chore(model): simplify FileInfoBatch size computation (#10776)

We used a size computation to generate reasonably sized batches, but the
ProtoSize call is fairly expensive as it requires a conversion to a wire
type etc. We don't need that much precision. Instead, just limit to 1000
files or 5000 blocks, which is likely approximately that much data
anyway.

Closes #10707

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/model/fileinfobatch.go

@@ -8,20 +8,19 @@ package model
 
 import (
 	"github.com/syncthing/syncthing/lib/protocol"
-	"google.golang.org/protobuf/proto"
 )
 
-// How many files to send in each Index/IndexUpdate message.
+// How many files & blocks to send in each Index/IndexUpdate message.
 const (
-	MaxBatchSizeBytes = 250 * 1024 // Aim for making index messages no larger than 250 KiB (uncompressed)
+	MaxBatchSizeFiles  = 1000
-	MaxBatchSizeFiles = 1000       // Either way, don't include more files than this
+	MaxBatchSizeBlocks = 5000
 )
 
 // FileInfoBatch is a utility to do file operations on the database in suitably
 // sized batches.
 type FileInfoBatch struct {
 	infos   []protocol.FileInfo
-	size    int
+	nblocks int
 	flushFn func([]protocol.FileInfo) error
 	error   error
 }
@@ -47,11 +46,11 @@ func (b *FileInfoBatch) Append(f protocol.FileInfo) {
 		b.infos = make([]protocol.FileInfo, 0, MaxBatchSizeFiles)
 	}
 	b.infos = append(b.infos, f)
-	b.size += proto.Size(f.ToWire(true))
+	b.nblocks += len(f.Blocks)
 }
 
 func (b *FileInfoBatch) Full() bool {
-	return len(b.infos) >= MaxBatchSizeFiles || b.size >= MaxBatchSizeBytes
+	return len(b.infos) >= MaxBatchSizeFiles || b.nblocks >= MaxBatchSizeBlocks
 }
 
 func (b *FileInfoBatch) FlushIfFull() error {
@@ -82,9 +81,5 @@ func (b *FileInfoBatch) Flush() error {
 func (b *FileInfoBatch) Reset() {
 	b.infos = nil
 	b.error = nil
-	b.size = 0
+	b.nblocks = 0
-}
-
-func (b *FileInfoBatch) Size() int {
-	return b.size
 }

lib/model/indexhandler.go

@@ -276,7 +276,7 @@ func (s *indexHandler) sendIndexTo(ctx context.Context) error {
 			// can't happen, once an error is returned the index sender exits
 			panic(fmt.Sprintf("bug: once failed it should stay failed (%v)", batchError))
 		}
-		l.Debugf("%v: Sending %d files (<%d bytes)", s, len(fs), batch.Size())
+		l.Debugf("%v: Sending %d files", s, len(fs))
 
 		lastSequence := fs[len(fs)-1].Sequence
 		var err error