credential: add support for multistage credential rounds

Over HTTP, NTLM and Kerberos require two rounds of authentication on the
client side.  It's possible that there are custom authentication schemes
that also implement this same approach.  Since these are tricky schemes
to implement and the HTTP library in use may not always handle them
gracefully on all systems, it would be helpful to allow the credential
helper to implement them instead for increased portability and
robustness.

To allow this to happen, add a boolean flag, continue, that indicates
that instead of failing when we get a 401, we should retry another round
of authentication.  However, this necessitates some changes in our
current credential code so that we can make this work.

Keep the state[] headers between iterations, but only use them to send
to the helper and only consider the new ones we read from the credential
helper to be valid on subsequent iterations.  That avoids us passing
stale data when we finally approve or reject the credential.  Similarly,
clear the multistage and wwwauth[] values appropriately so that we
don't pass stale data or think we're trying a multiround response when
we're not.  Remove the credential values so that we can actually fill a
second time with new responses.

Limit the number of iterations of reauthentication we do to 3.  This
means that if there's a problem, we'll terminate with an error message
instead of retrying indefinitely and not informing the user (and
possibly conducting a DoS on the server).

In our tests, handle creating multiple response output files from our
helper so we can verify that each of the messages sent is correct.

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

1 files changed, 31 insertions, 26 deletions

diff --git a/http.c b/http.c
index 906eb098c8..9a514404d0 100644
--- a/http.c
+++ b/http.c
@@ -1780,6 +1780,10 @@ static int handle_curl_result(struct slot_results *results)
 	else if (results->http_code == 401) {
 		if ((http_auth.username && http_auth.password) ||\
 		    (http_auth.authtype && http_auth.credential)) {
+			if (http_auth.multistage) {
+				credential_clear_secrets(&http_auth);
+				return HTTP_REAUTH;
+			}
 			credential_reject(&http_auth);
 			return HTTP_NOAUTH;
 		} else {
@@ -2177,6 +2181,7 @@ static int http_request_reauth(const char *url,
 			       void *result, int target,
 			       struct http_get_options *options)
 {
+	int i = 3;
 	int ret = http_request(url, result, target, options);
 
 	if (ret != HTTP_OK && ret != HTTP_REAUTH)
@@ -2190,35 +2195,35 @@ static int http_request_reauth(const char *url,
 		}
 	}
 
-	if (ret != HTTP_REAUTH)
-		return ret;
-
-	/*
-	 * The previous request may have put cruft into our output stream; we
-	 * should clear it out before making our next request.
-	 */
-	switch (target) {
-	case HTTP_REQUEST_STRBUF:
-		strbuf_reset(result);
-		break;
-	case HTTP_REQUEST_FILE:
-		if (fflush(result)) {
-			error_errno("unable to flush a file");
-			return HTTP_START_FAILED;
-		}
-		rewind(result);
-		if (ftruncate(fileno(result), 0) < 0) {
-			error_errno("unable to truncate a file");
-			return HTTP_START_FAILED;
+	while (ret == HTTP_REAUTH && --i) {
+		/*
+		 * The previous request may have put cruft into our output stream; we
+		 * should clear it out before making our next request.
+		 */
+		switch (target) {
+		case HTTP_REQUEST_STRBUF:
+			strbuf_reset(result);
+			break;
+		case HTTP_REQUEST_FILE:
+			if (fflush(result)) {
+				error_errno("unable to flush a file");
+				return HTTP_START_FAILED;
+			}
+			rewind(result);
+			if (ftruncate(fileno(result), 0) < 0) {
+				error_errno("unable to truncate a file");
+				return HTTP_START_FAILED;
+			}
+			break;
+		default:
+			BUG("Unknown http_request target");
 		}
-		break;
-	default:
-		BUG("Unknown http_request target");
-	}
 
-	credential_fill(&http_auth, 1);
+		credential_fill(&http_auth, 1);
 
-	return http_request(url, result, target, options);
+		ret = http_request(url, result, target, options);
+	}
+	return ret;
 }
 
 int http_get_strbuf(const char *url,