openssh unix dev - Oct 2025 - Terminal input lags behind when using mux with OpenSSH 10.1p1 + macOS 26.1b2

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.

Let me know how I can help debug this or if someone can replicate it.

Thanks
Jan

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 */