[PATCH v5 3/9] ssh signing: retrieve a default key from ssh-agent

["Fabian Stelzer via GitGitGadget" <gitgitgadget@gmail.com>]

From: Fabian Stelzer <fs@gigacodes.de>

if user.signingkey is not set and a ssh signature is requested we call
ssh-add -L and use the first key we get

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
---
 gpg-interface.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/gpg-interface.c b/gpg-interface.c
index c131977b347..3afacb48900 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -470,11 +470,35 @@ int git_gpg_config(const char *var, const char *value, void *cb)
 	return 0;
 }
 
+/* Returns the first public key from an ssh-agent to use for signing */
+static char *get_default_ssh_signing_key(void)
+{
+	struct child_process ssh_add = CHILD_PROCESS_INIT;
+	int ret = -1;
+	struct strbuf key_stdout = STRBUF_INIT;
+	struct strbuf **keys;
+
+	strvec_pushl(&ssh_add.args, "ssh-add", "-L", NULL);
+	ret = pipe_command(&ssh_add, NULL, 0, &key_stdout, 0, NULL, 0);
+	if (!ret) {
+		keys = strbuf_split_max(&key_stdout, '\n', 2);
+		if (keys[0])
+			return strbuf_detach(keys[0], NULL);
+	}
+
+	strbuf_release(&key_stdout);
+	return "";
+}
+
 const char *get_signing_key(void)
 {
 	if (configured_signing_key)
 		return configured_signing_key;
-	return git_committer_info(IDENT_STRICT|IDENT_NO_DATE);
+	if (!strcmp(use_format->name, "ssh")) {
+		return get_default_ssh_signing_key();
+	} else {
+		return git_committer_info(IDENT_STRICT | IDENT_NO_DATE);
+	}
 }
 
 int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *signing_key)
-- 
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