downloader: call Close on updates that are overwritten in `(*Downlo…

…ader).LoadOnce`

When an update is replaced in `(*Downloader).LoadOnce`,
the concurrency token issued by `d.r.decompressedSnapshotLimit`
is lost and can no longer be accessed, and thus be released.

syncer/receiver/downloader.go

@@ -130,6 +130,9 @@ func (d *Downloader) LoadOnce(ctx context.Context, ni snapshot.NameInfo) error {
 	// Make snapshot available to the syncer, replacing any previous one
 	// that has not been loaded yet.
 	d.r.mu.Lock()
+	// If we are about to replace an Update in the snapshotsByInstance map,
+	// we will need to Close it so as to release its concurrency limit token.
+	overwrittenSnap, hasOverwrittenSnap := d.r.snapshotsByInstance[d.instance]
 	// FIXME: use *snapshot.Update pointer in APIs with new tokens
 	d.r.snapshotsByInstance[d.instance] = snapshot.Update{
 		Snapshot: msg,
@@ -148,6 +151,12 @@ func (d *Downloader) LoadOnce(ctx context.Context, ni snapshot.NameInfo) error {
 	}
 	d.r.mu.Unlock()
 
+	if hasOverwrittenSnap {
+		d.l.WithField("overwritten_snapshot", overwrittenSnap.NameInfo.FullName).
+			Debug("Closing overwritten snapshot")
+		overwrittenSnap.Close()
+	}
+
 	t2 := time.Now()
 	d.l.WithFields(logrus.Fields{
 		"timestamp": ni.TimestampString,