Fix stdin chuck writing and add local perf cost

Author: cmoog

README.md

@@ -70,6 +70,26 @@ go run ./dev/client tty bash
 go run ./dev/client notty ls
 ```
 
+### Local performance cost
+
+Local `sh` through a local `wsep` connection
+
+```shellscript
+$ head -c 100000000 /dev/urandom > /tmp/random; cat /tmp/random | pv | time ./bin/client notty sh -c "cat > /dev/null"
+
+95.4MiB 0:00:00 [ 269MiB/s] [ <=>                                                                                  ]
+./bin/client notty sh -c "cat > /dev/null"  0.32s user 0.31s system 31% cpu 2.019 total
+```
+
+Local `sh` directly
+
+```shell script
+$ head -c 100000000 /dev/urandom > /tmp/random; cat /tmp/random | pv | time  sh -c "cat > /dev/null"
+
+95.4MiB 0:00:00 [1.73GiB/s] [ <=>                                                                                  ]
+sh -c "cat > /dev/null"  0.00s user 0.02s system 32% cpu 0.057 total
+```
+
 ### Performance Goals
 
 Test command

client.go

@@ -13,12 +13,15 @@ import (
 	"nhooyr.io/websocket"
 )
 
+const maxMessageSize = 64000
+
 type remoteExec struct {
 	conn *websocket.Conn
 }
 
 // RemoteExecer creates an execution interface from a WebSocket connection.
 func RemoteExecer(conn *websocket.Conn) Execer {
+	conn.SetReadLimit(maxMessageSize)
 	return remoteExec{conn: conn}
 }
 
@@ -94,7 +97,21 @@ func (r remoteStdin) Write(b []byte) (int, error) {
 		return 0, err
 	}
 	stdinWriter := proto.WithHeader(r.conn, headerByt)
-	return stdinWriter.Write(b)
+
+	maxBodySize := maxMessageSize - len(headerByt) - 1
+	var nn int
+	for len(b) > maxMessageSize {
+		n, err := stdinWriter.Write(b[:maxBodySize])
+		nn += n
+		if err != nil {
+			return nn, err
+		}
+		b = b[maxBodySize:]
+	}
+
+	n, err := stdinWriter.Write(b)
+	nn += n
+	return nn, err
 }
 
 func (r remoteStdin) Close() error {

server.go

@@ -20,6 +20,7 @@ import (
 // The execer may be another wsep connection for chaining.
 // Use LocalExecer for local command execution.
 func Serve(ctx context.Context, c *websocket.Conn, execer Execer) error {
+	c.SetReadLimit(maxMessageSize)
 	var (
 		header    proto.Header
 		process   Process