Fixes T12844. This code is misleading: the daemon insert is happening on a different connection, and is not inside the transaction on the Mail connection.
What actually happens is this:
- (Connection A) BEGIN
- (Connection A) INSERT INTO mail ...
- (Connection B) INSERT INTO worker ... <-- This is a different connection, and it is NOT in a transaction!
- There's a race window here: the worker row is globally visible but the mail row is still isolated inside the transaction.
- (Connection A) COMMIT
- Now we're clear: the mail row is globally visible.
Change this code to reflect what's actually happening.
This means that if the worker row insert fails for some reason, we'll now throw with a mail row written to the database. But this is fine: it doesn't send on its own (so it can't cause mail loops or anything) and it can be re-queued with bin/mail resend if necessary without too much trouble.