From 62985139c4fec069efaea3cce43d7457596f387e Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Sat, 28 Sep 2024 22:19:29 +1000
Subject: [PATCH] Rework handshake parsing

---
 go.mod                     |  2 +-
 go.sum                     |  4 ++--
 peer_protocol/handshake.go | 43 +++++++++++++++++++-------------------
 peer_protocol/protocol.go  | 10 +++++++++
 4 files changed, 35 insertions(+), 24 deletions(-)

diff --git a/go.mod b/go.mod
index fe0f108eeb..35cc26b65c 100644
--- a/go.mod
+++ b/go.mod
@@ -11,7 +11,7 @@ require (
 	github.com/anacrolix/dht/v2 v2.19.2-0.20221121215055-066ad8494444
 	github.com/anacrolix/envpprof v1.3.0
 	github.com/anacrolix/fuse v0.2.0
-	github.com/anacrolix/generics v0.0.2-0.20240227122613-f95486179cab
+	github.com/anacrolix/generics v0.0.3-0.20240902042256-7fb2702ef0ca
 	github.com/anacrolix/go-libutp v1.3.1
 	github.com/anacrolix/gostdapp v0.1.0
 	github.com/anacrolix/log v0.15.3-0.20240627045001-cd912c641d83
diff --git a/go.sum b/go.sum
index 52a865de24..2de5eff7e5 100644
--- a/go.sum
+++ b/go.sum
@@ -80,8 +80,8 @@ github.com/anacrolix/envpprof v1.3.0/go.mod h1:7QIG4CaX1uexQ3tqd5+BRa/9e2D02Wcer
 github.com/anacrolix/fuse v0.2.0 h1:pc+To78kI2d/WUjIyrsdqeJQAesuwpGxlI3h1nAv3Do=
 github.com/anacrolix/fuse v0.2.0/go.mod h1:Kfu02xBwnySDpH3N23BmrP3MDfwAQGRLUCj6XyeOvBQ=
 github.com/anacrolix/generics v0.0.0-20230113004304-d6428d516633/go.mod h1:ff2rHB/joTV03aMSSn/AZNnaIpUw0h3njetGsaXcMy8=
-github.com/anacrolix/generics v0.0.2-0.20240227122613-f95486179cab h1:MvuAC/UJtcohN6xWc8zYXSZfllh1LVNepQ0R3BCX5I4=
-github.com/anacrolix/generics v0.0.2-0.20240227122613-f95486179cab/go.mod h1:ff2rHB/joTV03aMSSn/AZNnaIpUw0h3njetGsaXcMy8=
+github.com/anacrolix/generics v0.0.3-0.20240902042256-7fb2702ef0ca h1:aiiGqSQWjtVNdi8zUMfA//IrM8fPkv2bWwZVPbDe0wg=
+github.com/anacrolix/generics v0.0.3-0.20240902042256-7fb2702ef0ca/go.mod h1:MN3ve08Z3zSV/rTuX/ouI4lNdlfTxgdafQJiLzyNRB8=
 github.com/anacrolix/go-libutp v1.3.1 h1:idJzreNLl+hNjGC3ZnUOjujEaryeOGgkwHLqSGoige0=
 github.com/anacrolix/go-libutp v1.3.1/go.mod h1:heF41EC8kN0qCLMokLBVkB8NXiLwx3t8R8810MTNI5o=
 github.com/anacrolix/gostdapp v0.1.0 h1:sZC+gSLhA7Hdalak5rPCkhO0YSEl0tt/lsovxh6qka4=
diff --git a/peer_protocol/handshake.go b/peer_protocol/handshake.go
index a87b7c9215..162047f3aa 100644
--- a/peer_protocol/handshake.go
+++ b/peer_protocol/handshake.go
@@ -3,12 +3,11 @@ package peer_protocol
 import (
 	"context"
 	"encoding/hex"
-	"errors"
 	"fmt"
+	"github.com/anacrolix/missinggo/v2/panicif"
 	"github.com/anacrolix/torrent/internal/ctxrw"
 	"io"
 	"math/bits"
-	"strconv"
 	"strings"
 	"unsafe"
 
@@ -153,37 +152,39 @@ func Handshake(
 	}()
 
 	post := func(bb []byte) {
-		select {
-		case postCh <- bb:
-		default:
-			panic("mustn't block while posting")
-		}
+		panicif.SendBlocks(postCh, bb)
 	}
 
-	post([]byte(Protocol))
+	post(protocolBytes())
 	post(extensions[:])
 	if ih != nil { // We already know what we want.
 		post(ih[:])
 		post(peerID[:])
 	}
-	var b [68]byte
-	_, err = io.ReadFull(sock, b[:68])
+
+	// Putting an array on the heap still escapes.
+	b := make([]byte, 68)
+	// Read in one hit to avoid potential overhead in underlying reader.
+	_, err = io.ReadFull(sock, b[:])
 	if err != nil {
 		return res, fmt.Errorf("while reading: %w", err)
 	}
-	if string(b[:20]) != Protocol {
-		return res, errors.New("unexpected protocol string")
-	}
 
-	copyExact := func(dst, src []byte) {
-		if dstLen, srcLen := uint64(len(dst)), uint64(len(src)); dstLen != srcLen {
-			panic("dst len " + strconv.FormatUint(dstLen, 10) + " != src len " + strconv.FormatUint(srcLen, 10))
-		}
-		copy(dst, src)
+	p := b[:len(Protocol)]
+	// This gets optimized to runtime.memequal
+	if string(p) != Protocol {
+		return res, fmt.Errorf("unexpected protocol string %q", string(p))
+	}
+	b = b[len(p):]
+	read := func(dst []byte) {
+		n := copy(dst, b)
+		panicif.NotEq(n, len(dst))
+		b = b[n:]
 	}
-	copyExact(res.PeerExtensionBits[:], b[20:28])
-	copyExact(res.Hash[:], b[28:48])
-	copyExact(res.PeerID[:], b[48:68])
+	read(res.PeerExtensionBits[:])
+	read(res.Hash[:])
+	read(res.PeerID[:])
+	panicif.NotEq(len(b), 0)
 	// peerExtensions.Add(res.PeerExtensionBits.String(), 1)
 
 	// TODO: Maybe we can just drop peers here if we're not interested. This
diff --git a/peer_protocol/protocol.go b/peer_protocol/protocol.go
index b1790d1c10..ebd0a7a17b 100644
--- a/peer_protocol/protocol.go
+++ b/peer_protocol/protocol.go
@@ -1,9 +1,19 @@
 package peer_protocol
 
+import (
+	"unsafe"
+)
+
 const (
 	Protocol = "\x13BitTorrent protocol"
 )
 
+var protocolStr = Protocol
+
+func protocolBytes() []byte {
+	return unsafe.Slice(unsafe.StringData(Protocol), len(Protocol))
+}
+
 type MessageType byte
 
 // golang.org/x/tools/cmd/stringer