Program hangs when modifying u->buffer while using proxy module to talk to upstream backend

Server NGINX
1

Hi,

I currently use upstream to talk to a back-end, which sends me some
response. In my module, i am trying to re-format the plain-text
upstream backend response, to XML, and I also add some new data to it,
based on an in-memory lookup within my module. What would be the best
way to do this?

In my setup, I am able to parse response from upstream (u->buffer),
construct a modified string and assign it back to u->buffer, and am I
setting u->headers_in.content_length_n and
r->headers_out.content_length_n correctly. The issue i face is, every
second request hangs, if i am trying the request through the
browser. (However, trying through command-line & curl, works
everytime). And I always see this error message in the logs: “client
prematurely closed connection, so upstream connection is closed too
while sending to client, client: 127.0.0.1”. What am i doing wrong?

Thanks,
Ashish

2

see the fastcgi module as an example

2012/4/10 Ashish S [email protected]:

3

Hello!

On Mon, Apr 09, 2012 at 07:02:10PM -0700, Ashish S wrote:

Hi,

I currently use upstream to talk to a back-end, which sends me some
response. In my module, i am trying to re-format the plain-text
upstream backend response, to XML, and I also add some new data to it,
based on an in-memory lookup within my module. What would be the best
way to do this?

Unless you are working on custom protocol module, you may want to
use filter module to re-format data instead.

In my setup, I am able to parse response from upstream (u->buffer),
construct a modified string and assign it back to u->buffer, and am I
setting u->headers_in.content_length_n and
r->headers_out.content_length_n correctly. The issue i face is, every
second request hangs, if i am trying the request through the
browser. (However, trying through command-line & curl, works
everytime). And I always see this error message in the logs: “client
prematurely closed connection, so upstream connection is closed too
while sending to client, client: 127.0.0.1”. What am i doing wrong?

It’s hard to say anything without seeing the code, but most likely
you fail to set u->length properly. See e.g. memcached module for
a simple example of non-buffered protocol handler.

Maxim D.

4

Hi, I figured the issue. Maxim’s intuition was right. u->length was
not set correctly (the u->length set in the code i copy-pasted
earlier, was modified later by a filter method). After fixing the
issue, the code works correctly, no more hangs.

Thanks,
Ashish

5

Hi Maxim,

Here is a simple test code i wrote, (for my process header method),
which shows the same behavior.

============
ngx_int_t
ngx_http_my_process_header(ngx_http_request_t *r)
{
ngx_http_upstream_t *u = r->upstream;

std::string tmpString ("Test string");
ngx_memcpy(u->buffer.start, tmpString.c_str(), tmpString.length());
u->buffer.pos = u->buffer.start;
u->buffer.last = u->buffer.pos + tmpString.length();

u->length = u->headers_in.content_length_n = tmpString.length();
r->headers_out.content_length_n = tmpString.length();
u->headers_in.status_n = NGX_HTTP_OK;
u->state->status = NGX_HTTP_OK;

return NGX_OK;

}

Thanks,
Ashish

6

Hi Maxim,

It’s hard to say anything without seeing the code, but most likely
you fail to set u->length properly. See e.g. memcached module for
a simple example of non-buffered protocol handler.

Is there an example i can follow, for buffered protocol handler?

Thanks
Ashish