Possible fix for crash bug related to event timeouts. [Bug 957]

If we ever add an event, then set it, then add it again, there will be
now two pointers to the event in the event base.  If we delete one and
free it, the first pointer will still be there, and possibly cause a
crash later.

This patch adds detection for this case to the code paths in
eventdns.c, and works around it.  If the warning message ever
displays, then a cleverer fix is in order.

{I am not too confident that this *is* the fix, since bug 957 is very
tricky.  If it is, it is a bugfix on 0.2.0.}

ChangeLog

@@ -25,6 +25,8 @@ Changes in version 0.2.1.15??? - ????-??-??
       descriptor as never-downloadable.  Bugfix on 0.2.1.9-alpha.
     - Fix a memory leak when v3 directory authorities load their keys
       and cert from disk. Bugfix on 0.2.0.1-alpha.
+    - Add a test and fix for a possible cause of bug 957.  If it's
+      the real cause, it should display some warning messages.
 
 
 Changes in version 0.2.1.14-rc - 2009-04-12

src/or/eventdns.c

@@ -490,7 +490,7 @@ _del_timeout_event(u16 *lineno, struct event *ev, int line)
 {
 	if (*lineno) {
 		log(EVDNS_LOG_WARN,
-			"Duplicate timeout event_del from line %d: first call "
+			"BUG: Duplicate timeout event_del from line %d: first call "
 			"was at %d.", line, (int)*lineno);
 		return 0;
 	} else {
@@ -501,6 +501,24 @@ _del_timeout_event(u16 *lineno, struct event *ev, int line)
 #define del_timeout_event(s)											\
 	(_del_timeout_event(&(s)->timeout_event_deleted, &(s)->timeout_event, \
 						__LINE__))
+/* For debugging bug 929/957. XXXX021 */
+static int
+_del_timeout_event_if_set(u16 *lineno, struct event *ev, int line)
+{
+	if (*lineno == 0) {
+		log(EVDNS_LOG_WARN,
+			"BUG: Event that I thought was non-added as of line %d "
+			"was actually added on line %d",
+			line, (int)*lineno);
+		*lineno = line;
+		return event_del(ev);
+	}
+	return 0;
+}
+#define del_timeout_event_if_set(s)				\
+	_del_timeout_event_if_set(&(s)->timeout_event_deleted,	\
+							  &(s)->timeout_event,			\
+							  __LINE__)
 
 /* This walks the list of inflight requests to find the */
 /* one with a matching transaction id. Returns NULL on */
@@ -549,6 +567,7 @@ nameserver_probe_failed(struct nameserver *const ns) {
 										global_nameserver_timeouts_length - 1)];
 	ns->failed_times++;
 
+	del_timeout_event_if_set(ns);
 	evtimer_set(&ns->timeout_event, nameserver_prod_callback, ns);
 	if (add_timeout_event(ns, (struct timeval *) timeout) < 0) {
 		log(EVDNS_LOG_WARN,
@@ -578,6 +597,7 @@ nameserver_failed(struct nameserver *const ns, const char *msg) {
 	ns->state = 0;
 	ns->failed_times = 1;
 
+	del_timeout_event_if_set(ns);
 	evtimer_set(&ns->timeout_event, nameserver_prod_callback, ns);
 	if (add_timeout_event(ns, (struct timeval *) &global_nameserver_timeouts[0]) < 0) {
 		log(EVDNS_LOG_WARN,
@@ -2089,6 +2109,7 @@ evdns_request_transmit(struct evdns_request *req) {
 		/* transmitted; we need to check for timeout. */
 		log(EVDNS_LOG_DEBUG,
 			"Setting timeout for request %lx", (unsigned long) req);
+		del_timeout_event_if_set(req);
 		evtimer_set(&req->timeout_event, evdns_request_timeout_callback, req);
 		if (add_timeout_event(req, &global_timeout) < 0) {
 			log(EVDNS_LOG_WARN,
@@ -2297,6 +2318,7 @@ _evdns_nameserver_add_impl(const struct sockaddr *address,
 	if (!ns) return -1;
 
 	memset(ns, 0, sizeof(struct nameserver));
+	ns->timeout_event_deleted = __LINE__;
 
 	ns->socket = socket(PF_INET, SOCK_DGRAM, 0);
 	if (ns->socket < 0) { err = 1; goto out1; }
@@ -2531,6 +2553,7 @@ request_new(int type, const char *name, int flags,
 	}
 
 	memset(req, 0, sizeof(struct evdns_request));
+	req->timeout_event_deleted = __LINE__;
 
 	if (global_randomize_case) {
 		unsigned i;