perf(p2p/channel): Speedup NewDelimitedWriter (backport cometbft#2949) (

cometbft#2969) (#40) (#49)

Speeds up 5% of the non-IO time overhead from
`channel.WritePacketMsgTo`. The CPU time overhead in this function is
quite significant, CPU time is more than 3 times the syscall time for
writing to the net buffer. Working on a github issue for more
substantial refactor / time eliminations, but this 3s is easy enough.

We don't even use this codepath, so this make slice is entirely wasted.
However we should do things that reduce this overhead further.

![image](https://github.com/cometbft/cometbft/assets/6440154/e02e45bf-d6ff-4e11-9983-e81ca1102dc8)

---

#### PR checklist

- [x] Tests written/updated
- [x] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [x] Updated relevant documentation (`docs/` or `spec/`) and code
comments
- [x] Title follows the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) spec
<hr>This is an automatic backport of pull request cometbft#2949 done by
[Mergify](https://mergify.com).

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Dev Ojha <ValarDragon@users.noreply.github.com>
(cherry picked from commit 3d1b9dc)

Co-authored-by: Adam Tucker <adam@osmosis.team>

.changelog/unreleased/improvements/2949-reduce-protoio-writer-creation-time.md

@@ -0,0 +1,2 @@
+- `[p2p/channel]` Speedup `ProtoIO` writer creation time, and thereby speedup channel writing by 5%.
+  ([\#2949](https://github.com/cometbft/cometbft/pull/2949))

libs/protoio/writer.go

@@ -42,7 +42,7 @@ import (
 // equivalent to the gogoproto NewDelimitedWriter, except WriteMsg() also returns the
 // number of bytes written, which is necessary in the p2p package.
 func NewDelimitedWriter(w io.Writer) WriteCloser {
-	return &varintWriter{w, make([]byte, binary.MaxVarintLen64), nil}
+	return &varintWriter{w, nil, nil}
 }
 
 type varintWriter struct {
@@ -69,6 +69,9 @@ func (w *varintWriter) WriteMsg(msg proto.Message) (int, error) {
 	}
 
 	// fallback
+	if w.lenBuf == nil {
+		w.lenBuf = make([]byte, binary.MaxVarintLen64)
+	}
 	data, err := proto.Marshal(msg)
 	if err != nil {
 		return 0, err