chore: remove weak hashing which does not pull its weight (#10005)

We've had weak/rolling hashing in the code for quite a while. It was a
popular request for a while, based on the belief that rsync does this
and we should too. However, the benefit is quite small; we save on
average about 0.8% of transferred blocks over the population as a whole:

<img width="974" alt="Screenshot 2025-03-28 at 17 09 02"
src="https://github.com/user-attachments/assets/bbe10dea-f85e-4043-9823-7cef1220b4a2"
/>

This would be fine if the cost was comparably low, however the downside
of attempting rolling hash matching is that we (by default) do a
complete file read on the destination in order to look for matches
before we starting pulling blocks for the file. For any larger file this
means a sometimes long, I/O-intensive pause before the file starts
syncing, for usually no benefit.

I propose we simply rip off the bandaid and save the effort.

lib/protocol/bep_fileinfo.go

@@ -544,32 +544,29 @@ func (f *FileInfo) setNoContent() {
 }
 
 type BlockInfo struct {
-	Hash     []byte
-	Offset   int64
-	Size     int
-	WeakHash uint32
+	Hash   []byte
+	Offset int64
+	Size   int
 }
 
 func (b BlockInfo) ToWire() *bep.BlockInfo {
 	return &bep.BlockInfo{
-		Hash:     b.Hash,
-		Offset:   b.Offset,
-		Size:     int32(b.Size),
-		WeakHash: b.WeakHash,
+		Hash:   b.Hash,
+		Offset: b.Offset,
+		Size:   int32(b.Size),
 	}
 }
 
 func BlockInfoFromWire(w *bep.BlockInfo) BlockInfo {
 	return BlockInfo{
-		Hash:     w.Hash,
-		Offset:   w.Offset,
-		Size:     int(w.Size),
-		WeakHash: w.WeakHash,
+		Hash:   w.Hash,
+		Offset: w.Offset,
+		Size:   int(w.Size),
 	}
 }
 
 func (b BlockInfo) String() string {
-	return fmt.Sprintf("Block{%d/%d/%d/%x}", b.Offset, b.Size, b.WeakHash, b.Hash)
+	return fmt.Sprintf("Block{%d/%d/%x}", b.Offset, b.Size, b.Hash)
 }
 
 // For each block size, the hash of a block of all zeroes
@@ -596,7 +593,6 @@ func BlocksHash(bs []BlockInfo) []byte {
 	h := sha256.New()
 	for _, b := range bs {
 		_, _ = h.Write(b.Hash)
-		_ = binary.Write(h, binary.BigEndian, b.WeakHash)
 	}
 	return h.Sum(nil)
 }

lib/protocol/bep_request_response.go

@@ -25,7 +25,6 @@ type Request struct {
 	Size          int
 	Hash          []byte
 	FromTemporary bool
-	WeakHash      uint32
 	BlockNo       int
 }
 
@@ -38,7 +37,6 @@ func (r *Request) toWire() *bep.Request {
 		Size:          int32(r.Size),
 		Hash:          r.Hash,
 		FromTemporary: r.FromTemporary,
-		WeakHash:      r.WeakHash,
 		BlockNo:       int32(r.BlockNo),
 	}
 }
@@ -52,7 +50,6 @@ func requestFromWire(w *bep.Request) *Request {
 		Size:          int(w.Size),
 		Hash:          w.Hash,
 		FromTemporary: w.FromTemporary,
-		WeakHash:      w.WeakHash,
 		BlockNo:       int(w.BlockNo),
 	}
 }

lib/protocol/common_test.go

@@ -17,7 +17,6 @@ type TestModel struct {
 	offset        int64
 	size          int32
 	hash          []byte
-	weakHash      uint32
 	fromTemporary bool
 	indexFn       func(string, []FileInfo)
 	ccFn          func(*ClusterConfig)
@@ -48,7 +47,6 @@ func (t *TestModel) Request(_ Connection, req *Request) (RequestResponse, error)
 	t.offset = req.Offset
 	t.size = int32(req.Size)
 	t.hash = req.Hash
-	t.weakHash = req.WeakHash
 	t.fromTemporary = req.FromTemporary
 	buf := make([]byte, len(t.data))
 	copy(buf, t.data)