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b1c8f88a44
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.
124 lines
2.9 KiB
Go
124 lines
2.9 KiB
Go
// Copyright (C) 2014 The Syncthing Authors.
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//
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// This Source Code Form is subject to the terms of the Mozilla Public
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// License, v. 2.0. If a copy of the MPL was not distributed with this file,
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// You can obtain one at https://mozilla.org/MPL/2.0/.
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package scanner
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import (
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"bytes"
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"context"
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"crypto/sha256"
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"io"
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"github.com/syncthing/syncthing/lib/protocol"
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)
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var SHA256OfNothing = []uint8{0xe3, 0xb0, 0xc4, 0x42, 0x98, 0xfc, 0x1c, 0x14, 0x9a, 0xfb, 0xf4, 0xc8, 0x99, 0x6f, 0xb9, 0x24, 0x27, 0xae, 0x41, 0xe4, 0x64, 0x9b, 0x93, 0x4c, 0xa4, 0x95, 0x99, 0x1b, 0x78, 0x52, 0xb8, 0x55}
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type Counter interface {
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Update(bytes int64)
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}
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// Blocks returns the blockwise hash of the reader.
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func Blocks(ctx context.Context, r io.Reader, blocksize int, sizehint int64, counter Counter) ([]protocol.BlockInfo, error) {
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if counter == nil {
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counter = &noopCounter{}
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}
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hf := sha256.New()
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const hashLength = sha256.Size
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var blocks []protocol.BlockInfo
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var hashes, thisHash []byte
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if sizehint >= 0 {
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// Allocate contiguous blocks for the BlockInfo structures and their
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// hashes once and for all, and stick to the specified size.
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r = io.LimitReader(r, sizehint)
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numBlocks := sizehint / int64(blocksize)
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remainder := sizehint % int64(blocksize)
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if remainder != 0 {
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numBlocks++
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}
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blocks = make([]protocol.BlockInfo, 0, numBlocks)
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hashes = make([]byte, 0, hashLength*numBlocks)
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}
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// A 32k buffer is used for copying into the hash function.
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buf := make([]byte, 32<<10)
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var offset int64
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lr := io.LimitReader(r, int64(blocksize)).(*io.LimitedReader)
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for {
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select {
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case <-ctx.Done():
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return nil, ctx.Err()
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default:
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}
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lr.N = int64(blocksize)
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n, err := io.CopyBuffer(hf, lr, buf)
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if err != nil {
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return nil, err
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}
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if n == 0 {
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break
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}
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counter.Update(n)
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// Carve out a hash-sized chunk of "hashes" to store the hash for this
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// block.
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hashes = hf.Sum(hashes)
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thisHash, hashes = hashes[:hashLength], hashes[hashLength:]
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b := protocol.BlockInfo{
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Size: int(n),
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Offset: offset,
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Hash: thisHash,
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}
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blocks = append(blocks, b)
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offset += n
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hf.Reset()
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}
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if len(blocks) == 0 {
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// Empty file
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blocks = append(blocks, protocol.BlockInfo{
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Offset: 0,
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Size: 0,
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Hash: SHA256OfNothing,
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})
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}
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return blocks, nil
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}
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// Validate validates the hash, if len(hash)>0.
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func Validate(buf, hash []byte) bool {
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if len(hash) > 0 {
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hbuf := sha256.Sum256(buf)
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return bytes.Equal(hbuf[:], hash)
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}
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return true
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}
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type noopHash struct{}
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func (noopHash) Sum32() uint32 { return 0 }
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func (noopHash) BlockSize() int { return 0 }
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func (noopHash) Size() int { return 0 }
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func (noopHash) Reset() {}
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func (noopHash) Sum([]byte) []byte { return nil }
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func (noopHash) Write([]byte) (int, error) { return 0, nil }
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type noopCounter struct{}
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func (*noopCounter) Update(_ int64) {}
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