Clear web terminal when session ends (#8850)

This change clears the screen when an ssh session ends (only in FIPS mode). Note: This doesn't currently do anything in `tsh` on Windows since BoringCrypto isn't supported, but once it is supported, the behavior will match Unix and web.

Co-authored-by: Grzegorz <[email protected]>
Co-authored-by: Russell Jones <[email protected]>

lib/client/session.go

@@ -26,6 +26,7 @@ import (
 	"os"
 	"os/signal"
 	"strings"
+	"sync"
 	"syscall"
 	"time"
 
@@ -68,6 +69,10 @@ type NodeSession struct {
 	// this session. It's also used to wait for everyone to close
 	closer *utils.CloseBroadcaster
 
+	// closeWait is used to wait for cleanup-related goroutines created by
+	// this session to close.
+	closeWait *sync.WaitGroup
+
 	ExitMsg string
 
 	enableEscapeSequences bool
@@ -108,6 +113,7 @@ func newSession(client *NodeClient,
 		stderr:                stderr,
 		namespace:             client.Namespace,
 		closer:                utils.NewCloseBroadcaster(),
+		closeWait:             &sync.WaitGroup{},
 		enableEscapeSequences: enableEscapeSequences,
 	}
 	// if we're joining an existing session, we need to assume that session's
@@ -140,6 +146,24 @@ func newSession(client *NodeClient,
 		ns.id = session.ID(sid)
 	}
 	ns.env[sshutils.SessionEnvVar] = string(ns.id)
+
+	// Close the Terminal when finished.
+	ns.closeWait.Add(1)
+	go func() {
+		defer ns.closeWait.Done()
+
+		<-ns.closer.C
+		if isFIPS() {
+			// \x1b[3J - clears scrollback (it is needed at least for the Mac terminal) -
+			// https://newbedev.com/how-do-i-reset-the-scrollback-in-the-terminal-via-a-shell-command
+			// \x1b\x63 - clears current screen - same as '\0033\0143' from https://superuser.com/a/123007
+			const resetPattern = "\x1b[3J\x1b\x63\n"
+			if _, err := ns.stdout.Write([]byte(resetPattern)); err != nil {
+				log.Warnf("Failed to clear screen: %v.", err)
+			}
+		}
+	}()
+
 	return ns, nil
 }
 

lib/client/terminal/terminal_common.go

@@ -0,0 +1,69 @@
+/*
+Copyright 2021 Gravitational, Inc.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+package terminal
+
+import (
+	"sync"
+
+	"github.com/gravitational/teleport"
+	"github.com/gravitational/trace"
+	"github.com/sirupsen/logrus"
+)
+
+var log = logrus.WithFields(logrus.Fields{
+	trace.Component: teleport.ComponentClient,
+})
+
+// ResizeEvent is emitted when a terminal window is resized.
+type ResizeEvent struct{}
+
+// StopEvent is emitted when the user sends a SIGSTOP
+type StopEvent struct{}
+
+type signalEmitter struct {
+	subscribers      []chan interface{}
+	subscribersMutex sync.Mutex
+}
+
+// Subscribe creates a channel that will receive terminal events.
+func (e *signalEmitter) Subscribe() chan interface{} {
+	e.subscribersMutex.Lock()
+	defer e.subscribersMutex.Unlock()
+
+	ch := make(chan interface{})
+	e.subscribers = append(e.subscribers, ch)
+
+	return ch
+}
+
+func (e *signalEmitter) writeEvent(event interface{}) {
+	e.subscribersMutex.Lock()
+	defer e.subscribersMutex.Unlock()
+
+	for _, sub := range e.subscribers {
+		sub <- event
+	}
+}
+
+func (e *signalEmitter) clearSubscribers() {
+	e.subscribersMutex.Lock()
+	defer e.subscribersMutex.Unlock()
+
+	for _, ch := range e.subscribers {
+		close(ch)
+	}
+	e.subscribers = e.subscribers[:0]
+}