request pool cleanup handler

Maxim Dounin mdounin at
Mon Oct 19 03:48:35 MSD 2009


On Sun, Oct 18, 2009 at 07:02:17PM -0400, Leo P. wrote:

> Good day. I'm working on
> . and I've got this
> one nasty bug that I'm having a bit of trouble with.
> An aside first, though. The way this thing works is by setting up an
> rbtree in a shared memory zone. Waiting listener requests are queued
> up in channels (represented by nodes on the shared rbtree).
> Naturally, when a listener request is finished or aborted, I want it
> dequeued from a channel's "waiting list". I am doing this with a
> request pool cleanup handler, using the following piece of code to
> set it up:
> //test to see if the connection was closed or something.
> r->read_event_handler = ngx_http_test_reading; //r is a long-polling
> listener request.
> //attach a cleaner to remove the request from the channel, if need
> be (if the connection goes dead or something)
> ngx_http_push_listener_cleanup_t *clndata;
> ngx_pool_cleanup_t     *cln = ngx_pool_cleanup_add(r->pool,
> sizeof(*clndata));
> if (cln == NULL) { //make sure we can.
>   return NGX_ERROR;
> }
> cln->handler = (ngx_pool_cleanup_pt) ngx_http_push_listener_cleanup;
> clndata = (ngx_http_push_listener_cleanup_t *) cln->data;
> clndata->channel=channel;
> clndata->listener=listener;
> ngx_shmtx_lock(&shpool->mutex);
> listener->cleanup = clndata;
> ngx_shmtx_unlock(&shpool->mutex);
> Trouble is, I'm getting (occasional) segfaults from accessing said
> ling-polling listener requests that have already been freed.
> Meaning, I suspect, the request pool cleanup handler was not called.
> Also, I am unable to reproduce this when worker_processes = 1;

As far as I see, you store pointers to request structure in shared 
memory and later use them without checking if you are in the 
process they belong to.

This seems to be more or less obvious way to SIGSEGV once you have 
more than 1 worker process (which may happen either due to 
worker_processes > 1 or configuration reload recently happended).

Maxim Dounin

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