[bugfix beta] Fetch cancels unload

When fetching records, cancel internal model destruction if it is
scheduled, ie if the model is dematerializing because it was unloaded.

It is not possible to construct snapshots for dematerializing internal
models.  Prior to this commit, this could happen when fetching a record
in the same runloop that it was unloaded.  A straightfoward way of
getting into this state was via

```
store.findRecord('book', 1).then(b => b.unloadRecord)
```

when the model is already cached in the store.  Under these conditions,
the fetch is scheduled, then the promise fulfills with the cached record
and is unloaded and *then* the scheduled fetch is flushed.

[fix #5343]

addon/-private/system/store.js

@@ -793,6 +793,10 @@ Store = Service.extend({
     assert(`You tried to find a record but you have no adapter (for ${modelName})`, adapter);
     assert(`You tried to find a record but your adapter (for ${modelName}) does not implement 'findRecord'`, typeof adapter.findRecord === 'function');
 
+    if (internalModel.hasScheduledDestroy()) {
+      internalModel.cancelDestroy();
+    }
+
     return _find(adapter, this, internalModel.type, internalModel.id, internalModel, options);
   },
 
@@ -921,6 +925,9 @@ Store = Service.extend({
       // will once again convert the records to snapshots for adapter.findMany()
       let snapshots = new Array(totalItems);
       for (let i = 0; i < totalItems; i++) {
+        if (internalModels[i].hasScheduledDestroy()) {
+          internalModels[i].cancelDestroy();
+        }
         snapshots[i] = internalModels[i].createSnapshot();
       }
 

tests/integration/records/unload-test.js

@@ -2008,3 +2008,31 @@ test('1 sync : many async unload sync side', function(assert) {
     })
   );
 });
+
+test('fetching records cancels unloading', function(assert) {
+  env.adapter.findRecord = (store, type, id) => {
+    assert.equal(type, Person, 'findRecord(_, type) is correct');
+    assert.deepEqual(id, '1', 'findRecord(_, _, id) is correct');
+
+    return  {
+      data: {
+        id: 1,
+        type: 'person'
+      }
+    }
+  };
+
+  run(() =>
+    env.store.push({
+      data: {
+        id: 1,
+        type: 'person'
+      }
+    })
+  );
+
+  return run(() =>
+    env.store.findRecord('person', 1, { backgroundReload: true })
+      .then(person => person.unloadRecord())
+  );
+});