restore: separately restore blobs that are frequently referenced

Writing these blobs to their files can take a long time and consequently
cause the backend connection to time out. Avoid that by retrieving these
blobs separately.

internal/restorer/filerestorer.go

@@ -242,8 +242,33 @@ func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) error {
 
 	// track already processed blobs for precise error reporting
 	processedBlobs := restic.NewBlobSet()
-	err := r.downloadBlobs(ctx, pack.id, blobs, processedBlobs)
+	for _, entry := range blobs {
+		occurrences := 0
+		for _, offsets := range entry.files {
+			occurrences += len(offsets)
+		}
+		// With a maximum blob size of 8MB, the normal blob streaming has to write
+		// at most 800MB for a single blob. This should be short enough to avoid
+		// network connection timeouts. Based on a quick test, a limit of 100 only
+		// selects a very small number of blobs (the number of references per blob
+		// - aka. `count` - seem to follow a expontential distribution)
+		if occurrences > 100 {
+			// process frequently referenced blobs first as these can take a long time to write
+			// which can cause backend connections to time out
+			delete(blobs, entry.blob.ID)
+			partialBlobs := blobToFileOffsetsMapping{entry.blob.ID: entry}
+			err := r.downloadBlobs(ctx, pack.id, partialBlobs, processedBlobs)
+			if err := r.reportError(blobs, processedBlobs, err); err != nil {
+				return err
+			}
+		}
+	}
 
+	if len(blobs) == 0 {
+		return nil
+	}
+
+	err := r.downloadBlobs(ctx, pack.id, blobs, processedBlobs)
 	return r.reportError(blobs, processedBlobs, err)
 }