mosh server: disable scrollback in VT emulator (saves ~200MB/session)

cmd/mosh-wasm/main.go

@@ -158,7 +158,8 @@ func (s *session) recvLoop() {
 		t := s.client.Transport()
 		oldNum := t.LastRecvOldNum()
 		newNum := t.LastRecvNewNum()
-		s.tracker.applyDiff(diff, oldNum, newNum)
+		throwawayNum := t.ThrowawayNum()
+		s.tracker.applyDiff(diff, oldNum, newNum, throwawayNum)
 	}
 }
 

cmd/mosh-wasm/state.go

@@ -41,7 +41,8 @@ func newStateTracker(cols, rows int) *stateTracker {
 }
 
 // applyDiff processes an incoming server diff using state tracking.
-func (st *stateTracker) applyDiff(diff []byte, oldNum, newNum uint64) {
+// throwawayNum is the server's indication of which states it no longer references.
+func (st *stateTracker) applyDiff(diff []byte, oldNum, newNum, throwawayNum uint64) {
 	st.mu.Lock()
 	defer st.mu.Unlock()
 
@@ -93,9 +94,13 @@ func (st *stateTracker) applyDiff(diff []byte, oldNum, newNum uint64) {
 		st.latestState = newNum
 	}
 
-	// Bound states list.
-	if len(st.states) > 128 {
-		st.pruneStates()
+	// Prune states the server no longer references.
+	if throwawayNum > 0 {
+		for n := range st.states {
+			if n < throwawayNum {
+				delete(st.states, n)
+			}
+		}
 	}
 
 	// Display diff: compute ANSI difference between displayed and latest.
@@ -133,24 +138,3 @@ func (st *stateTracker) resize(cols, rows int) {
 	st.displayedFB = nil
 }
 
-func (st *stateTracker) pruneStates() {
-	if len(st.states) <= 64 {
-		return
-	}
-	// Keep the 64 most recent states.
-	var nums []uint64
-	for n := range st.states {
-		nums = append(nums, n)
-	}
-	// Simple sort for small lists.
-	for i := range nums {
-		for j := i + 1; j < len(nums); j++ {
-			if nums[j] < nums[i] {
-				nums[i], nums[j] = nums[j], nums[i]
-			}
-		}
-	}
-	for _, n := range nums[:len(nums)-64] {
-		delete(st.states, n)
-	}
-}

go.mod

@@ -5,7 +5,7 @@ go 1.26.0
 require (
 	github.com/charmbracelet/x/ansi v0.11.6
 	github.com/creack/pty v1.1.24
-	github.com/unixshells/vt-go v0.1.0
+	github.com/unixshells/vt-go v0.2.0
 	golang.org/x/crypto v0.49.0
 	golang.org/x/term v0.41.0
 )

go.sum

@@ -28,8 +28,8 @@ github.com/muesli/cancelreader v0.2.2 h1:3I4Kt4BQjOR54NavqnDogx/MIoWBFa0StPA8ELU
 github.com/muesli/cancelreader v0.2.2/go.mod h1:3XuTXfFS2VjM+HTLZY9Ak0l6eUKfijIfMUZ4EgX0QYo=
 github.com/rivo/uniseg v0.4.7 h1:WUdvkW8uEhrYfLC4ZzdpI2ztxP1I582+49Oc5Mq64VQ=
 github.com/rivo/uniseg v0.4.7/go.mod h1:FN3SvrM+Zdj16jyLfmOkMNblXMcoc8DfTHruCPUcx88=
-github.com/unixshells/vt-go v0.1.0 h1:HWILcExV8MHc6o2YqIRMIM/KrjH31kPzZrlvLS9BG6Y=
-github.com/unixshells/vt-go v0.1.0/go.mod h1:dZpOXxVyO7LG1uxkVMixX7HwUf2ZbUkEKhtHHJTTIBI=
+github.com/unixshells/vt-go v0.2.0 h1:fKOHymso7q4mE/TTRPBZMWtM5kZFoHdPUsk1kkLtypA=
+github.com/unixshells/vt-go v0.2.0/go.mod h1:dZpOXxVyO7LG1uxkVMixX7HwUf2ZbUkEKhtHHJTTIBI=
 github.com/xo/terminfo v0.0.0-20220910002029-abceb7e1c41e h1:JVG44RsyaB9T2KIHavMF/ppJZNG9ZpyihvCd0w101no=
 github.com/xo/terminfo v0.0.0-20220910002029-abceb7e1c41e/go.mod h1:RbqR21r5mrJuqunuUZ/Dhy/avygyECGrLceyNeo4LiM=
 golang.org/x/crypto v0.49.0 h1:+Ng2ULVvLHnJ/ZFEq4KdcDd/cfjrrjjNSXNzxg0Y4U4=

server.go

@@ -150,7 +150,7 @@ func (s *Server) Serve() error {
 		Cols: uint16(s.cols),
 	})
 
-	s.emu = vt.NewEmulator(s.cols, s.rows)
+	s.emu = vt.NewEmulatorNoScrollback(s.cols, s.rows)
 	s.curVisible.Store(true)
 	s.emu.SetCallbacks(vt.Callbacks{
 		CursorVisibility: func(visible bool) { s.curVisible.Store(visible) },
@@ -199,7 +199,7 @@ 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.emu = vt.NewEmulatorNoScrollback(s.cols, s.rows)
 	s.curVisible.Store(true)
 	s.emu.SetCallbacks(vt.Callbacks{
 		CursorVisibility: func(visible bool) { s.curVisible.Store(visible) },

transport.go

@@ -379,6 +379,14 @@ func (t *Transport) LastRecvNewNum() uint64 {
 	return t.lastRecvNewNum
 }
 
+// ThrowawayNum returns the server's throwaway number — states below this
+// are no longer referenced by the server and can be safely pruned.
+func (t *Transport) ThrowawayNum() uint64 {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	return t.throwawayNum
+}
+
 // LastRecv returns the time of the last received datagram.
 func (t *Transport) LastRecv() time.Time {
 	t.mu.Lock()