Terminal input lags behind when using mux with OpenSSH 10.1p1 + macOS 26.1b2

[Damien Miller]

On Tue, 7 Oct 2025, Jan Schermer wrote:

> Hello, I just upgraded openssh on my laptop using homebrew to 10.1p1
> and macOS to developer beta 2 and it no longer works right.
>
> At first I thought it was lag, but it is actually ssh waiting for
>input of several (more) characters before it both displays output or
>executes remote commands (input).
>
> It onlly happens if 10.1p1 is the version running the master mux.
> When macOS default client (OpenSSH_10.0p2 in this version) is the
> one hosting the master, then everything works fine. Vice versa, when
> OpenSSH_10.0p2 is the one attaching to 10.1p1 socket running the
> master then it lags.
>
> I have narrowed the problem down to simply using or not using
> ControlMaster (ControlPath=socket vs ControlPath=none). Tested with
> several versions of remote sshd on Debian, Ubuntu and FreeBSD and ssh
> client in iTerm2 and Terminal.app without change. Recompiling from
> source with newest Xcode on 26.1b2 didn’t fix it. Unfortunately I
> don’t have a non-beta OS to test.

Yes, this is https://bugzilla.mindrot.org/show_bug.cgi?id=3872

I've just committed a fix on the V_10_1 branch to fix this, also
below.


commit beae06f56e0d0a66ca535896149d5fb0b2e8a1b4
Author: djm at openbsd.org <djm at openbsd.org>
Date:   Tue Oct 7 08:02:32 2025 +0000

    upstream: don't reuse c->isatty for signalling that the remote channel
    
    has a tty attached as this causes side effects, e.g. in channel_handle_rfd().
    bz3872
    
    ok markus@
    
    OpenBSD-Commit-ID: 4cd8a9f641498ca6089442e59bad0fd3dcbe85f8

diff --git a/channels.c b/channels.c
index f1d7bcf34..80014ff34 100644
--- a/channels.c
+++ b/channels.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: channels.c,v 1.451 2025/09/25 06:33:19 djm Exp $ */
+/* $OpenBSD: channels.c,v 1.452 2025/10/07 08:02:32 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo at cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo at cs.hut.fi>, Espoo, Finland
@@ -362,7 +362,7 @@ channel_classify(struct ssh *ssh, Channel *c)
 {
 	struct ssh_channels *sc = ssh->chanctxt;
 	const char *type = c->xctype == NULL ? c->ctype : c->xctype;
-	const char *classifier = c->isatty ?
+	const char *classifier = (c->isatty || c->remote_has_tty) ?
 	    sc->bulk_classifier_tty : sc->bulk_classifier_notty;
 
 	c->bulk = type != NULL && match_pattern_list(type, classifier, 0) == 1;
@@ -566,7 +566,7 @@ channel_new(struct ssh *ssh, char *ctype, int type, int rfd, int wfd, int efd,
 void
 channel_set_tty(struct ssh *ssh, Channel *c)
 {
-	c->isatty = 1;
+	c->remote_has_tty = 1;
 	channel_classify(ssh, c);
 }
 
@@ -1078,7 +1078,8 @@ channel_format_status(const Channel *c)
 	    c->rfd, c->wfd, c->efd, c->sock, c->ctl_chan,
 	    c->have_ctl_child_id ? "c" : "nc", c->ctl_child_id,
 	    c->io_want, c->io_ready,
-	    c->isatty ? "T" : "", c->bulk ? "B" : "I");
+	    c->isatty ? "T" : (c->remote_has_tty ? "RT" : ""),
+	    c->bulk ? "B" : "I");
 	return ret;
 }
 
diff --git a/channels.h b/channels.h
index df7c7f364..7456541f8 100644
--- a/channels.h
+++ b/channels.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: channels.h,v 1.161 2025/09/25 06:33:19 djm Exp $ */
+/* $OpenBSD: channels.h,v 1.162 2025/10/07 08:02:32 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo at cs.hut.fi>
@@ -145,6 +145,7 @@ struct Channel {
 	int     ctl_chan;	/* control channel (multiplexed connections) */
 	uint32_t ctl_child_id;	/* child session for mux controllers */
 	int	have_ctl_child_id;/* non-zero if ctl_child_id is valid */
+	int     remote_has_tty;	/* remote side has a tty */
 	int     isatty;		/* rfd is a tty */
 #ifdef _AIX
 	int     wfd_isatty;	/* wfd is a tty */