Alerting: Fix state transition from Recovering back to Alerting (#103286)

What is this feature?

This PR fixes a state transition issue where alerts transitioning from the Recovering state back to the Alerting state incorrectly entered the Pending state first if the rule had a For duration configured.

Why do we need this feature?

When an alert goes from Alerting to Recovering (when using the Keep firing for) and then back to Alerting, the existing logic would incorrectly put the alert into Pending state while it should be alerting and still sending notifications to the Alertmanager.

pkg/services/ngalert/state/manager_test.go

@@ -1201,6 +1201,40 @@ func TestProcessEvalResults(t *testing.T) {
 				},
 			},
 		},
+		{
+			desc:      "normal -> alerting (pending) -> alerting -> normal (recovering) -> alerting when both KeepFiringFor and For are set",
+			alertRule: baseRuleWith(m.WithKeepFiringForNTimes(2), m.WithForNTimes(1)),
+			evalResults: map[time.Time]eval.Results{
+				t1: {
+					newResult(eval.WithState(eval.Normal), eval.WithLabels(labels1)),
+				},
+				t2: {
+					newResult(eval.WithState(eval.Alerting), eval.WithLabels(labels1)),
+				},
+				t3: {
+					newResult(eval.WithState(eval.Alerting), eval.WithLabels(labels1)),
+				},
+				tn(4): {
+					newResult(eval.WithState(eval.Normal), eval.WithLabels(labels1)),
+				},
+				tn(5): {
+					newResult(eval.WithState(eval.Alerting), eval.WithLabels(labels1)),
+				},
+			},
+			expectedAnnotations: 4,
+			expectedStates: []*state.State{
+				{
+					Labels:             labels["system + rule + labels1"],
+					ResultFingerprint:  labels1.Fingerprint(),
+					State:              eval.Alerting,
+					LatestResult:       newEvaluation(tn(5), eval.Alerting),
+					StartsAt:           tn(5),
+					EndsAt:             tn(5).Add(state.ResendDelay * 4),
+					LastEvaluationTime: tn(5),
+					LastSentAt:         util.Pointer(t3),
+				},
+			},
+		},
 	}
 
 	for _, tc := range testCases {