Adjust collector to use separate connection per scrape (prometheus-community#931)

Fixes prometheus-community#921 Signed-off-by: Joe Adams <github@joeadams.io>

Author: sysadmind

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 {