BUG/MEDIUM: h2/threads: never release the task outside of the task handler

Currently, h2_release() will release all resources assigned to the h2
connection, including the timeout task if any. But since the multi-threaded
scheduler, the timeout task could very well be queued in the thread-local
list of running tasks without any way to remove it, so task_delete() will
have no effect and task_free() will cause this undefined object to be
dereferenced.

In order to prevent this from happening, we never release the task in
h2_release(), instead we wake it up after marking its context NULL so that
the task handler can release the task.

Future improvements could consist in modifying the scheduler so that a
task_wakeup() has to be done on any task having to be killed, letting
the scheduler take care of it.

This fix must be backported to 1.8. This bug was apparently not reported
so far.

diff --git a/src/mux_h2.c b/src/mux_h2.c
index 505f5e372d5ea49b69962a34527640fbc46d9f46..8a0c4a3301f11e7eb8ca235db7ad9c47d961ed10 100644 (file)
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -433,8 +433,8 @@ static void h2_release(struct connection *conn)
                h2_release_buf(h2c, &h2c->mbuf);
 
                if (h2c->task) {
-                       task_delete(h2c->task);
-                       task_free(h2c->task);
+                       h2c->task->context = NULL;
+                       task_wakeup(h2c->task, TASK_WOKEN_OTHER);
                        h2c->task = NULL;
                }
 
@@ -2329,9 +2329,18 @@ static struct task *h2_timeout_task(struct task *t)
        struct h2c *h2c = t->context;
        int expired = tick_is_expired(t->expire, now_ms);
 
-       if (!expired)
+       if (!expired && h2c)
                return t;
 
+       task_delete(t);
+       task_free(t);
+
+       if (!h2c) {
+               /* resources were already deleted */
+               return NULL;
+       }
+
+       h2c->task = NULL;
        h2c_error(h2c, H2_ERR_NO_ERROR);
        h2_wake_some_streams(h2c, 0, 0);
 
@@ -2348,17 +2357,12 @@ static struct task *h2_timeout_task(struct task *t)
        if (h2c->mbuf->o && !(h2c->flags & H2_CF_GOAWAY_FAILED) && conn_xprt_ready(h2c->conn))
                h2c->conn->xprt->snd_buf(h2c->conn, h2c->mbuf, 0);
 
-       if (!eb_is_empty(&h2c->streams_by_id))
-               goto wait;
-
-       h2_release(h2c->conn);
-       return NULL;
+       /* either we can release everything now or it will be done later once
+        * the last stream closes.
+        */
+       if (eb_is_empty(&h2c->streams_by_id))
+               h2_release(h2c->conn);
 
- wait:
-       /* the streams have been notified, we must let them finish and close */
-       h2c->task = NULL;
-       task_delete(t);
-       task_free(t);
        return NULL;
 }