[dbnode] Only extend reverse index retention up (#2745)

Author: linasm

src/dbnode/storage/index.go

@@ -2181,7 +2181,9 @@ func (i *nsIndex) SetExtendedRetentionPeriod(period time.Duration) {
 	i.state.Lock()
 	defer i.state.Unlock()
 
-	i.extendedRetentionPeriod = period
+	if period > i.extendedRetentionPeriod {
+		i.extendedRetentionPeriod = period
+	}
 }
 
 func (i *nsIndex) effectiveRetentionPeriodWithLock() time.Duration {

src/dbnode/storage/index_test.go

@@ -391,9 +391,9 @@ func TestNamespaceIndexSetExtendedRetentionPeriod(t *testing.T) {
 	idx.SetExtendedRetentionPeriod(longerRetention)
 	assert.Equal(t, longerRetention, idx.effectiveRetentionPeriodWithLock())
 
-	shorterRetention := originalRetention - time.Minute
+	shorterRetention := longerRetention - time.Second
 	idx.SetExtendedRetentionPeriod(shorterRetention)
-	assert.Equal(t, originalRetention, idx.effectiveRetentionPeriodWithLock())
+	assert.Equal(t, longerRetention, idx.effectiveRetentionPeriodWithLock())
 }
 
 func verifyFlushForShards(