[Bug 3872] Interactive input excessively buffered when using a mux socket if ControlPersist is set

[bugzilla-daemon at mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=3872

--- Comment #10 from Darren Tucker <dtucker at dtucker.net> ---
Given there's a reproducer and a relatively small range of changes, I
bisected it down to one of the following (the SIGINFO changes don't
compile until the ifdef wrapping):

There are only 'skip'ped commits left to test.
The first bad commit could be any of:
dc5147028ff19213a32281dad07bba02e58da3fa
289239046b2c4b0076c14394ae9703a879e78706
f807a598c96be683d97810481e954ec9db6b0027
b7ee13fbbb4ebafcf71f29685f053ecb97d1bcef
We cannot bisect more!

commit b7ee13fbbb4ebafcf71f29685f053ecb97d1bcef
Author: Damien Miller <djm at mindrot.org>
Date:   Mon Aug 18 14:22:18 2025 +1000

    wrap SIGINFO in ifdef

commit 289239046b2c4b0076c14394ae9703a879e78706 (HEAD)
Author: djm at openbsd.org <djm at openbsd.org>
Date:   Mon Aug 18 03:43:01 2025 +0000

    upstream: Make ssh(1) and sshd(8) set IP QoS (aka IP_TOS,
IPV6_TCLASS)

    continually at runtime based on what sessions/channels are open.

    Previously, ssh(1) and sshd(8) would pick a QoS value when they
    were started and use it for the whole connection. This could
    produce suboptimal choices for the QoS value, e.g. for multiplexed
    sessions that started interactive but picked up a sftp client,
    or sessions that moved large amounts of data via port forwarding.

    Now the QoS value will change to the non-interactive IPQoS whenever
    a "non-interactive" channel is open; basically any channel that
lacks
    a tty other than agent forwarding.

    This is important now that the default interactive IPQoS is EF
    (Expedited Forwarding), as many networks are configured to allow
    only relatively small amounts of traffic of this class and they
will
    aggressively deprioritise the entire connection if this is
exceeded.

    NB. because ssh(1) and sshd(8) now change IP_TOS/IPV6_TCLASS
    continually via setsockopt(), this commit requires a recent
pledge(2)
    change that landed recently in the OpenBSD kernel. Please ensure
    you have updated to a kernel from within the last two weeks before
    updating OpenSSH.

    with job@ deraadt@

    OpenBSD-Commit-ID: 325fc41717eecdf5e4b534bfa8d66817425b840f

commit dc5147028ff19213a32281dad07bba02e58da3fa
Author: djm at openbsd.org <djm at openbsd.org>
Date:   Mon Aug 18 03:29:11 2025 +0000

    upstream: SIGINFO handler for sshd(8) to dump active

    channels/sessions ok deraadt@

    OpenBSD-Commit-ID: 9955cb6d157c6d7aa23a819e8ef61b1edabc8b7d

commit f807a598c96be683d97810481e954ec9db6b0027
Author: djm at openbsd.org <djm at openbsd.org>
Date:   Mon Aug 18 03:28:36 2025 +0000

    upstream: SIGINFO handler for ssh(1) to dump active

    channels/sessions ok deraadt@

    OpenBSD-Commit-ID: 12f88a5044bca40ef5f41ff61b1755d0e25df901

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