Get non-cached reports in parallel

app/multitenant/dynamo_collector.go

@@ -240,21 +240,38 @@ func (c *dynamoDBCollector) getCached(reportKeys []string) ([]report.Report, []s
 	return foundReports, missingReports
 }
 
+// Fetch multiple reports in parallel from S3.
 func (c *dynamoDBCollector) getNonCached(reportKeys []string) ([]report.Report, error) {
-	reports := []report.Report{}
+	type result struct {
+		key    string
+		report *report.Report
+		err    error
+	}
+
+	ch := make(chan result, len(reportKeys))
+
 	for _, reportKey := range reportKeys {
-		rep, err := c.getNonCachedReport(reportKey)
-		if err != nil {
-			return nil, err
+		go func(reportKey string) {
+			r := result{key: reportKey}
+			r.report, r.err = c.getNonCachedReport(reportKey)
+			ch <- r
+		}(reportKey)
+	}
+
+	reports := []report.Report{}
+	for range reportKeys {
+		r := <-ch
+		if r.err != nil {
+			return nil, r.err
 		}
-		reports = append(reports, rep)
-		c.cache.Set(reportKey, rep)
+		reports = append(reports, *r.report)
+		c.cache.Set(r.key, *r.report)
 	}
 	return reports, nil
 }
 
 // Fetch a single report from S3.
-func (c *dynamoDBCollector) getNonCachedReport(reportKey string) (report.Report, error) {
+func (c *dynamoDBCollector) getNonCachedReport(reportKey string) (*report.Report, error) {
 	var resp *s3.GetObjectOutput
 	err := timeRequest("Get", s3RequestDuration, func() error {
 		var err error
@@ -265,17 +282,17 @@ func (c *dynamoDBCollector) getNonCachedReport(reportKey string) (report.Report,
 		return err
 	})
 	if err != nil {
-		return report.Report{}, err
+		return nil, err
 	}
 	reader, err := gzip.NewReader(resp.Body)
 	if err != nil {
-		return report.Report{}, err
+		return nil, err
 	}
 	rep := report.MakeReport()
 	if err := codec.NewDecoder(reader, &codec.MsgpackHandle{}).Decode(&rep); err != nil {
-		return report.Report{}, err
+		return nil, err
 	}
-	return rep, nil
+	return &rep, nil
 }
 
 func (c *dynamoDBCollector) getReports(userid string, row int64, start, end time.Time) ([]report.Report, error) {