ssh: fix spinloop in channel SendRequest drain on closed channel

The drain loop added in 3c7c869 ("ssh: fix deadlock on unexpected
channel responses") receives from ch.msg without checking the comma-ok
flag. Once ch.msg is closed by channel.close(), receives succeed
immediately with the zero value, so the default arm is never taken
and the loop spins forever, pinning a CPU core for every concurrent
SendRequest caller on a torn-down channel.

Use the comma-ok idiom to detect the closed channel and break out.

Fixes golang/go#79658

Change-Id: Id80aa5d8429550c7a68e8759aed9ffba3aabb006
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/783680
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Neal Patel <neal@golang.org>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Neal Patel <nealpatel@google.com>

ssh/channel.go

@@ -634,7 +634,10 @@ func (ch *channel) SendRequest(name string, wantReply bool, payload []byte) (boo
 	drain:
 		for {
 			select {
-			case <-ch.msg:
+			case _, ok := <-ch.msg:
+				if !ok {
+					break drain
+				}
 			default:
 				break drain
 			}

ssh/mux_test.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"sync"
 	"testing"
+	"time"
 )
 
 func muxPair() (*mux, *mux) {
@@ -1366,3 +1367,41 @@ func TestChannelResponseAcceptedWhilePending(t *testing.T) {
 	writer.Close()
 	<-serverDone
 }
+
+func TestChannelSendRequestAfterCloseNoSpinloop(t *testing.T) {
+	writer, reader, mux := channelPair(t)
+	defer writer.Close()
+	defer mux.Close()
+
+	// Close the channel, this closes reader.msg.
+	if err := reader.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+	// Wait for reader.msg to actually be closed by the mux loop.
+	for {
+		select {
+		case _, ok := <-reader.msg:
+			if !ok {
+				goto closed
+			}
+		case <-time.After(2 * time.Second):
+			t.Fatal("reader.msg was not closed")
+		}
+	}
+closed:
+
+	done := make(chan error, 1)
+	go func() {
+		_, err := reader.SendRequest("ping", true, nil)
+		done <- err
+	}()
+
+	select {
+	case err := <-done:
+		if !errors.Is(err, io.EOF) {
+			t.Fatalf("SendRequest after close: got %v, want io.EOF", err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("SendRequest spinloop on closed ch.msg")
+	}
+}