ssh/tailssh: use context.WithCancelCause

It was using a custom implmentation of the context.WithCancelCause,
replace usage with stdlib.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ssh/tailssh/context.go

@@ -1,63 +0,0 @@
-// Copyright (c) Tailscale Inc & AUTHORS
-// SPDX-License-Identifier: BSD-3-Clause
-
-package tailssh
-
-import (
-	"context"
-	"sync"
-	"time"
-)
-
-// sshContext is the context.Context implementation we use for SSH
-// that adds a CloseWithError method. Otherwise it's just a normalish
-// Context.
-type sshContext struct {
-	underlying context.Context
-	cancel     context.CancelFunc // cancels underlying
-	mu         sync.Mutex
-	closed     bool
-	err        error
-}
-
-func newSSHContext(ctx context.Context) *sshContext {
-	ctx, cancel := context.WithCancel(ctx)
-	return &sshContext{underlying: ctx, cancel: cancel}
-}
-
-func (ctx *sshContext) CloseWithError(err error) {
-	ctx.mu.Lock()
-	defer ctx.mu.Unlock()
-	if ctx.closed {
-		return
-	}
-	ctx.closed = true
-	ctx.err = err
-	ctx.cancel()
-}
-
-func (ctx *sshContext) Err() error {
-	ctx.mu.Lock()
-	defer ctx.mu.Unlock()
-	return ctx.err
-}
-
-func (ctx *sshContext) Done() <-chan struct{}                   { return ctx.underlying.Done() }
-func (ctx *sshContext) Deadline() (deadline time.Time, ok bool) { return }
-func (ctx *sshContext) Value(k any) any                         { return ctx.underlying.Value(k) }
-
-// userVisibleError is a wrapper around an error that implements
-// SSHTerminationError, so msg is written to their session.
-type userVisibleError struct {
-	msg string
-	error
-}
-
-func (ue userVisibleError) SSHTerminationMessage() string { return ue.msg }
-
-// SSHTerminationError is implemented by errors that terminate an SSH
-// session and should be written to user's sessions.
-type SSHTerminationError interface {
-	error
-	SSHTerminationMessage() string
-}

ssh/tailssh/tailssh.go

@@ -787,7 +787,8 @@ type sshSession struct {
 	sharedID string // ID that's shared with control
 	logf     logger.Logf
 
-	ctx           *sshContext // implements context.Context
+	ctx           context.Context
+	cancelCtx     context.CancelCauseFunc
 	conn          *conn
 	agentListener net.Listener // non-nil if agent-forwarding requested+allowed
 
@@ -812,12 +813,14 @@ func (ss *sshSession) vlogf(format string, args ...interface{}) {
 func (c *conn) newSSHSession(s ssh.Session) *sshSession {
 	sharedID := fmt.Sprintf("sess-%s-%02x", c.srv.now().UTC().Format("20060102T150405"), randBytes(5))
 	c.logf("starting session: %v", sharedID)
+	ctx, cancel := context.WithCancelCause(s.Context())
 	return &sshSession{
-		Session:  s,
-		sharedID: sharedID,
-		ctx:      newSSHContext(s.Context()),
-		conn:     c,
-		logf:     logger.WithPrefix(c.srv.logf, "ssh-session("+sharedID+"): "),
+		Session:   s,
+		sharedID:  sharedID,
+		ctx:       ctx,
+		cancelCtx: cancel,
+		conn:      c,
+		logf:      logger.WithPrefix(c.srv.logf, "ssh-session("+sharedID+"): "),
 	}
 }
 
@@ -844,7 +847,7 @@ func (c *conn) checkStillValid() {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	for _, s := range c.sessions {
-		s.ctx.CloseWithError(userVisibleError{
+		s.cancelCtx(userVisibleError{
 			fmt.Sprintf("Access revoked.\r\n"),
 			context.Canceled,
 		})
@@ -897,7 +900,7 @@ func (ss *sshSession) killProcessOnContextDone() {
 	// Either the process has already exited, in which case this does nothing.
 	// Or, the process is still running in which case this will kill it.
 	ss.exitOnce.Do(func() {
-		err := ss.ctx.Err()
+		err := context.Cause(ss.ctx)
 		if serr, ok := err.(SSHTerminationError); ok {
 			msg := serr.SSHTerminationMessage()
 			if msg != "" {
@@ -997,7 +1000,7 @@ var recordSSH = envknob.RegisterBool("TS_DEBUG_LOG_SSH")
 func (ss *sshSession) run() {
 	metricActiveSessions.Add(1)
 	defer metricActiveSessions.Add(-1)
-	defer ss.ctx.CloseWithError(errSessionDone)
+	defer ss.cancelCtx(errSessionDone)
 
 	if attached := ss.conn.srv.attachSessionToConnIfNotShutdown(ss); !attached {
 		fmt.Fprintf(ss, "Tailscale SSH is shutting down\r\n")
@@ -1011,7 +1014,7 @@ func (ss *sshSession) run() {
 
 	if ss.conn.finalAction.SessionDuration != 0 {
 		t := time.AfterFunc(ss.conn.finalAction.SessionDuration, func() {
-			ss.ctx.CloseWithError(userVisibleError{
+			ss.cancelCtx(userVisibleError{
 				fmt.Sprintf("Session timeout of %v elapsed.", ss.conn.finalAction.SessionDuration),
 				context.DeadlineExceeded,
 			})
@@ -1066,7 +1069,7 @@ func (ss *sshSession) run() {
 		defer ss.stdin.Close()
 		if _, err := io.Copy(rec.writer("i", ss.stdin), ss); err != nil {
 			logf("stdin copy: %v", err)
-			ss.ctx.CloseWithError(err)
+			ss.cancelCtx(err)
 		}
 	}()
 	var openOutputStreams atomic.Int32
@@ -1080,7 +1083,7 @@ func (ss *sshSession) run() {
 		_, err := io.Copy(rec.writer("o", ss), ss.stdout)
 		if err != nil && !errors.Is(err, io.EOF) {
 			logf("stdout copy: %v", err)
-			ss.ctx.CloseWithError(err)
+			ss.cancelCtx(err)
 		}
 		if openOutputStreams.Add(-1) == 0 {
 			ss.CloseWrite()
@@ -1489,3 +1492,19 @@ var (
 	metricSFTP                 = clientmetric.NewCounter("ssh_sftp_requests")
 	metricLocalPortForward     = clientmetric.NewCounter("ssh_local_port_forward_requests")
 )
+
+// userVisibleError is a wrapper around an error that implements
+// SSHTerminationError, so msg is written to their session.
+type userVisibleError struct {
+	msg string
+	error
+}
+
+func (ue userVisibleError) SSHTerminationMessage() string { return ue.msg }
+
+// SSHTerminationError is implemented by errors that terminate an SSH
+// session and should be written to user's sessions.
+type SSHTerminationError interface {
+	error
+	SSHTerminationMessage() string
+}