credential: add method for querying capabilities

Right now, there's no specific way to determine whether a credential
helper or git credential itself supports a given set of capabilities.
It would be helpful to have such a way, so let's let credential helpers
and git credential take an argument, "capability", which has it list the
capabilities and a version number on standard output.

Specifically choose a format that is slightly different from regular
credential output and assume that no capabilities are supported if a
non-zero exit status occurs or the data deviates from the format.  It is
common for users to write small shell scripts as the argument to
credential.helper, which will almost never be designed to emit
capabilities.  We want callers to gracefully handle this case by
assuming that they are not capable of extended support because that is
almost certainly the case, and specifying the error behavior up front
does this and preserves backwards compatibility in a graceful way.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 16 insertions, 0 deletions

diff --git a/builtin/credential-cache.c b/builtin/credential-cache.c
index bba96d4ffd..f5c989e2b2 100644
--- a/builtin/credential-cache.c
+++ b/builtin/credential-cache.c
@@ -1,4 +1,5 @@
 #include "builtin.h"
+#include "credential.h"
 #include "gettext.h"
 #include "parse-options.h"
 #include "path.h"
@@ -127,6 +128,13 @@ static char *get_socket_path(void)
 	return socket;
 }
 
+static void announce_capabilities(void)
+{
+	struct credential c = CREDENTIAL_INIT;
+	c.capa_authtype.request_initial = 1;
+	credential_announce_capabilities(&c, stdout);
+}
+
 int cmd_credential_cache(int argc, const char **argv, const char *prefix)
 {
 	char *socket_path = NULL;
@@ -160,6 +168,8 @@ int cmd_credential_cache(int argc, const char **argv, const char *prefix)
 		do_cache(socket_path, op, timeout, FLAG_RELAY);
 	else if (!strcmp(op, "store"))
 		do_cache(socket_path, op, timeout, FLAG_RELAY|FLAG_SPAWN);
+	else if (!strcmp(op, "capability"))
+		announce_capabilities();
 	else
 		; /* ignore unknown operation */
 
diff --git a/builtin/credential.c b/builtin/credential.c
index 3568e57025..5100d441f2 100644
--- a/builtin/credential.c
+++ b/builtin/credential.c
@@ -17,6 +17,12 @@ int cmd_credential(int argc, const char **argv, const char *prefix UNUSED)
 		usage(usage_msg);
 	op = argv[1];
 
+	if (!strcmp(op, "capability")) {
+		credential_set_all_capabilities(&c, CREDENTIAL_OP_INITIAL);
+		credential_announce_capabilities(&c, stdout);
+		return 0;
+	}
+
 	if (credential_read(&c, stdin, CREDENTIAL_OP_INITIAL) < 0)
 		die("unable to read credential from stdin");