vmalert: always re-evaluate Annotations (#3119)

* vmalert: always re-evaluate Annotations

Previously, Annotations were evaluated only:
1. On alert creating.
2. On alert's value change.

This is premature optimization. It was assumed that since annotations
could contain only text with alert's labels or value - there is no need
in spending resources to re-compile Annotations.

Later, template function `query` was added, which can execute
arbitrary queries and return different results on every evaluation.
So if it was used in annotations, it would be executed only on init
or value change.

Another case when optimization caused an issue - annotations hot reload.
In this case, annotations of the active alert won't change even if Rule's
annotations were changed.

This fix enables Annotations re-evaluation on each iteration to resolve
issues above. It would have some impact on performance, but it is unlikely
it will be noticeable.

Signed-off-by: hagen1778 <roman@victoriametrics.com>

* vmalert: add tp Changelog

Signed-off-by: hagen1778 <roman@victoriametrics.com>

Signed-off-by: hagen1778 <roman@victoriametrics.com>

app/vmalert/alerting.go

@@ -285,15 +285,11 @@ func (ar *AlertingRule) Exec(ctx context.Context, ts time.Time, limit int) ([]pr
 				a.State = notifier.StatePending
 				a.ActiveAt = ts
 			}
-			if a.Value != m.Values[0] {
+			a.Value = m.Values[0]
-				// update Value field with latest value
+			// re-exec template since Value or query can be used in annotations
-				a.Value = m.Values[0]
+			a.Annotations, err = a.ExecTemplate(qFn, ls.origin, ar.Annotations)
-				// and re-exec template since Value can be used
+			if err != nil {
-				// in annotations
+				return nil, err
-				a.Annotations, err = a.ExecTemplate(qFn, ls.origin, ar.Annotations)
-				if err != nil {
-					return nil, err
-				}
 			}
 			continue
 		}

docs/CHANGELOG.md

@@ -24,6 +24,8 @@ The following tip changes can be tested by building VictoriaMetrics components f
 * BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): properly calculate `increase(m[d])` over slow-changing counters with values smaller than 100. Previously [increase](https://docs.victoriametrics.com/MetricsQL.html#increase) could return unexpectedly big results in this case. See [the related issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/962) and [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/3163).
 * BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): ignore empty series when applying [limit_offset](https://docs.victoriametrics.com/MetricsQL.html#limit_offset). It should improve queries with additional filters by value in expressions like `limit_offset(1,1, foo > 1)`.
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): fix `RangeError: Maximum call stack size exceeded` error when the query returns too many data points at `Table` view. See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/3092/files).
+* BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert.html): re-evaluate annotations per each alert evaluation. Previously, annotations were evaluated only on alert's value ch
+ange. This could result in stale annotations in some cases described in [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/3119).
 
 
 ## [v1.79.3](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.79.3)