How to abort a Firestore transaction in case of error

Question

I'm using Firestore runTransaction method in a https Cloud function (running Express). What I want is ensure that if any of the read or write fails the other transaction reads or writes won't run or will rollback if needed.

Transaction

admin.firestore().runTransaction(async (t) => {
  const { uid, artworkUid, tagLabel } = req.body;

  if (!tagLabel) {
    return Promise.reject('Missing parameters: "tagLabel"');
  }

  if (!artworkUid) {
    return Promise.reject('Missing parameters: "artworkUid"');
  }

  if (tagLabel.length < 3) {
    return Promise.reject('The tag must be at least 3 characters long');
  }

  const [ user, artwork ] = await Promise.all([
    getUser(uid),
    getArtwork(artworkUid)
  ]);

  return Promise.all([
    addArtworkTag({
      artwork,
      tagLabel,
      proposer: user
    }, t),
    giveXpFor({
      user,
      action: 'add-artwork-tags',
      type: user.can('add-artwork-tags') ? 'effective' : 'temporary'
    }, t)
  ])
})
.catch(err => res.status(403).send(err))
.then(() => res.status(200).send());

As you can see the addArtworkTag and giveXpFor functions take the t in parameter.

addArtworkTag

export const addArtworkTag = function(params: { artwork: any, tagLabel: string, proposer: ShadraUser }, t?: FirebaseFirestore.Transaction): Promise<any> {
  const { artwork, tagLabel, proposer } = params;

  if (!artwork || !proposer || typeof tagLabel !== 'string') {
    return Promise.reject('Can\'t add the tag. Bad/missing datas');
  }

  const tag = <any>{
    slug: slugify(tagLabel),
    label: tagLabel,
    status: proposer.can('add-artwork-tags') ? 'validated' : 'proposed',
    proposerUid: proposer.uid
  };

  const tags = artwork.tags || [];
  tags.push(tag);

  const artworkRef = admin.firestore().doc(`artworks/${artwork.uid}`);

  t.set(artworkRef, { tags }, { merge: true });
  return Promise.resolve();
}

My question is: If the addArtworkTag function fails (because of a bad parameter for instance) how can I abort (or even rollback) the transaction so giveXpFor is not called

Thanks a lot

PS: I think I've misused transactions... What I should do is probably just use sequential promises instead of Promise.all, right ?

Answer 1

The API documentation for runTransaction states:

If the transaction completed successfully or was explicitly aborted (by the updateFunction returning a failed Promise), the Promise returned by the updateFunction will be returned here. Else if the transaction failed, a rejected Promise with the corresponding failure error will be returned.

All you have to do the fail a transaction correctly is return a rejected promise. It doesn't matter what you've done in that transaction - all of it will be rolled back.