termd/internal/server/attach.go

package server

import (
	"encoding/json"
	"net/http"
	"time"

	"github.com/coder/websocket"

	"termd/internal/api"
)

// Attach websocket protocol: binary frames are raw terminal bytes in both
// directions; text frames are JSON control messages — client sends
// {"resize":{"cols":N,"rows":N}}, server sends {"hello":{...}} on connect and
// {"exited":{"code":N}} when the process dies.

type attachControl struct {
	Hello  *api.SessionInfo `json:"hello,omitempty"`
	Resize *api.ResizeRequest `json:"resize,omitempty"`
	Exited *attachExited    `json:"exited,omitempty"`
}

type attachExited struct {
	Code int `json:"code"`
}

func (s *Server) handleAttach(w http.ResponseWriter, r *http.Request) {
	sess := s.getSession(w, r)
	if sess == nil {
		return
	}
	readonly := r.URL.Query().Get("readonly") != ""
	conn, err := websocket.Accept(w, r, &websocket.AcceptOptions{
		// Local research tool; the bearer token (when set) is the auth story.
		OriginPatterns: []string{"*"},
	})
	if err != nil {
		// Accept has already written the HTTP error.
		return
	}
	defer conn.CloseNow()

	ctx := r.Context()
	writeControl := func(c attachControl) error {
		b, err := json.Marshal(c)
		if err != nil {
			return err
		}
		return conn.Write(ctx, websocket.MessageText, b)
	}

	info := sess.Info()
	if err := writeControl(attachControl{Hello: &info}); err != nil {
		return
	}

	// view=render streams debounced emulator-rendered frames (absolute
	// positioning, lines cleared to the viewer's edge) instead of the raw
	// byte stream — for viewers whose terminal is larger than the session.
	render := r.URL.Query().Get("view") == "render"

	snapshot, ch, cancel := sess.Subscribe()
	defer cancel()
	if render {
		snapshot = sess.RenderFrame(true)
	}
	if err := conn.Write(ctx, websocket.MessageBinary, snapshot); err != nil {
		return
	}

	// Client-to-session direction.
	go func() {
		for {
			typ, data, err := conn.Read(ctx)
			if err != nil {
				cancel() // closes ch, ends the send loop below
				return
			}
			switch typ {
			case websocket.MessageBinary:
				if readonly {
					// Read-only connections must not reach the PTY.
					continue
				}
				if err := sess.WriteHuman(data); err != nil {
					cancel()
					return
				}
			case websocket.MessageText:
				var c attachControl
				if err := json.Unmarshal(data, &c); err == nil && c.Resize != nil {
					// Last-writer-wins, tmux "latest client" style.
					_ = sess.Resize(c.Resize.Cols, c.Resize.Rows)
				}
			}
		}
	}()

	exitedCh, _ := sess.Exited()
	// In render mode, output chunks only mark the frame dirty; the actual
	// frame is sent when the debounce window closes, coalescing bursts.
	var frameDue <-chan time.Time
	for {
		select {
		case b, ok := <-ch:
			if !ok {
				// Session output ended (teardown or we were dropped as a slow
				// client). If the process exited, tell the client the code.
				if render {
					_ = conn.Write(ctx, websocket.MessageBinary, sess.RenderFrame(false))
				}
				select {
				case <-exitedCh:
					_, code := sess.Exited()
					_ = writeControl(attachControl{Exited: &attachExited{Code: code}})
				case <-time.After(time.Second):
				}
				_ = conn.Close(websocket.StatusNormalClosure, "session ended")
				return
			}
			if render {
				if frameDue == nil {
					frameDue = time.After(25 * time.Millisecond)
				}
			} else if err := conn.Write(ctx, websocket.MessageBinary, b); err != nil {
				return
			}
		case <-frameDue:
			frameDue = nil
			if err := conn.Write(ctx, websocket.MessageBinary, sess.RenderFrame(false)); err != nil {
				return
			}
		case <-exitedCh:
			_, code := sess.Exited()
			_ = writeControl(attachControl{Exited: &attachExited{Code: code}})
			exitedCh = nil // fire once; keep draining trailing output
		case <-ctx.Done():
			return
		}
	}
}