lib/model: Add support for different puller block ordering (#6587)

* WIP

* Tests

* Header and format

* WIP

* Fix tests

* Goland you disappoint me

* Remove CC storage

* Update blockpullreorderer.go

lib/model/folder_sendrecv.go

@@ -25,7 +25,6 @@ import (
 	"github.com/syncthing/syncthing/lib/ignore"
 	"github.com/syncthing/syncthing/lib/osutil"
 	"github.com/syncthing/syncthing/lib/protocol"
-	"github.com/syncthing/syncthing/lib/rand"
 	"github.com/syncthing/syncthing/lib/scanner"
 	"github.com/syncthing/syncthing/lib/sha256"
 	"github.com/syncthing/syncthing/lib/sync"
@@ -104,8 +103,9 @@ type sendReceiveFolder struct {
 	fs        fs.Filesystem
 	versioner versioner.Versioner
 
-	queue        *jobQueue
-	writeLimiter *byteSemaphore
+	queue              *jobQueue
+	blockPullReorderer blockPullReorderer
+	writeLimiter       *byteSemaphore
 
 	pullErrors    map[string]string // errors for most recent/current iteration
 	oldPullErrors map[string]string // errors from previous iterations for log filtering only
@@ -114,12 +114,13 @@ type sendReceiveFolder struct {
 
 func newSendReceiveFolder(model *model, fset *db.FileSet, ignores *ignore.Matcher, cfg config.FolderConfiguration, ver versioner.Versioner, fs fs.Filesystem, evLogger events.Logger, ioLimiter *byteSemaphore) service {
 	f := &sendReceiveFolder{
-		folder:        newFolder(model, fset, ignores, cfg, evLogger, ioLimiter),
-		fs:            fs,
-		versioner:     ver,
-		queue:         newJobQueue(),
-		writeLimiter:  newByteSemaphore(cfg.MaxConcurrentWrites),
-		pullErrorsMut: sync.NewMutex(),
+		folder:             newFolder(model, fset, ignores, cfg, evLogger, ioLimiter),
+		fs:                 fs,
+		versioner:          ver,
+		queue:              newJobQueue(),
+		blockPullReorderer: newBlockPullReorderer(cfg.BlockPullOrder, model.id, cfg.DeviceIDs()),
+		writeLimiter:       newByteSemaphore(cfg.MaxConcurrentWrites),
+		pullErrorsMut:      sync.NewMutex(),
 	}
 	f.folder.puller = f
 	f.folder.Service = util.AsService(f.serve, f.String())
@@ -1071,8 +1072,8 @@ func (f *sendReceiveFolder) handleFile(file protocol.FileInfo, snap *db.Snapshot
 		blocks = append(blocks, file.Blocks...)
 	}
 
-	// Shuffle the blocks
-	rand.Shuffle(blocks)
+	// Reorder blocks
+	blocks = f.blockPullReorderer.Reorder(blocks)
 
 	f.evLogger.Log(events.ItemStarted, map[string]string{
 		"folder": f.folderID,