Database once() should only fire once

.changeset/many-turtles-try.md

@@ -0,0 +1,5 @@
+---
+"@firebase/database": patch
+---
+
+Fixed an issue that could cause `once()` to fire more than once if the value was modified inside its callback.

packages/database/src/api/Reference.ts

@@ -326,7 +326,7 @@ export class Query implements Compat<ExpQuery> {
     validateCallback('Query.once', 'callback', callback, true);
 
     const ret = Query.getCancelAndContextArgs_(
-      'Query.on',
+      'Query.once',
       failureCallbackOrContext,
       context
     );

packages/database/src/exp/Reference_impl.ts

@@ -959,8 +959,8 @@ function addEventListener(
   if (options && options.onlyOnce) {
     const userCallback = callback;
     const onceCallback: UserCallback = (dataSnapshot, previousChildName) => {
-      userCallback(dataSnapshot, previousChildName);
       repoRemoveEventCallbackForQuery(query._repo, query, container);
+      userCallback(dataSnapshot, previousChildName);
     };
     onceCallback.userCallback = callback.userCallback;
     onceCallback.context = callback.context;

packages/database/test/helpers/EventAccumulator.ts

@@ -27,6 +27,23 @@ export const EventAccumulatorFactory = {
       count++;
     });
     return ea;
+  },
+  waitsForExactCount: maxCount => {
+    let count = 0;
+    const condition = () => {
+      if (count > maxCount) {
+        throw new Error('Received more events than expected');
+      }
+      return count === maxCount;
+    };
+    const ea = new EventAccumulator(condition);
+    ea.onReset(() => {
+      count = 0;
+    });
+    ea.onEvent(() => {
+      count++;
+    });
+    return ea;
   }
 };
 

packages/database/test/query.test.ts

@@ -2330,6 +2330,32 @@ describe('Query Tests', () => {
     await ea.promise;
   });
 
+  it('Query.once() only fires once', async () => {
+    const node = getRandomNode() as Reference;
+
+    let count = 1;
+    node.set(count);
+
+    const valueEvent = EventAccumulatorFactory.waitsForCount(3);
+    node.on('value', () => {
+      if (count < 3) {
+        ++count;
+        node.set(count);
+      }
+      valueEvent.addEvent();
+    });
+
+    const onceEvent = EventAccumulatorFactory.waitsForExactCount(1);
+    node.once('value', () => {
+      ++count;
+      node.set(count);
+      onceEvent.addEvent();
+    });
+
+    await valueEvent.promise;
+    await onceEvent.promise;
+  });
+
   it('Ensure on() returns callback function.', () => {
     const node = getRandomNode() as Reference;
     const callback = function () {};