Fix transaction.set() failure without retry on "already-exists" error

.changeset/young-knives-shake.md

@@ -0,0 +1,6 @@
+---
+'@firebase/firestore': patch
+'firebase': patch
+---
+
+Fix transaction.set() failure without retry on "already-exists" error.

packages/firestore/src/core/transaction_runner.ts

@@ -120,6 +120,7 @@ export class TransactionRunner<T> {
       return (
         code === 'aborted' ||
         code === 'failed-precondition' ||
+        code === 'already-exists' ||
         !isPermanentError(code)
       );
     }

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

@@ -629,6 +629,30 @@ apiDescribe('Database transactions', (persistence: boolean) => {
     });
   });
 
+  it('retries when document already exists', () => {
+    return withTestDb(persistence, async db => {
+      let retryCounter = 0;
+      const docRef = doc(collection(db, 'nonexistent'));
+
+      await runTransaction(db, async transaction => {
+        ++retryCounter;
+        const snap = await transaction.get(docRef);
+
+        if (retryCounter === 1) {
+          expect(snap.exists()).to.be.false;
+          // On the first attempt, create a doc before transaction.set(), so that
+          // the transaction fails with "already-exists" error, and retries.
+          await setDoc(docRef, { count: 1 });
+        }
+
+        transaction.set(docRef, { count: 2 });
+      });
+      expect(retryCounter).to.equal(2);
+      const snap = await getDoc(docRef);
+      expect(snap.get('count')).to.equal(2);
+    });
+  });
+
   it('are successful with no transaction operations', () => {
     return withTestDb(persistence, db => runTransaction(db, async () => {}));
   });