Adjust collector to use separate connection per scrape

Fixes prometheus-community#921

Signed-off-by: Joe Adams <github@joeadams.io>

collector/collector.go

@@ -167,19 +167,22 @@ func (p PostgresCollector) Describe(ch chan<- *prometheus.Desc) {
 func (p PostgresCollector) Collect(ch chan<- prometheus.Metric) {
 	ctx := context.TODO()
 
+	// copy the instance so that concurrent scrapes have independent instances
+	inst := p.instance.copy()
+
 	// Set up the database connection for the collector.
-	err := p.instance.setup()
+	err := inst.setup()
 	if err != nil {
 		level.Error(p.logger).Log("msg", "Error opening connection to database", "err", err)
 		return
 	}
-	defer p.instance.Close()
+	defer inst.Close()
 
 	wg := sync.WaitGroup{}
 	wg.Add(len(p.Collectors))
 	for name, c := range p.Collectors {
 		go func(name string, c Collector) {
-			execute(ctx, name, c, p.instance, ch, p.logger)
+			execute(ctx, name, c, inst, ch, p.logger)
 			wg.Done()
 		}(name, c)
 	}

collector/instance.go

@@ -43,6 +43,13 @@ func newInstance(dsn string) (*instance, error) {
 	return i, nil
 }
 
+// copy returns a copy of the instance.
+func (i *instance) copy() *instance {
+	return &instance{
+		dsn: i.dsn,
+	}
+}
+
 func (i *instance) setup() error {
 	db, err := sql.Open("postgres", i.dsn)
 	if err != nil {