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>

diff --git a/gpg-interface.c b/gpg-interface.c
index f877a1ea56460de8e7c831b58ab4cfbda837a304..687236430bf21c7c64d9a13128af016cf579d3c3 100644 (file)
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -998,6 +998,7 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
        char *ssh_signing_key_file = NULL;
        struct strbuf ssh_signature_filename = STRBUF_INIT;
        const char *literal_key = NULL;
+       int literal_ssh_key = 0;
 
        if (!signing_key || signing_key[0] == '\0')
                return error(
@@ -1005,6 +1006,7 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
 
        if (is_literal_ssh_key(signing_key, &literal_key)) {
                /* A literal ssh key */
+               literal_ssh_key = 1;
                key_file = mks_tempfile_t(".git_signing_key_tmpXXXXXX");
                if (!key_file)
                        return error_errno(
@@ -1039,8 +1041,10 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
                     "-Y", "sign",
                     "-n", "git",
                     "-f", ssh_signing_key_file,
-                    buffer_file->filename.buf,
                     NULL);
+       if (literal_ssh_key)
+               strvec_push(&signer.args, "-U");
+       strvec_push(&signer.args, buffer_file->filename.buf);
 
        sigchain_push(SIGPIPE, SIG_IGN);
        ret = pipe_command(&signer, NULL, 0, NULL, 0, &signer_stderr, 0);