HTTP/2: fixed posted streams handling.

A bug was introduced by 82efcedb310b that could lead to timing out of
responses or segmentation fault, when accept_mutex was enabled.

The output queue in HTTP/2 can contain frames from different streams.
When the queue is sent, all related write handlers need to be called.
In order to do so, the streams were added to the h2c->posted queue
after handling sent frames.  Then this queue was processed in
ngx_http_v2_write_handler().

If accept_mutex is enabled, the event's "ready" flag is set but its
handler is not called immediately.  Instead, the event is added to
the ngx_posted_events queue.  At the same time in this queue can be
events from upstream connections.  Such events can result in sending
output queue before ngx_http_v2_write_handler() is triggered.  And
at the time ngx_http_v2_write_handler() is called, the output queue
can be already empty with some streams added to h2c->posted.

But after 82efcedb310b, these streams weren't processed if all frames
have already been sent and the output queue was empty.  This might lead
to a situation when a number of streams were get stuck in h2c->posted
queue for a long time.  Eventually these streams might get closed by
the send timeout.

In the worst case this might also lead to a segmentation fault, if
already freed stream was left in the h2c->posted queue.  This could
happen if one of the streams was terminated but wasn't closed, due to
the HEADERS frame or a partially sent DATA frame left in the output
queue.  If this happened the ngx_http_v2_filter_cleanup() handler
removed the stream from the h2c->waiting or h2c->posted queue on
termination stage, before the frame has been sent, and the stream
was again added to the h2c->posted queue after the frame was sent.

In order to fix all these problems and simplify the code, write
events of fake stream connections are now added to ngx_posted_events
instead of using a custom h2c->posted queue.

src/http/v2/ngx_http_v2.c

@@ -286,7 +286,6 @@ ngx_http_v2_init(ngx_event_t *rev)
                                             : ngx_http_v2_state_preface;
 
     ngx_queue_init(&h2c->waiting);
-    ngx_queue_init(&h2c->posted);
     ngx_queue_init(&h2c->dependencies);
     ngx_queue_init(&h2c->closed);
 
@@ -420,9 +419,7 @@ static void
 ngx_http_v2_write_handler(ngx_event_t *wev)
 {
     ngx_int_t                  rc;
-    ngx_queue_t               *q;
     ngx_connection_t          *c;
-    ngx_http_v2_stream_t      *stream;
     ngx_http_v2_connection_t  *h2c;
 
     c = wev->data;
@@ -457,26 +454,6 @@ ngx_http_v2_write_handler(ngx_event_t *wev)
         return;
     }
 
-    while (!ngx_queue_empty(&h2c->posted)) {
-        q = ngx_queue_head(&h2c->posted);
-
-        ngx_queue_remove(q);
-
-        stream = ngx_queue_data(q, ngx_http_v2_stream_t, queue);
-
-        stream->handled = 0;
-
-        ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
-                       "run http2 stream %ui", stream->node->id);
-
-        wev = stream->request->connection->write;
-
-        wev->active = 0;
-        wev->ready = 1;
-
-        wev->handler(wev);
-    }
-
     h2c->blocked = 0;
 
     if (rc == NGX_AGAIN) {
@@ -2254,7 +2231,7 @@ ngx_http_v2_state_window_update(ngx_http_v2_connection_t *h2c, u_char *pos,
 
         stream = ngx_queue_data(q, ngx_http_v2_stream_t, queue);
 
-        stream->handled = 0;
+        stream->waiting = 0;
 
         wev = stream->request->connection->write;
 
@@ -4274,7 +4251,7 @@ ngx_http_v2_finalize_connection(ngx_http_v2_connection_t *h2c,
                 continue;
             }
 
-            stream->handled = 0;
+            stream->waiting = 0;
 
             r = stream->request;
             fc = r->connection;

src/http/v2/ngx_http_v2.h

@@ -137,7 +137,6 @@ struct ngx_http_v2_connection_s {
 
     ngx_http_v2_out_frame_t         *last_out;
 
-    ngx_queue_t                      posted;
     ngx_queue_t                      dependencies;
     ngx_queue_t                      closed;
 
@@ -192,7 +191,7 @@ struct ngx_http_v2_stream_s {
 
     ngx_pool_t                      *pool;
 
-    unsigned                         handled:1;
+    unsigned                         waiting:1;
     unsigned                         blocked:1;
     unsigned                         exhausted:1;
     unsigned                         in_closed:1;

src/http/v2/ngx_http_v2_filter_module.c

@@ -1104,11 +1104,11 @@ ngx_http_v2_waiting_queue(ngx_http_v2_connection_t *h2c,
     ngx_queue_t           *q;
     ngx_http_v2_stream_t  *s;
 
-    if (stream->handled) {
+    if (stream->waiting) {
         return;
     }
 
-    stream->handled = 1;
+    stream->waiting = 1;
 
     for (q = ngx_queue_last(&h2c->waiting);
          q != ngx_queue_sentinel(&h2c->waiting);
@@ -1298,20 +1298,29 @@ static ngx_inline void
 ngx_http_v2_handle_stream(ngx_http_v2_connection_t *h2c,
     ngx_http_v2_stream_t *stream)
 {
+    ngx_event_t       *wev;
     ngx_connection_t  *fc;
 
-    if (stream->handled || stream->blocked) {
+    if (stream->waiting || stream->blocked) {
         return;
     }
 
     fc = stream->request->connection;
 
-    if (!fc->error && (stream->exhausted || fc->write->delayed)) {
+    if (!fc->error && stream->exhausted) {
         return;
     }
 
-    stream->handled = 1;
+    wev = fc->write;
-    ngx_queue_insert_tail(&h2c->posted, &stream->queue);
+
+    wev->active = 0;
+    wev->ready = 1;
+
+    if (!fc->error && wev->delayed) {
+        return;
+    }
+
+    ngx_post_event(wev, &ngx_posted_events);
 }
 
 
@@ -1321,11 +1330,13 @@ ngx_http_v2_filter_cleanup(void *data)
     ngx_http_v2_stream_t *stream = data;
 
     size_t                     window;
+    ngx_event_t               *wev;
+    ngx_queue_t               *q;
     ngx_http_v2_out_frame_t   *frame, **fn;
     ngx_http_v2_connection_t  *h2c;
 
-    if (stream->handled) {
+    if (stream->waiting) {
-        stream->handled = 0;
+        stream->waiting = 0;
         ngx_queue_remove(&stream->queue);
     }
 
@@ -1359,9 +1370,26 @@ ngx_http_v2_filter_cleanup(void *data)
         fn = &frame->next;
     }
 
-    if (h2c->send_window == 0 && window && !ngx_queue_empty(&h2c->waiting)) {
+    if (h2c->send_window == 0 && window) {
-        ngx_queue_add(&h2c->posted, &h2c->waiting);
+
-        ngx_queue_init(&h2c->waiting);
+        while (!ngx_queue_empty(&h2c->waiting)) {
+            q = ngx_queue_head(&h2c->waiting);
+
+            ngx_queue_remove(q);
+
+            stream = ngx_queue_data(q, ngx_http_v2_stream_t, queue);
+
+            stream->waiting = 0;
+
+            wev = stream->request->connection->write;
+
+            wev->active = 0;
+            wev->ready = 1;
+
+            if (!wev->delayed) {
+                ngx_post_event(wev, &ngx_posted_events);
+            }
+        }
     }
 
     h2c->send_window += window;