Fix posting Slack message when route is deleted (#3702)

# What this PR does

Fixes https://github.com/grafana/oncall/issues/3646

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Fixed Webhooks UI not allowing simple webhooks to be created ([#3691](https://github.com/grafana/oncall/pull/3691))
+- Fix posting Slack message when route is deleted by @vadimkerr ([#3702](https://github.com/grafana/oncall/pull/3702))
 
 ## v1.3.88 (2024-01-16)
 

engine/apps/slack/scenarios/distribute_alerts.py

@@ -76,7 +76,12 @@ class AlertShootingStep(scenario_step.ScenarioStep):
 
         if num_updated_rows == 1:
             try:
-                channel_id = alert.group.channel_filter.slack_channel_id_or_general_log_id
+                channel_id = (
+                    alert.group.channel_filter.slack_channel_id_or_general_log_id
+                    if alert.group.channel_filter
+                    # if channel filter is deleted mid escalation, use default Slack channel
+                    else alert.group.channel.organization.general_log_channel_id
+                )
                 self._send_first_alert(alert, channel_id)
             except SlackAPIError:
                 AlertGroup.objects.filter(pk=alert.group.pk).update(slack_message_sent=False)

engine/apps/slack/tests/test_scenario_steps/test_distribute_alerts.py

@@ -5,6 +5,7 @@ import pytest
 from apps.alerts.models import AlertGroup
 from apps.slack.errors import SlackAPIRestrictedActionError
 from apps.slack.models import SlackMessage
+from apps.slack.scenarios.distribute_alerts import AlertShootingStep
 from apps.slack.scenarios.scenario_step import ScenarioStep
 from apps.slack.tests.conftest import build_slack_response
 
@@ -36,3 +37,30 @@ def test_restricted_action_error(
     assert alert_group.slack_message is None
     assert SlackMessage.objects.count() == 0
     assert not alert.delivered
+
+
+@patch.object(AlertShootingStep, "_post_alert_group_to_slack")
+@pytest.mark.django_db
+def test_alert_shooting_no_channel_filter(
+    mock_post_alert_group_to_slack,
+    make_slack_team_identity,
+    make_organization,
+    make_alert_receive_channel,
+    make_alert_group,
+    make_alert,
+):
+    slack_team_identity = make_slack_team_identity()
+    organization = make_organization(
+        slack_team_identity=slack_team_identity, general_log_channel_id="DEFAULT_CHANNEL_ID"
+    )
+    alert_receive_channel = make_alert_receive_channel(organization)
+
+    # simulate an alert group with channel filter deleted in the middle of the escalation
+    alert_group = make_alert_group(alert_receive_channel, channel_filter=None)
+    alert = make_alert(alert_group, raw_request_data={})
+
+    step = AlertShootingStep(slack_team_identity, organization)
+    step.process_signal(alert)
+
+    mock_post_alert_group_to_slack.assert_called_once()
+    assert mock_post_alert_group_to_slack.call_args[1]["channel_id"] == "DEFAULT_CHANNEL_ID"