old value for this key.
-            if (this._modified.has(key)) {
+            if (this._modified.has(key) || this._removed.has(key)) {
                 continue;
             }
-            let oldValue = await this.get(key);
+            const oldValue = await this.get(key);
             // If this key is newly introduced,
             // we have to mark it as removed to maintain our state.
             if (!oldValue) {
@@ -70,11 +70,11 @@ class Snapshot extends Transaction {
         }
         for (const key of tx._removed) {
             // Continue if we already have the old value for this key.
-            if (this._modified.has(key)) {
+            if (this._modified.has(key) || this._removed.has(key)) {
                 continue;
             }
             // Removed values have to be remembered.
-            let oldValue = await this.get(key);
+            const oldValue = await this.get(key);
             this._put(key, oldValue);
         }
     }

src/test/generic/Snapshot.spec.js

@@ -58,6 +58,35 @@ describe('Snapshot', () => {
         })().then(done, done.fail);
     });
 
+    it('works with adding and removing', (done) => {
+        (async function () {
+            const snap = objectStore.snapshot();
+            for (let i=0; i<10; ++i) {
+                expect(await snap.get(`key${i}`)).toBe(`value${i}`);
+            }
+
+            // Add an item first.
+            await objectStore.put('something_new', 'test');
+            expect(await objectStore.get('something_new')).toBe('test');
+
+            expect(await snap.get('something_new')).toBe(undefined);
+            let keys = await snap.keys();
+            expect(keys.size).toBe(10);
+            expect(keys.has('something_new')).toBe(false);
+
+            // Then remove it.
+            await objectStore.remove('something_new');
+            expect(await objectStore.get('something_new')).toBe(undefined);
+
+            expect(await snap.get('something_new')).toBe(undefined);
+            keys = await snap.keys();
+            expect(keys.size).toBe(10);
+            expect(keys.has('something_new')).toBe(false);
+
+            await snap.abort();
+        })().then(done, done.fail);
+    });
+
     it('works on unflushed transaction', (done) => {
         (async function () {
             const tx1 = objectStore.transaction();