don't measure archive lag on replicas

collector/pg_stat_archiver_lag.go

@@ -46,7 +46,7 @@ var (
 	statArchiverLagQuery = `
     SELECT
       last_archived_wal,
-      pg_current_wal_lsn() AS current_lsn
+      CASE WHEN pg_is_in_recovery() THEN NULL ELSE pg_current_wal_lsn() END AS current_lsn
     FROM pg_stat_archiver
     WHERE last_archived_wal IS NOT NULL
       AND last_archived_wal != ''

collector/pg_stat_archiver_lag_test.go

@@ -263,3 +263,43 @@ func TestBytesBetweenLSN(t *testing.T) {
 		})
 	}
 }
+
+func TestPGStatArchiverLagCollectorReplica(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	if err != nil {
+		t.Fatalf("Error opening a stub db connection: %s", err)
+	}
+	defer db.Close()
+
+	inst := &Instance{db: db}
+
+	columns := []string{"last_archived_wal", "current_lsn"}
+	// Simulate replica where current_lsn is NULL (which is what the query returns when in recovery)
+	rows := sqlmock.NewRows(columns).
+		AddRow("000000010000000000000001", nil)
+	mock.ExpectQuery(sanitizeQuery(statArchiverLagQuery)).WillReturnRows(rows)
+
+	ch := make(chan prometheus.Metric)
+	go func() {
+		defer close(ch)
+		c := PGStatArchiverLagCollector{}
+
+		if err := c.Update(context.Background(), inst, ch); err != nil {
+			t.Errorf("Error calling PGStatArchiverLagCollector.Update: %s", err)
+		}
+	}()
+
+	expected := []MetricResult{
+		{labels: labelMap{}, value: 0, metricType: dto.MetricType_GAUGE},
+	}
+
+	convey.Convey("Metrics comparison", t, func() {
+		for _, expect := range expected {
+			m := readMetric(<-ch)
+			convey.So(expect, convey.ShouldResemble, m)
+		}
+	})
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("there were unfulfilled exceptions: %s", err)
+	}
+}