Fix pg_up never being set to 1 (introduced by #162).

Correct fix for #160.

cmd/postgres_exporter/postgres_exporter.go

@@ -20,6 +20,7 @@ import (
 	"gopkg.in/yaml.v2"
 
 	"crypto/sha256"
+
 	"github.com/blang/semver"
 	_ "github.com/lib/pq"
 	"github.com/prometheus/client_golang/prometheus"
@@ -971,10 +972,6 @@ func (e *Exporter) getDB(conn string) (*sql.DB, error) {
 		if err != nil {
 			return nil, err
 		}
-		err = d.Ping()
-		if err != nil {
-			return nil, err
-		}
 
 		d.SetMaxOpenConns(1)
 		d.SetMaxIdleConns(1)
@@ -983,6 +980,15 @@ func (e *Exporter) getDB(conn string) (*sql.DB, error) {
 		log.Infoln("Established new database connection.")
 	}
 
+	// Always send a ping and possibly invalidate the connection if it fails
+	if err := e.dbConnection.Ping(); err != nil {
+		cerr := e.dbConnection.Close()
+		log.Infoln("Error while closing non-pinging DB connection:", cerr)
+		e.dbConnection = nil
+		e.psqlUp.Set(0)
+		return nil, err
+	}
+
 	return e.dbConnection, nil
 }
 
@@ -1007,11 +1013,14 @@ func (e *Exporter) scrape(ch chan<- prometheus.Metric) {
 			loggableDsn = pDsn.String()
 		}
 		log.Infof("Error opening connection to database (%s): %s", loggableDsn, err)
+		e.psqlUp.Set(0)
 		e.error.Set(1)
-		e.psqlUp.Set(0) // Force "up" to 0 here.
 		return
 	}
 
+	// Didn't fail, can mark connection as up for this scrape.
+	e.psqlUp.Set(1)
+
 	// Check if map versions need to be updated
 	if err := e.checkMapVersions(ch, db); err != nil {
 		log.Warnln("Proceeding with outdated query maps, as the Postgres version could not be determined:", err)

cmd/postgres_exporter/tests/test-smoke

@@ -108,6 +108,13 @@ smoketest_postgres() {
         exit 1
     fi
 
+    # HACK test: check pg_up is a 1 - TODO: expand integration tests to include metric consumption
+    if ! grep 'pg_up.* 1' $METRICS_DIR/.metrics.single.$version.prom ; then
+        echo "pg_up metric was not 1 despite exporter and database being up"
+        kill $exporter_pid
+        exit 1
+    fi
+
     kill $exporter_pid
     docker kill $CONTAINER_NAME
     docker rm -v $CONTAINER_NAME