[PATCH v3] ssh signing: better error message when key not in agent

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-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1270/radicle-dev/maint-v3
Pull-Request: https://github.com/git/git/pull/1270

Range-diff vs v2:

 1:  03dfca79387 ! 1:  dc7acff3b95 ssh signing: better error message when key not in agent
     @@ gpg-interface.c: static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf
       		if (!key_file)
       			return error_errno(
      @@ gpg-interface.c: static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
     - 	}
     - 
     - 	strvec_pushl(&signer.args, use_format->program,
     --		     "-Y", "sign",
     --		     "-n", "git",
     --		     "-f", ssh_signing_key_file,
     + 		     "-Y", "sign",
     + 		     "-n", "git",
     + 		     "-f", ssh_signing_key_file,
      -		     buffer_file->filename.buf,
     --		     NULL);
     -+			"-Y", "sign",
     -+			"-n", "git",
     -+			"-f", ssh_signing_key_file,
     -+			NULL);
     -+	if (literal_ssh_key) {
     + 		     NULL);
     ++	if (literal_ssh_key)
      +		strvec_push(&signer.args, "-U");
     -+	}
      +	strvec_push(&signer.args, buffer_file->filename.buf);
       
       	sigchain_push(SIGPIPE, SIG_IGN);

 gpg-interface.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)