mpm_event: Fix a possible listener deadlock. PR 65769.

When the listener starts accepting more connections than the number of workers
already started (due to scheduling), the listening sockets gets disabled (per
AH03269) but nothing was re-enabling them before the end of the connections,
despite the creation of more idle/available workers in the meantime.
In the wost case there is no idle worker when the listener accepts the first
connection thus nothing to wake up the listener blocked in poll() with no
socket, hence a deadlock.

Fix this by waking up the listener when a worker becomes idle and this unblocks
connections_above_limit(). This is also worthwhile when all the workers are
started (fully initialized runtime) since the number of idle workers is a
condition for connections_above_limit() anyway so the sooner the listeners are
re-enabled the better (the other condition is the number of connections which
is unblocked appropriately by decrement_connection_count() already).

Also when a child exists with ps->quiescing == 1 and it's caught by
server_main_loop() before perform_idle_server_maintenance(), active_daemons was
not decrement as needed (including accross restarts), leading to an invalid
active_daemons accounting.

* server/mpm/event/event.c(should_enable_listensocks):
  New helper that returns whether listenning sockets can be poll()ed again.

* server/mpm/event/event.c(decrement_connection_count, listener_thread):
  Use should_enable_listensocks() where previously open-coded.

* server/mpm/event/event.c(worker_thread):
  Wake up the listener when is_idle => 1 and should_enable_listensocks().
  Have a single point of exit when workers_may_exit to make sure that the
  wake always occurs (even when exiting).

* server/mpm/event/event.c(server_main_loop):
  Decrement active_daemons not only when !ps->quiescing but also when
  ps->quiescing == 1, i.e. all the cases not handled by
  perform_idle_server_maintenance() already.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1896505 13f79535-47bb-0310-9956-ffa450edef68

2 files changed, 17 insertions, 6 deletions

diff --git a/changes-entries/bz_65769.txt b/changes-entries/bz_65769.txt
new file mode 100644
index 0000000000..7fede1a211
--- /dev/null
+++ b/changes-entries/bz_65769.txt
@@ -0,0 +1,2 @@
+  *) mpm_event: Fix a possible listener deadlock on heavy load when restarting
+     and/or reaching MaxConnectionsPerChild.  PR 65769.  [Yann Ylavic]
diff --git a/server/mpm/event/event.c b/server/mpm/event/event.c
index 8e5bc8e0fe..34762f9df4 100644
--- a/server/mpm/event/event.c
+++ b/server/mpm/event/event.c
@@ -561,6 +561,11 @@ static APR_INLINE int connections_above_limit(int *busy)
     return 1;
 }
 
+static APR_INLINE int should_enable_listensocks(void)
+{
+    return !dying && listeners_disabled() && !connections_above_limit(NULL);
+}
+
 static void close_socket_nonblocking_(apr_socket_t *csd,
                                       const char *from, int line)
 {
@@ -816,7 +821,7 @@ static apr_status_t decrement_connection_count(void *cs_)
     is_last_connection = !apr_atomic_dec32(&connection_count);
     if (listener_is_wakeable
             && ((is_last_connection && listener_may_exit)
-                || (listeners_disabled() && !connections_above_limit(NULL)))) {
+                || should_enable_listensocks())) {
         apr_pollset_wakeup(event_pollset);
     }
     if (dying) {
@@ -2308,9 +2313,7 @@ do_maintenance:
             }
         }
 
-        if (listeners_disabled()
-                && !workers_were_busy
-                && !connections_above_limit(NULL)) {
+        if (!workers_were_busy && should_enable_listensocks()) {
             enable_listensocks();
         }
     } /* listener main loop */
@@ -2372,7 +2375,7 @@ static void *APR_THREAD_FUNC worker_thread(apr_thread_t * thd, void *dummy)
     ap_update_child_status_from_indexes(process_slot, thread_slot,
                                         SERVER_STARTING, NULL);
 
-    while (!workers_may_exit) {
+    for (;;) {
         apr_socket_t *csd = NULL;
         event_conn_state_t *cs;
         timer_event_t *te = NULL;
@@ -2388,6 +2391,12 @@ static void *APR_THREAD_FUNC worker_thread(apr_thread_t * thd, void *dummy)
                 signal_threads(ST_GRACEFUL);
                 break;
             }
+            /* A new idler may have changed connections_above_limit(),
+             * let the listener know and decide.
+             */
+            if (listener_is_wakeable && should_enable_listensocks()) {
+                apr_pollset_wakeup(event_pollset);
+            }
             is_idle = 1;
         }
 
@@ -3359,7 +3368,7 @@ static void server_main_loop(int remaining_children_to_start)
 
                 event_note_child_killed(child_slot, 0, 0);
                 ps = &ap_scoreboard_image->parent[child_slot];
-                if (!ps->quiescing)
+                if (ps->quiescing != 2)
                     retained->active_daemons--;
                 ps->quiescing = 0;
                 /* NOTE: We don't dec in the (child_slot < 0) case! */