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

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

            Bug ID: 3872
           Summary: Interactive input excessively buffered when using a
                    mux socket if ControlPersist is set
           Product: Portable OpenSSH
           Version: 10.0p2
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: grawity at gmail.com

With 10.0, connecting to an SSH server with a mux socket active results
in all interactive input being *heavily* buffered. For example, hitting
'Enter' once shows no response; hitting 'Enter' a second time sends
both keypresses. Sometimes 3-4 keypresses are buffered up.

But this only occurs if ControlPersist is set to something other than
'no'. (I have it set to "5s" in my config, due to historical reasons.)

good: -S none
good: -S /tmp/sock -o ControlPersist=no
bad: -S /tmp/sock -o ControlPersist=5s

Git bisect points towards 289239046b2c "upstream: Make ssh(1) and
sshd(8) set IP QoS (aka IP_TOS, IPV6_TCLASS)". I'm pretty sure it is
not the usual "DSCP intolerant network" issue since it also happens
with `ssh localhost`.

Though just in case, I tried "-o IPQoS=none" which is documented as
using the OS default, and got:

debug2: client_loop: session QoS is now interactive
debug2: fd 3 setting TCP_NODELAY
debug1: set_sock_tos: invalid TOS 2147483647

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