fix: update endsAt when resolving alerts via API

When an alert is posted via the API with only endsAt set (to resolve
an existing alert), the endsAt field was not being updated due to two
issues:

1. The merge condition in provider/mem/mem.go only merged alerts when
   there was an overlap in activity range. This prevented merging when
   resolving an alert (where the new endsAt is before the old endsAt
   timeout).

2. The Merge function in types/types.go didn't properly handle the case
   where a new resolved alert is merged with an existing unresolved alert.

This fix:
- Updates the merge condition to also merge when updating an existing
  alert (same fingerprint), allowing updates like setting endsAt to
  resolve an alert even without overlap.
- Updates the Merge function to always use the new alert's EndsAt when
  it is resolved, ensuring that resolving an alert via API properly
  updates the endsAt timestamp.

Fixes #4554

Signed-off-by: tennisleng <tennisleng@users.noreply.github.com>

Author: tennisleng

provider/mem/mem.go

@@ -242,9 +242,17 @@ func (a *Alerts) Put(alerts ...*types.Alert) error {
 		if old, err := a.alerts.Get(fp); err == nil {
 			existing = true
 
-			// Merge alerts if there is an overlap in activity range.
-			if (alert.EndsAt.After(old.StartsAt) && alert.EndsAt.Before(old.EndsAt)) ||
-				(alert.StartsAt.After(old.StartsAt) && alert.StartsAt.Before(old.EndsAt)) {
+			// Merge alerts if there is an overlap in activity range, or if
+			// the new alert is updating the existing alert (e.g., setting endsAt
+			// to resolve it, or updating annotations/labels).
+			hasOverlap := (alert.EndsAt.After(old.StartsAt) && alert.EndsAt.Before(old.EndsAt)) ||
+				(alert.StartsAt.After(old.StartsAt) && alert.StartsAt.Before(old.EndsAt))
+			
+			// Also merge if updating an existing alert (same fingerprint) to allow
+			// updates like setting endsAt to resolve an alert, even without overlap.
+			isUpdate := !alert.UpdatedAt.Before(old.UpdatedAt)
+			
+			if hasOverlap || isUpdate {
 				alert = old.Merge(alert)
 			}
 		}

types/types.go

@@ -453,6 +453,9 @@ func (a *Alert) Merge(o *Alert) *Alert {
 	}
 
 	if o.Resolved() {
+		// If the new alert is resolved, use its EndsAt timestamp.
+		// This handles the case where an alert is being resolved via API.
+		res.EndsAt = o.EndsAt
 		// The latest explicit resolved timestamp wins if both alerts are effectively resolved.
 		if a.Resolved() && a.EndsAt.After(o.EndsAt) {
 			res.EndsAt = a.EndsAt