dynamoevents: Fix panic if cursor is outside of [fromUTC,toUTC] range (…

…#42247)

If clients send a request where the cursor falls outside of the [From.UTC,To.UTC] interval, dynamodbevents will panic after consuming all possible values in `dates`.
This happens because `dates` is an array of full days between
`[from,to]`.

The patched for loop tried to discard days that were already consumed by
the latest request but if the cursor fall outside of this window, the
loop consumed all days and eventually panic.

Signed-off-by: Tiago Silva <tiago.silva@goteleport.com>

lib/events/dynamoevents/dynamoevents.go

@@ -648,6 +648,17 @@ func (l *Log) searchEventsRaw(ctx context.Context, fromUTC, toUTC time.Time, nam
 		return nil, "", trace.Wrap(err)
 	}
 
+	if checkpoint.Date != "" {
+		if t, err := time.Parse(time.DateOnly, checkpoint.Date); err == nil {
+			d := fromUTC.Unix()
+			// if fromUTC at 00:00:00 is bigger than the cursor,
+			// reset the cursor and advance to next day.
+			if time.Unix(d-d%(24*3600), 0).After(t) {
+				checkpoint = checkpointKey{}
+			}
+		}
+	}
+
 	totalSize := 0
 	dates := daysBetween(fromUTC, toUTC)
 	if order == types.EventOrderDescending {
@@ -668,9 +679,15 @@ func (l *Log) searchEventsRaw(ctx context.Context, fromUTC, toUTC time.Time, nam
 	// We need to perform a guard check on the length of `dates` here in case a query is submitted with
 	// `toUTC` occurring before `fromUTC`.
 	if checkpoint.Date != "" && len(dates) > 0 {
-		for dates[0] != checkpoint.Date {
+		for len(dates) > 0 && dates[0] != checkpoint.Date {
 			dates = dates[1:]
 		}
+		// if the initial data wasn't found in [fromUTC,toUTC]
+		// dates will be empty and we can return early since we
+		// won't find any events.
+		if len(dates) == 0 {
+			return nil, "", nil
+		}
 	}
 
 	foundStart := checkpoint.EventKey == ""

lib/events/dynamoevents/dynamoevents_test.go

@@ -20,6 +20,7 @@ package dynamoevents
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"math/rand"
 	"net/url"
@@ -116,6 +117,32 @@ func TestSearchSessionEvensBySessionID(t *testing.T) {
 	tt.suite.SearchSessionEventsBySessionID(t)
 }
 
+// TestCheckpointOutsideOfWindow tests if [Log] doesn't panic
+// if checkpoint date is outside of the window [fromUTC,toUTC].
+func TestCheckpointOutsideOfWindow(t *testing.T) {
+	tt := &Log{}
+
+	key := checkpointKey{
+		Date: "2022-10-02",
+	}
+	keyB, err := json.Marshal(key)
+	require.NoError(t, err)
+
+	results, nextKey, err := tt.SearchEvents(
+		context.Background(),
+		events.SearchEventsRequest{
+			From:     time.Date(2021, 10, 10, 0, 0, 0, 0, time.UTC),
+			To:       time.Date(2021, 11, 10, 0, 0, 0, 0, time.UTC),
+			Limit:    100,
+			StartKey: string(keyB),
+			Order:    types.EventOrderAscending,
+		},
+	)
+	require.NoError(t, err)
+	require.Empty(t, results)
+	require.Empty(t, nextKey)
+}
+
 func TestSizeBreak(t *testing.T) {
 	tt := setupDynamoContext(t)