ngx_http_sub_module causes requests to hang on a simple match.

[ptcell]

I've built with the sub filter enabled and I'm finding it hangs requests if
there is a match.   It is a very simple substitution/replace.   I've
resorted to following the request in GDB and the sub module completes and
calls the next body filter (which in my case appears to be the charset
module).   I have no other odd modules enabled other than using threads with
a thread pool size of two (shouldn't matter, right?).  Pausing all the
threads in GDB shows no obvious place it is hanging.

If I change the match string to something that doesn't match anything, the
request works fine.

Here is my config:

        location / {
            root   html;
            index  index.html index.htm;
            sub_filter '</title>' 'xxx</title>';
            sub_filter_once on;
        }

nginx -V
nginx version: nginx/1.7.11
built by gcc 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4) 
configure arguments: --with-http_sub_module --with-debug --with-threads
--with-cc-opt='-O0 -g'

Thanks!

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,275245,275245#msg-275245