transports: allow the creds callback to say it doesn't exist

Allow the credentials callback to return GIT_PASSTHROUGH to make the
transports code behave as though none was set.

This should make it easier for bindings to behave closer to the C code
when there is no credentials callback set at their level.

include/git2/remote.h

@@ -468,6 +468,9 @@ struct git_remote_callbacks {
 	/**
 	 * This will be called if the remote host requires
 	 * authentication in order to connect to it.
+	 *
+	 * Returning GIT_PASSTHROUGH will make libgit2 behave as
+	 * though this field isn't set.
 	 */
 	int (*credentials)(git_cred **cred, const char *url, const char *username_from_url, unsigned int allowed_types,	void *data);
 

src/transports/http.c

@@ -248,6 +248,7 @@ static int on_headers_complete(http_parser *parser)
 	http_subtransport *t = ctx->t;
 	http_stream *s = ctx->s;
 	git_buf buf = GIT_BUF_INIT;
+	int error = 0, no_callback = 0;
 
 	/* Both parse_header_name and parse_header_value are populated
 	 * and ready for consumption. */
@@ -256,9 +257,12 @@ static int on_headers_complete(http_parser *parser)
 			return t->parse_error = PARSE_ERROR_GENERIC;
 
 	/* Check for an authentication failure. */
+
 	if (parser->status_code == 401 &&
-		get_verb == s->verb &&
-		t->owner->cred_acquire_cb) {
+	    get_verb == s->verb) {
+		if (!t->owner->cred_acquire_payload) {
+			no_callback = 1;
+		} else {
 			int allowed_types = 0;
 
 			if (parse_unauthorized_response(&t->www_authenticate,
@@ -268,19 +272,30 @@ static int on_headers_complete(http_parser *parser)
 			if (allowed_types &&
 			    (!t->cred || 0 == (t->cred->credtype & allowed_types))) {
 
-			if (t->owner->cred_acquire_cb(&t->cred,
+				error = t->owner->cred_acquire_cb(&t->cred,
 								  t->owner->url,
 								  t->connection_data.user,
 								  allowed_types,
-					t->owner->cred_acquire_payload) < 0)
-				return PARSE_ERROR_GENERIC;
+								  t->owner->cred_acquire_payload);
 
+				if (error == GIT_PASSTHROUGH) {
+					no_callback = 1;
+				} else if (error < 0) {
+					return PARSE_ERROR_GENERIC;
+				} else {
 					assert(t->cred);
 
 					/* Successfully acquired a credential. */
 					return t->parse_error = PARSE_ERROR_REPLAY;
 				}
 			}
+		}
+
+		if (no_callback) {
+			giterr_set(GITERR_NET, "authentication required but no callback set");
+			return t->parse_error = PARSE_ERROR_GENERIC;
+		}
+	}
 
 	/* Check for a redirect.
 	 * Right now we only permit a redirect to the same hostname. */

src/transports/ssh.c

@@ -387,6 +387,7 @@ static int _git_ssh_setup_conn(
 {
 	char *host=NULL, *port=NULL, *path=NULL, *user=NULL, *pass=NULL;
 	const char *default_port="22";
+	int no_callback = 0;
 	ssh_stream *s;
 	LIBSSH2_SESSION* session=NULL;
 	LIBSSH2_CHANNEL* channel=NULL;
@@ -413,24 +414,31 @@ static int _git_ssh_setup_conn(
 	if (user && pass) {
 		if (git_cred_userpass_plaintext_new(&t->cred, user, pass) < 0)
 			goto on_error;
-	} else if (t->owner->cred_acquire_cb) {
-		if (t->owner->cred_acquire_cb(
-				&t->cred, t->owner->url, user,
+	} else if (!t->owner->cred_acquire_cb) {
+		no_callback = 1;
+	} else {
+		int error;
+		error = t->owner->cred_acquire_cb(&t->cred, t->owner->url, user,
 			GIT_CREDTYPE_USERPASS_PLAINTEXT |
-				GIT_CREDTYPE_SSH_KEY |
-				GIT_CREDTYPE_SSH_INTERACTIVE |
-				GIT_CREDTYPE_SSH_CUSTOM,
-				t->owner->cred_acquire_payload) < 0)
-			goto on_error;
+			GIT_CREDTYPE_SSH_KEY | GIT_CREDTYPE_SSH_CUSTOM |
+			GIT_CREDTYPE_SSH_INTERACTIVE,
+			t->owner->cred_acquire_payload);
 
-		if (!t->cred) {
+		if (error == GIT_PASSTHROUGH)
+			no_callback = 1;
+		else if (error < 0)
+			goto on_error;
+		else if (!t->cred) {
 			giterr_set(GITERR_SSH, "Callback failed to initialize SSH credentials");
 			goto on_error;
 		}
-	} else {
-		giterr_set(GITERR_SSH, "Cannot set up SSH connection without credentials");
+	}
+
+	if (no_callback) {
+		giterr_set(GITERR_SSH, "authentication required but no callback set");
 		goto on_error;
 	}
+
 	assert(t->cred);
 
 	if (_git_ssh_session_create(&session, s->socket) < 0)