http: send probe packets

When we're authenticating with a connection-based authentication scheme
(NTLM, Negotiate), we need to make sure that we're still connected
between the initial GET where we did the authentication and the POST
that we're about to send.  Our keep-alive session may have not kept
alive, but more likely, some servers do not authenticate the entire
keep-alive connection and may have "forgotten" that we were
authenticated, namely Apache and nginx.

Send a "probe" packet, that is an HTTP POST request to the upload-pack
or receive-pack endpoint, that consists of an empty git pkt ("0000").
If we're authenticated, we'll get a 200 back.  If we're not, we'll get a
401 back, and then we'll resend that probe packet with the first step of
our authentication (asking to start authentication with the given
scheme).  We expect _yet another_ 401 back, with the authentication
challenge.

Finally, we will send our authentication response with the actual POST
data.  This will allow us to authenticate without draining the POST data
in the initial request that gets us a 401.

src/transports/auth_ntlm.h

@@ -11,6 +11,9 @@
 #include "git2.h"
 #include "auth.h"
 
+/* NTLM requires a full request/challenge/response */
+#define GIT_AUTH_STEPS_NTLM 2
+
 #ifdef GIT_NTLM
 
 #if defined(GIT_OPENSSL)

src/transports/http.c

@@ -432,6 +432,56 @@ done:
 	return error;
 }
 
+static bool needs_probe(http_stream *stream)
+{
+	http_subtransport *transport = OWNING_SUBTRANSPORT(stream);
+
+	return (transport->server.auth_schemetypes == GIT_HTTP_AUTH_NTLM ||
+	        transport->server.auth_schemetypes == GIT_HTTP_AUTH_NEGOTIATE);
+}
+
+static int send_probe(http_stream *stream)
+{
+	http_subtransport *transport = OWNING_SUBTRANSPORT(stream);
+	git_http_client *client = transport->http_client;
+	const char *probe = "0000";
+	size_t len = 4;
+	git_net_url url = GIT_NET_URL_INIT;
+	git_http_request request = {0};
+	git_http_response response = {0};
+	bool complete = false;
+	size_t step, steps = 1;
+	int error;
+
+	/* NTLM requires a full challenge/response */
+	if (transport->server.auth_schemetypes == GIT_HTTP_AUTH_NTLM)
+		steps = GIT_AUTH_STEPS_NTLM;
+
+	/*
+	 * Send at most two requests: one without any authentication to see
+	 * if we get prompted to authenticate.  If we do, send a second one
+	 * with the first authentication message.  The final authentication
+	 * message with the response will occur with the *actual* POST data.
+	 */
+	for (step = 0; step < steps && !complete; step++) {
+		git_net_url_dispose(&url);
+		git_http_response_dispose(&response);
+
+		if ((error = generate_request(&url, &request, stream, len)) < 0 ||
+		    (error = git_http_client_send_request(client, &request)) < 0 ||
+		    (error = git_http_client_send_body(client, probe, len)) < 0 ||
+		    (error = git_http_client_read_response(&response, client)) < 0 ||
+		    (error = git_http_client_skip_body(client)) < 0 ||
+		    (error = handle_response(&complete, stream, &response, true)) < 0)
+			goto done;
+	}
+
+done:
+	git_http_response_dispose(&response);
+	git_net_url_dispose(&url);
+	return error;
+}
+
 /*
 * Write to an HTTP transport - for the first invocation of this function
 * (ie, when stream->state == HTTP_STATE_NONE), we'll send a POST request
@@ -458,6 +508,20 @@ static int http_stream_write(
 		git_net_url_dispose(&url);
 		git_http_response_dispose(&response);
 
+		/*
+		 * If we're authenticating with a connection-based mechanism
+		 * (NTLM, Kerberos), send a "probe" packet.  Servers SHOULD
+		 * authenticate an entire keep-alive connection, so ideally
+		 * we should not need to authenticate but some servers do
+		 * not support this.  By sending a probe packet, we'll be
+		 * able to follow up with a second POST using the actual
+		 * data (and, in the degenerate case, the authentication
+		 * header as well).
+		 */
+		if (needs_probe(stream) && (error = send_probe(stream)) < 0)
+			goto done;
+
+		/* Send the regular POST request. */
 		if ((error = generate_request(&url, &request, stream, len)) < 0 ||
 		    (error = git_http_client_send_request(
 			transport->http_client, &request)) < 0)
@@ -511,6 +575,7 @@ static int http_stream_read_response(
 {
 	http_stream *stream = (http_stream *)s;
 	http_subtransport *transport = OWNING_SUBTRANSPORT(stream);
+	git_http_client *client = transport->http_client;
 	git_http_response response = {0};
 	bool complete;
 	int error;
@@ -518,7 +583,7 @@ static int http_stream_read_response(
 	*out_len = 0;
 
 	if (stream->state == HTTP_STATE_SENDING_REQUEST) {
-		if ((error = git_http_client_read_response(&response, transport->http_client)) < 0 ||
+		if ((error = git_http_client_read_response(&response, client)) < 0 ||
 		    (error = handle_response(&complete, stream, &response, false)) < 0)
 		    goto done;
 
@@ -526,7 +591,7 @@ static int http_stream_read_response(
 		stream->state = HTTP_STATE_RECEIVING_RESPONSE;
 	}
 
-	error = git_http_client_read_body(transport->http_client, buffer, buffer_size);
+	error = git_http_client_read_body(client, buffer, buffer_size);
 
 	if (error > 0) {
 		*out_len = error;