Sorry, I spoke too soon on that last message, the bug was still there, I was
just running the agent at version 2.1.7... not awake yet.

Anyway, your patch solved the issue of not being able to add new keys to the
agent via ssh-add, though it may have raised another issue.

I successfully added a new key to the agent, then I removed it from the
ssh-control file and added it again. When trying to readd it after restarting
the agent, it did not show a password prompt to set the password. Instead it
returned a successful message without actually adding the key to the agent.

% ssh-add foo
Identity added: foo (foo)

backtrace without the arch patch, for RSA key

#0 ssh_identity_register (confirm=0, ttl=0, key=0x7ff366864800,
spec=0x7ff36577adb0, ctrl=0x14e0da0) at command-ssh.c:3112
#1 ssh_handler_add_identity (ctrl=0x14e0da0, request=<optimized out>,
response=0x7ff360004d20) at command-ssh.c:3247
#2 0x0000000000417948 in ssh_request_process (stream_sock=0x7ff3600009c0,
ctrl=0x14e0da0) at command-ssh.c:3513
#3 start_command_handler_ssh (ctrl=ctrl@entry=0x14e0da0, sock_client=<optimized
out>) at command-ssh.c:3619
#4 0x0000000000409c3b in start_connection_thread_ssh (arg=0x14e0da0) at
gpg-agent.c:2278
#5 0x00007ff365d3ee5c in ?? () from /usr/lib/libnpth.so.0
#6 0x00007ff365b274a4 in start_thread () from /usr/lib/libpthread.so.0
#7 0x00007ff36586513d in clone () from /usr/lib/libc.so.6

This patch was applied to the build

https://projects.archlinux.org/svntogit/packages.git/tree/trunk/ssh-ed25519.patch?h=packages/gnupg

Here is a gdb backtrace of the segfault

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Type "apropos word" to search for commands related to "word"...
Reading symbols from /usr/bin/gpg-agent...done.
[New LWP 7934]
[New LWP 7066]

warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
Core was generated by `/usr/bin/gpg-agent --daemon --debug-level 99
--enable-ssh-support --log-file /h'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 ssh_identity_register (confirm=0, ttl=0, key=0x7f9a398ee800,
spec=0x7f9a38804db0, ctrl=0x11fdda0) at command-ssh.c:3114
3114 command-ssh.c: No such file or directory.
[Current thread is 1 (Thread 0x7f9a38805700 (LWP 7934))]
(gdb) bt
#0 ssh_identity_register (confirm=0, ttl=0, key=0x7f9a398ee800,
spec=0x7f9a38804db0, ctrl=0x11fdda0) at command-ssh.c:3114
#1 ssh_handler_add_identity (ctrl=0x11fdda0, request=<optimized out>,
response=0x7f9a34004d20) at command-ssh.c:3249
#2 0x0000000000417948 in ssh_request_process (stream_sock=0x7f9a340009c0,
ctrl=0x11fdda0) at command-ssh.c:3515
#3 start_command_handler_ssh (ctrl=ctrl@entry=0x11fdda0, sock_client=<optimized
out>) at command-ssh.c:3621
#4 0x0000000000409c3b in start_connection_thread_ssh (arg=0x11fdda0) at
gpg-agent.c:2278
#5 0x00007f9a38dc8e5c in ?? () from /usr/lib/libnpth.so.0
#6 0x00007f9a38bb14a4 in start_thread () from /usr/lib/libpthread.so.0
#7 0x00007f9a388ef13d in clone () from /usr/lib/libc.so.6

It seems this is related to T2096 but the key in question here is rsa, not
ed25519. Downgrading to 2.1.7 worked for me as well.