potiuk opened a new pull request, #69877:
URL: https://github.com/apache/airflow/pull/69877
## What
Restore the ability to deliver `email_on_failure` / `email_on_retry` task
alerts through a **pluggable backend** (SES, SendGrid, or any custom notifier),
which regressed in #57354.
## Why
`email_on_failure` / `email_on_retry` are deprecated (slated for Airflow 4,
migrate to `SmtpNotifier`) but still functional. Since #57354, task
failure/retry emails are routed **unconditionally** through `SmtpNotifier`,
silently ignoring the `[email] email_backend` configuration. Deployments using
a custom `email_backend` (Amazon SES, SendGrid, org-internal) stopped receiving
failure/retry alerts even though the config option still exists and the
deprecated params still work. This restores that behaviour while keeping the
single-notifier design #57354 introduced.
## How
`_send_error_email_notification` (shared by both the worker task-runner path
and the DAG-processor callback path) now resolves the notifier by precedence:
1. **`[email] email_notifier`** (new option, default `SmtpNotifier`) — any
`BaseNotifier` whose constructor accepts `to` / `from_email` / `subject` /
`html_content`.
2. **Legacy `[email] email_backend`** (non-default) — transparently wrapped
in a new `LegacyEmailBackendNotifier` (added to
`apache-airflow-providers-common-compat`) so existing SES/SendGrid setups keep
working unchanged. The backend is loaded lazily from config at notify time, so
the Task SDK keeps no static dependency on `airflow.utils.email`.
3. Otherwise the default `SmtpNotifier`.
Because both failure-email entry points funnel through the same function,
the selected backend is used **consistently** regardless of how the task failed
(normal failure vs. zombie/killed).
## Tests
- `TestEmailNotifications` (task-sdk): default still uses `SmtpNotifier`
(unchanged regression tests), custom `email_notifier` selected, legacy
`email_backend` wrapped and invoked, explicit `email_notifier` overrides legacy
backend, unresolvable notifier logged (no raise).
- `TestExecuteEmailCallbacks` (DAG-processor path): unchanged and green —
confirms both entry points stay consistent.
- `test_notifier.py` (common.compat): unit tests for
`LegacyEmailBackendNotifier` dispatch.
## Notes
- The deprecated `email_on_*` params are **not** un-deprecated; this only
keeps their existing behaviour pluggable until removal in Airflow 4.
- No newsfragment yet — will add `{PR_NUMBER}.improvement.rst` once this PR
number is known.
---
##### Was generative AI tooling used to co-author this PR?
- [X] Yes — Claude Code (Opus 4.8)
Generated-by: Claude Code (Opus 4.8) following [the
guidelines](https://github.com/apache/airflow/blob/main/contributing-docs/05_pull_requests.rst#gen-ai-assisted-contributions)
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