fix: skip autovacuum metric when xact_start is NULL (#1381)

The stat_activity_autovacuum collector scanned xact_start into a plain
float64, so a NULL value (possible for autovacuum workers) failed the
whole collector with "converting NULL to float64 is unsupported". Scan
into sql.NullFloat64 and skip the row when it is NULL, matching the
pg_stat_walreceiver / pg_stat_database collectors.

Fixes #952

Signed-off-by: Dean Chen <862469039@qq.com>

collector/pg_stat_activity_autovacuum.go

@@ -15,6 +15,7 @@ package collector
 
 import (
 	"context"
+	"database/sql"
 	"log/slog"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -51,7 +52,7 @@ var (
 	`
 )
 
-func (PGStatActivityAutovacuumCollector) Update(ctx context.Context, instance *instance, ch chan<- prometheus.Metric) error {
+func (c PGStatActivityAutovacuumCollector) Update(ctx context.Context, instance *instance, ch chan<- prometheus.Metric) error {
 	db := instance.getDB()
 	rows, err := db.QueryContext(ctx,
 		statActivityAutovacuumQuery)
@@ -63,16 +64,21 @@ func (PGStatActivityAutovacuumCollector) Update(ctx context.Context, instance *i
 
 	for rows.Next() {
 		var relname string
-		var ageInSeconds float64
+		var ageInSeconds sql.NullFloat64
 
 		if err := rows.Scan(&relname, &ageInSeconds); err != nil {
 			return err
 		}
 
+		if !ageInSeconds.Valid {
+			c.log.Debug("Skipping collecting metric because it has no xact_start")
+			continue
+		}
+
 		ch <- prometheus.MustNewConstMetric(
 			statActivityAutovacuumAgeInSeconds,
 			prometheus.GaugeValue,
-			ageInSeconds, relname,
+			ageInSeconds.Float64, relname,
 		)
 	}
 	if err := rows.Err(); err != nil {

collector/pg_stat_activity_autovacuum_test.go

@@ -19,6 +19,7 @@ import (
 	"github.com/DATA-DOG/go-sqlmock"
 	"github.com/prometheus/client_golang/prometheus"
 	dto "github.com/prometheus/client_model/go"
+	"github.com/prometheus/common/promslog"
 	"github.com/smartystreets/goconvey/convey"
 )
 
@@ -60,3 +61,43 @@ func TestPGStatActivityAutovacuumCollector(t *testing.T) {
 		t.Errorf("there were unfulfilled exceptions: %s", err)
 	}
 }
+
+func TestPGStatActivityAutovacuumCollectorNullTimestamp(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{
+		"relname",
+		"timestamp_seconds",
+	}
+	rows := sqlmock.NewRows(columns).
+		AddRow("no_xact_start", nil).
+		AddRow("test", 3600)
+
+	mock.ExpectQuery(sanitizeQuery(statActivityAutovacuumQuery)).WillReturnRows(rows)
+
+	ch := make(chan prometheus.Metric)
+	go func() {
+		defer close(ch)
+		c := PGStatActivityAutovacuumCollector{log: promslog.NewNopLogger()}
+
+		if err := c.Update(context.Background(), inst, ch); err != nil {
+			t.Errorf("Error calling PGStatActivityAutovacuumCollector.Update: %s", err)
+		}
+	}()
+	expected := []MetricResult{
+		{labels: labelMap{"relname": "test"}, value: 3600, 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)
+	}
+}