eiac/mosh-go
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server: VT emulator framebuffer diffing

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Replace raw PTY passthrough with proper VT emulator processing.
PTY output feeds into charmbracelet/x/vt, snapshots are diffed
against the last-acked framebuffer using CUP-based positioning.
Fixes character doubling during fast typing.
This commit is contained in:
realrasengan
2026-03-14 20:34:30 -07:00
parent bb08f71e30
commit b8b5994527
6 changed files with 296 additions and 44 deletions
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server.go
+70 -32
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@@ -10,8 +10,10 @@ import (
"os"
"os/exec"
"sync"
"sync/atomic"
"time"
"github.com/charmbracelet/x/vt"
"github.com/creack/pty"
)
@@ -27,8 +29,8 @@ const (
// Maximum payload per UDP datagram.
maxPayload = 16384
// Heartbeat / tick interval for the SSP send loop.
tickInterval = 20 * time.Millisecond
// Tick interval matches mosh's SEND_MINDELAY (8ms) for keystroke batching.
tickInterval = 8 * time.Millisecond
// Server gives up waiting for client after this.
associationTimeout = 60 * time.Second
@@ -59,6 +61,12 @@ type Server struct {
// Transport handles SSP sequencing, fragmentation, and crypto.
transport *Transport
// VT emulator and framebuffer state for CUP-based diffing.
emu *vt.Emulator
baseFB *Framebuffer // what client has (last-acked)
sentFB *Framebuffer // what we last sent (pending ack)
curVisible atomic.Bool
// Remote client address — set on first received datagram.
mu sync.Mutex
clientAddr *net.UDPAddr
@@ -154,6 +162,13 @@ func (s *Server) Serve() error {
Cols: uint16(s.cols),
})
s.emu = vt.NewEmulator(s.cols, s.rows)
s.curVisible.Store(true)
s.emu.SetCallbacks(vt.Callbacks{
CursorVisibility: func(visible bool) { s.curVisible.Store(visible) },
})
s.baseFB = NewFramebuffer(s.cols, s.rows)
var wg sync.WaitGroup
wg.Add(3)
@@ -196,6 +211,13 @@ func (s *Server) Done() <-chan struct{} {
func (s *Server) ServeRW(rw io.ReadWriteCloser, resize func(cols, rows uint16)) error {
close(s.started)
s.emu = vt.NewEmulator(s.cols, s.rows)
s.curVisible.Store(true)
s.emu.SetCallbacks(vt.Callbacks{
CursorVisibility: func(visible bool) { s.curVisible.Store(visible) },
})
s.baseFB = NewFramebuffer(s.cols, s.rows)
var wg sync.WaitGroup
wg.Add(3)
@@ -265,7 +287,7 @@ func (s *Server) mainLoopRW(rw io.Writer, resize func(cols, rows uint16), ioOutp
ticker := time.NewTicker(tickInterval)
defer ticker.Stop()
var pending []byte
dirty := false
for {
select {
@@ -276,7 +298,8 @@ func (s *Server) mainLoopRW(rw io.Writer, resize func(cols, rows uint16), ioOutp
if !ok {
return
}
pending = append(pending, data...)
s.emu.Write(data)
dirty = true
case ui := <-userInput:
if len(ui.Keys) > 0 {
@@ -290,18 +313,29 @@ func (s *Server) mainLoopRW(rw io.Writer, resize func(cols, rows uint16), ioOutp
if resize != nil {
resize(uint16(ui.Width), uint16(ui.Height))
}
s.emu.Resize(int(ui.Width), int(ui.Height))
s.baseFB = NewFramebuffer(int(ui.Width), int(ui.Height))
s.sentFB = nil
dirty = true
}
case <-ticker.C:
if len(pending) > 0 {
hi := HostInstruction{
Hoststring: pending,
EchoAckNum: -1,
}
diff := marshalHostMessage([]HostInstruction{hi})
s.transport.SetPending(diff)
pending = nil
if s.sentFB != nil && s.transport.AckedByRemote() >= s.transport.SentNum() {
s.baseFB = s.sentFB
s.sentFB = nil
}
if dirty {
currentFB := SnapshotEmulator(s.emu, s.curVisible.Load())
diffBytes := currentFB.Diff(s.baseFB)
if len(diffBytes) > 0 {
hi := HostInstruction{Hoststring: diffBytes, EchoAckNum: -1}
s.transport.SetPending(marshalHostMessage([]HostInstruction{hi}))
s.sentFB = currentFB
}
dirty = false
}
datagrams := s.transport.Tick()
s.sendDatagrams(datagrams)
}
@@ -331,8 +365,9 @@ func (s *Server) Close() {
}
}
// mainLoop is the SSP event loop. It maintains the terminal framebuffer,
// processes user input, diffs the screen, and drives the transport.
// mainLoop is the SSP event loop. It feeds PTY output to a VT emulator,
// snapshots the framebuffer, diffs against the last-acked state, and
// sends CUP-based updates via the transport.
func (s *Server) mainLoop(ptyOutput <-chan []byte, userInput <-chan UserInstruction) {
// Wait for client to connect before processing.
for {
@@ -349,14 +384,10 @@ func (s *Server) mainLoop(ptyOutput <-chan []byte, userInput <-chan UserInstruct
}
}
// Terminal state: we maintain a simple passthrough buffer.
// PTY output is accumulated and sent as HostBytes diffs.
// For full mosh compatibility, the diff is the raw terminal output
// wrapped in the HostMessage protobuf.
ticker := time.NewTicker(tickInterval)
defer ticker.Stop()
var pending []byte
dirty := false
for {
select {
@@ -367,14 +398,13 @@ func (s *Server) mainLoop(ptyOutput <-chan []byte, userInput <-chan UserInstruct
if !ok {
return
}
pending = append(pending, data...)
s.emu.Write(data)
dirty = true
case ui := <-userInput:
// Handle keystrokes.
if len(ui.Keys) > 0 {
s.ptmx.Write(ui.Keys)
}
// Handle resize.
if ui.Width > 0 && ui.Height > 0 {
s.mu.Lock()
s.cols = int(ui.Width)
@@ -384,20 +414,28 @@ func (s *Server) mainLoop(ptyOutput <-chan []byte, userInput <-chan UserInstruct
Cols: uint16(ui.Width),
Rows: uint16(ui.Height),
})
s.emu.Resize(int(ui.Width), int(ui.Height))
s.baseFB = NewFramebuffer(int(ui.Width), int(ui.Height))
s.sentFB = nil
dirty = true
}
case <-ticker.C:
// Build diff and send via transport.
if len(pending) > 0 {
hi := HostInstruction{
Hoststring: pending,
EchoAckNum: -1,
}
diff := marshalHostMessage([]HostInstruction{hi})
// Advance base when client acks all pending.
if s.sentFB != nil && s.transport.AckedByRemote() >= s.transport.SentNum() {
s.baseFB = s.sentFB
s.sentFB = nil
}
// Wrap in TransportInstruction via transport.
s.transport.SetPending(diff)
pending = nil
if dirty {
currentFB := SnapshotEmulator(s.emu, s.curVisible.Load())
diffBytes := currentFB.Diff(s.baseFB)
if len(diffBytes) > 0 {
hi := HostInstruction{Hoststring: diffBytes, EchoAckNum: -1}
s.transport.SetPending(marshalHostMessage([]HostInstruction{hi}))
s.sentFB = currentFB
}
dirty = false
}
datagrams := s.transport.Tick()