http: add callback to allow server to decline (and thereby close) inc…

…oming connections.

This is important, as otherwise clients can easily exhaust the file
descriptors available on a libevent HTTP server, which can cause
problems in other code which does not handle EMFILE well: for example,
see bitcoin/bitcoin#11368

Closes: libevent#578 (patch cherry picked)

http-internal.h

@@ -170,6 +170,8 @@ struct evhttp {
 	void *gencbarg;
 	struct bufferevent* (*bevcb)(struct event_base *, void *);
 	void *bevcbarg;
+	int (*newreqcb)(struct evhttp_request *req, void *);
+	void *newreqcbarg;
 
 	struct event_base *base;
 };

http.c

@@ -3929,6 +3929,14 @@ evhttp_set_bevcb(struct evhttp *http,
 	http->bevcbarg = cbarg;
 }
 
+void
+evhttp_set_newreqcb(struct evhttp *http,
+    int (*cb)(struct evhttp_request *, void *), void *cbarg)
+{
+	http->newreqcb = cb;
+	http->newreqcbarg = cbarg;
+}
+
 /*
  * Request related functions
  */
@@ -4239,17 +4247,20 @@ evhttp_associate_new_request_with_connection(struct evhttp_connection *evcon)
 	req->evcon = evcon;	/* the request ends up owning the connection */
 	req->flags |= EVHTTP_REQ_OWN_CONNECTION;
 
-	/* We did not present the request to the user user yet, so treat it as
-	 * if the user was done with the request.  This allows us to free the
-	 * request on a persistent connection if the client drops it without
-	 * sending a request.
+	/* We did not present the request to the user yet, so treat it
+	 * as if the user was done with the request.  This allows us
+	 * to free the request on a persistent connection if the
+	 * client drops it without sending a request.
 	 */
 	req->userdone = 1;
-
-	TAILQ_INSERT_TAIL(&evcon->requests, req, next);
-
 	req->kind = EVHTTP_REQUEST;
 
+	if (http->newreqcb && http->newreqcb(req, http->newreqcbarg) == -1) {
+		evhttp_request_free(req);
+		return (-1);
+	}
+
+	TAILQ_INSERT_TAIL(&evcon->requests, req, next);
 
 	evhttp_start_read_(evcon);
 

include/event2/http.h

@@ -298,6 +298,24 @@ EVENT2_EXPORT_SYMBOL
 void evhttp_set_bevcb(struct evhttp *http,
     struct bufferevent *(*cb)(struct event_base *, void *), void *arg);
 
+
+/**
+   Set a callback which allows the user to note or throttle incoming requests.
+
+   The requests are not populated with HTTP level information. They
+   are just associated to a connection.
+
+   If the callback returns -1, the associated connection is terminated
+   and the request is closed.
+
+   @param http the evhttp server object for which to set the callback
+   @param cb the callback to invoke for incoming connections
+   @param arg an context argument for the callback
+ */
+EVENT2_EXPORT_SYMBOL
+void evhttp_set_newreqcb(struct evhttp *http,
+    int (*cb)(struct evhttp_request*, void *), void *arg);
+
 /**
    Adds a virtual host to the http server.
 

test/regress_http.c

@@ -4604,6 +4604,129 @@ http_request_extra_body_test(void *arg)
 		evbuffer_free(body);
 }
 
+struct http_newreqcb_test_state
+{
+	int connections_started;
+	int connections_noticed;
+	int connections_throttled;
+	int connections_good;
+	int connections_error;
+	int connections_done;
+};
+
+static void
+http_newreqcb_test_state_check(struct http_newreqcb_test_state* state)
+{
+	tt_int_op(state->connections_started, >=, 0);
+	tt_int_op(state->connections_started, >=, state->connections_noticed);
+	tt_int_op(state->connections_throttled, >=, state->connections_error);
+
+	tt_int_op(state->connections_done, <=, state->connections_started);
+	if (state->connections_good + state->connections_error == state->connections_started) {
+		tt_int_op(state->connections_throttled, ==, state->connections_error);
+		tt_int_op(state->connections_good + state->connections_error, ==, state->connections_done);
+		event_base_loopexit(exit_base, NULL);
+	}
+
+	return;
+end:
+	tt_fail();
+	exit(17);
+}
+
+static void
+http_request_done_newreqcb(struct evhttp_request *req, void *arg)
+{
+	struct http_newreqcb_test_state* state = arg;
+	if (req && evhttp_request_get_response_code(req) == HTTP_OK) {
+		++state->connections_good;
+		evhttp_request_set_error_cb(req, NULL);
+	}
+	++state->connections_done;
+
+	http_newreqcb_test_state_check(state);
+}
+
+static void
+http_request_error_newreqcb(enum evhttp_request_error err, void *arg)
+{
+	struct http_newreqcb_test_state* state = arg;
+	++state->connections_error;
+
+	http_newreqcb_test_state_check(state);
+}
+
+static int
+http_newreqcb(struct evhttp_request* req, void *arg)
+{
+	struct http_newreqcb_test_state* state = arg;
+	++state->connections_noticed;
+	http_newreqcb_test_state_check(state);
+	if (1 == state->connections_noticed % 7) {
+		state->connections_throttled++;
+		return -1;
+	}
+	return 0;
+}
+
+
+static void
+http_newreqcb_test(void *arg)
+{
+	struct basic_test_data *data = arg;
+	ev_uint16_t port = 0;
+	struct evhttp *http = http_setup(&port, data->base, 0);
+	struct evhttp_connection *evcons[100];
+	struct http_newreqcb_test_state newreqcb_test_state;
+	unsigned n;
+
+	exit_base = data->base;
+	test_ok = 0;
+
+	memset(&newreqcb_test_state, 0, sizeof(newreqcb_test_state));
+	memset(evcons, 0, sizeof(evcons));
+
+	evhttp_set_newreqcb(http, http_newreqcb, &newreqcb_test_state);
+
+	for (n = 0; n < sizeof(evcons)/sizeof(evcons[0]); ++n) {
+		struct evhttp_connection* evcon = NULL;
+		struct evhttp_request *req = NULL;
+		evcons[n] = evhttp_connection_base_new(data->base, NULL, "127.0.0.1", port);
+		evcon = evcons[n];
+		evhttp_connection_set_retries(evcon, 0);
+
+		tt_assert(evcon);
+
+		req = evhttp_request_new(http_request_done_newreqcb, &newreqcb_test_state);
+		evhttp_add_header(evhttp_request_get_output_headers(req), "Connection", "close");
+		evhttp_request_set_error_cb(req, http_request_error_newreqcb);
+
+		/* We give ownership of the request to the connection */
+		if (evhttp_make_request(evcon, req, EVHTTP_REQ_GET, "/test") == -1) {
+			tt_abort_msg("Couldn't make request");
+		}
+
+		++newreqcb_test_state.connections_started;
+		http_newreqcb_test_state_check(&newreqcb_test_state);
+	}
+
+	event_base_dispatch(data->base);
+
+	http_newreqcb_test_state_check(&newreqcb_test_state);
+	tt_int_op(newreqcb_test_state.connections_throttled, >, 0);
+
+ end:
+	evhttp_free(http);
+
+	for (n = 0; n < sizeof(evcons)/sizeof(evcons[0]); ++n) {
+		if (evcons[n])
+			evhttp_connection_free(evcons[n]);
+
+	}
+
+}
+
+
 #define HTTP_LEGACY(name)						\
 	{ #name, run_legacy_test_fn, TT_ISOLATED|TT_LEGACY, &legacy_setup, \
 		    http_##name##_test }
@@ -4725,6 +4848,8 @@ struct testcase_t http_testcases[] = {
 
 	HTTP(request_extra_body),
 
+	HTTP(newreqcb),
+
 #ifdef EVENT__HAVE_OPENSSL
 	HTTPS(basic),
 	HTTPS(filter_basic),