ssh/tailssh: close sshContext on context cancellation

This was preventing tailscaled from shutting down properly if there were
active sessions in certain states (e.g. waiting in check mode).

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

ssh/tailssh/context.go

@@ -5,6 +5,7 @@
 package tailssh
 
 import (
+	"context"
 	"sync"
 	"time"
 )
@@ -13,14 +14,16 @@
 // that adds a CloseWithError method. Otherwise it's just a normalish
 // Context.
 type sshContext struct {
-	mu     sync.Mutex
-	closed bool
-	done   chan struct{}
-	err    error
+	underlying context.Context
+	cancel     context.CancelFunc // cancels underlying
+	mu         sync.Mutex
+	closed     bool
+	err        error
 }
 
-func newSSHContext() *sshContext {
-	return &sshContext{done: make(chan struct{})}
+func newSSHContext(ctx context.Context) *sshContext {
+	ctx, cancel := context.WithCancel(ctx)
+	return &sshContext{underlying: ctx, cancel: cancel}
 }
 
 func (ctx *sshContext) CloseWithError(err error) {
@@ -31,7 +34,7 @@ func (ctx *sshContext) CloseWithError(err error) {
 	}
 	ctx.closed = true
 	ctx.err = err
-	close(ctx.done)
+	ctx.cancel()
 }
 
 func (ctx *sshContext) Err() error {
@@ -40,9 +43,9 @@ func (ctx *sshContext) Err() error {
 	return ctx.err
 }
 
-func (ctx *sshContext) Done() <-chan struct{}                   { return ctx.done }
+func (ctx *sshContext) Done() <-chan struct{}                   { return ctx.underlying.Done() }
 func (ctx *sshContext) Deadline() (deadline time.Time, ok bool) { return }
-func (ctx *sshContext) Value(any) any                           { return nil }
+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.

ssh/tailssh/tailssh.go

@@ -770,7 +770,7 @@ func (c *conn) newSSHSession(s ssh.Session) *sshSession {
 	return &sshSession{
 		Session:  s,
 		sharedID: sharedID,
-		ctx:      newSSHContext(),
+		ctx:      newSSHContext(s.Context()),
 		conn:     c,
 		logf:     logger.WithPrefix(c.srv.logf, "ssh-session("+sharedID+"): "),
 	}