ssh signing: better error message when key not in agent
When signing a commit with a SSH key, with the private key missing from
ssh-agent, a confusing error message is produced:
error: Load key
"/var/folders/t5/cscwwl_n3n1_8_5j_00x_3t40000gn/T//.git_signing_key_tmpkArSj7":
invalid format? fatal: failed to write commit object
The temporary file .git_signing_key_tmpkArSj7 created by git contains a
valid *public* key. The error message comes from `ssh-keygen -Y sign' and
is caused by a fallback mechanism in ssh-keygen whereby it tries to
interpret .git_signing_key_tmpkArSj7 as a *private* key if it can't find in
the agent [1]. A fix is scheduled to be released in OpenSSH 9.1. All that
needs to be done is to pass an additional backward-compatible option -U to
'ssh-keygen -Y sign' call. With '-U', ssh-keygen always interprets the file
as public key and expects to find the private key in the agent.
As a result, when the private key is missing from the agent, a more accurate
error message gets produced:
error: Couldn't find key in agent
[1] https://bugzilla.mindrot.org/show_bug.cgi?id=3429
Signed-off-by: Adam Szkoda <adaszko@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>