add Reporter.HasReports() for cheap report availability checking

This requires no report reading / merging.

We plan to expose this in the HTTP API, so the UI gets a cheap way of
checking whether the app is currently receiving data from probes.

app/collector.go

@@ -29,6 +29,7 @@ const reportQuantisationInterval = 3 * time.Second
 // interface for parts of the app, and several experimental components.
 type Reporter interface {
 	Report(context.Context, time.Time) (report.Report, error)
+	HasReports(context.Context, time.Time) (bool, error)
 	HasHistoricReports() bool
 	WaitOn(context.Context, chan struct{})
 	UnWait(context.Context, chan struct{})
@@ -161,6 +162,19 @@ func (c *collector) Report(_ context.Context, timestamp time.Time) (report.Repor
 	return rpt, nil
 }
 
+// HasReports indicates whether the collector contains reports between
+// timestamp-app.window and timestamp.
+func (c *collector) HasReports(ctx context.Context, timestamp time.Time) (bool, error) {
+	c.mtx.Lock()
+	defer c.mtx.Unlock()
+
+	if len(c.timestamps) < 1 {
+		return false, nil
+	}
+
+	return !c.timestamps[0].After(timestamp) && !c.timestamps[len(c.reports)-1].Before(timestamp.Add(-c.window)), nil
+}
+
 // HasHistoricReports indicates whether the collector contains reports
 // older than now-app.window.
 func (c *collector) HasHistoricReports() bool {
@@ -223,6 +237,12 @@ func (c StaticCollector) Report(context.Context, time.Time) (report.Report, erro
 	return report.Report(c), nil
 }
 
+// HasReports indicates whether the collector contains reports between
+// timestamp-app.window and timestamp.
+func (c StaticCollector) HasReports(context.Context, time.Time) (bool, error) {
+	return true, nil
+}
+
 // HasHistoricReports indicates whether the collector contains reports
 // older than now-app.window.
 func (c StaticCollector) HasHistoricReports() bool {

app/multitenant/aws_collector.go

@@ -349,6 +349,11 @@ func (c *awsCollector) Report(ctx context.Context, timestamp time.Time) (report.
 	return c.merger.Merge(reports), nil
 }
 
+func (c *awsCollector) HasReports(ctx context.Context, timestamp time.Time) (bool, error) {
+	reportKeys, err := c.getReportKeys(ctx, timestamp)
+	return len(reportKeys) > 0, err
+}
+
 func (c *awsCollector) HasHistoricReports() bool {
 	return true
 }