Add test for postmaster

Signed-off-by: Felix Yuan <felix.yuan@reddit.com>

Author: Sticksman

collector/pg_postmaster.go

@@ -37,11 +37,11 @@ var pgPostMasterStartTimeSeconds = prometheus.NewDesc(
 []string{}, nil,
 )
 
+var pgPostmasterQuery = "SELECT pg_postmaster_start_time from pg_postmaster_start_time();"
+
 func (c *PGPostmasterCollector) Update(ctx context.Context, db *sql.DB, ch chan<- prometheus.Metric) error {
 row := db.QueryRowContext(ctx,
-`SELECT
-pg_postmaster_start_time
-from pg_postmaster_start_time();`)
+pgPostmasterQuery)
 
 var startTimeSeconds float64
 err := row.Scan(&startTimeSeconds)

collector/pg_postmaster_test.go

@@ -0,0 +1,45 @@
+package collector
+
+import (
+"context"
+"testing"
+
+"github.com/DATA-DOG/go-sqlmock"
+"github.com/prometheus/client_golang/prometheus"
+dto "github.com/prometheus/client_model/go"
+"github.com/smartystreets/goconvey/convey"
+)
+
+func TestPgPostmasterCollector(t *testing.T) {
+db, mock, err := sqlmock.New()
+if err != nil {
+t.Fatalf("Error opening a stub db connection: %s", err)
+}
+defer db.Close()
+
+mock.ExpectQuery(sanitizeQuery(pgPostmasterQuery)).WillReturnRows(sqlmock.NewRows([]string{"pg_postmaster_start_time"}).
+AddRow(1685739904))
+
+ch := make(chan prometheus.Metric)
+go func() {
+defer close(ch)
+c := PGPostmasterCollector{}
+
+if err := c.Update(context.Background(), db, ch); err != nil {
+t.Errorf("Error calling PGPostmasterCollector.Update: %s", err)
+}
+}()
+
+expected := []MetricResult{
+{labels: labelMap{}, value: 1685739904, 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)
+}
+}