[PATCH] ssh signing: better error message when key not in agent

["Adam Szkoda via GitGitGadget" <gitgitgadget@gmail.com>]

From: Adam Szkoda <adaszko@gmail.com>

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>
---
    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

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1270%2Fradicle-dev%2Fmaint-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1270/radicle-dev/maint-v1
Pull-Request: https://github.com/git/git/pull/1270

 gpg-interface.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/gpg-interface.c b/gpg-interface.c
index 280f1fa1a58..4a5913ae942 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -1022,6 +1022,7 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
 	strvec_pushl(&signer.args, use_format->program,
 		     "-Y", "sign",
 		     "-n", "git",
+		     "-U",
 		     "-f", ssh_signing_key_file,
 		     buffer_file->filename.buf,
 		     NULL);

base-commit: e54793a95afeea1e10de1e5ad7eab914e7416250
-- 
gitgitgadget