Firestore transaction fails with "Transaction failed all retries"

Question

I'm running a very simple Firestore transaction which checks for the presence of a document, before writing to it if absent.

(The use case is registering a username - if it's not already registered, the current user gets to grab it)

Here's a snippet of the relevant Flutter code:

DocumentReference usernameDocRef =
    Firestore.instance.collection(_USERNAMES).document(username);

await Firestore.instance.runTransaction((transaction) async {
  var snapshot = await transaction.get(usernameDocRef);
  if (!snapshot.exists) {
    transaction.set(usernameDocRef, {
      _UsernamesKey.userid: _user.id,
    });
  }
});

This is failing with an exception "Transaction failed all retries".

Based on the Firestore documentation, failure can occur for two reasons:

• The transaction contains read operations after write operations. Read operations must always come before any write operations.
• The transaction read a document that was modified outside of the transaction. In this case, the transaction automatically runs again. The transaction is retried a finite number of times.

I don't think I trigger either of those. Any suggestions?

Solution

This has recently been fixed - https://github.com/flutter/plugins/pull/1206.

If you are using the master channel, the fix should be available already. For the other channels (dev, beta, stable) YMMV.