Example body filter hangs when modified a little bit - request is not terminating.

[Roman Arutyunyan]

Hi,

On Wed, Jan 10, 2018 at 08:05:15PM -0500, ptcell wrote:
> I tried out this nginx example ngx_http_foo_body_filter body filter (here
> http://nginx.org/en/docs/dev/development_guide.html#http_body_buffers_reuse
> ) and got that to work just fine.  It inserts a "foo" string before each
> incoming buffer.
> 
> I tried modifying a little bit so that it puts the foo string AFTER each
> incoming buffer chain in the list.   The pages render correctly, but the
> request never terminates and the browser just sits there spinning.   I don't
> know what I'm doing wrong.   I'm pretty sure it is not module ordering
> because the original version works ok.   I also set the content length to -1
> in the header filter, etc.
> 
> In the debugger, the out linked chain looks right with no cycles or
> anything.
> 
> I appreciate any help you can give.  Thank you.

The last buffer in chain should normally have the last_buf flag set.
This is not changed by the devguide example, but in your code it's your buffer
which is the last one, and it obviously does not have the right flag.
This is why ngx_http_chunked_filter does not end the HTTP chunked response
with the final empty chunk, but your client is expecting it.

> Here is my version:
> 
> ngx_int_t
> ngx_http_foo_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
> {
>     ngx_int_t                   rc;
>     ngx_buf_t                  *b;
>     ngx_chain_t                *cl, *tl, *out, **ll;
>     ngx_http_foo_filter_ctx_t  *ctx;
> 
>     ctx = ngx_http_get_module_ctx(r, ngx_http_foo_filter_module);
>     if (ctx == NULL) {
>         return ngx_http_next_body_filter(r, in);
>     }
> 
>     /* create a new chain "out" from "in" with all the changes */
> 
>     ll = &out;
> 
>     for (cl = in; cl; cl = cl->next) {
> 
>         /* append the next incoming buffer */
> 
>         tl = ngx_alloc_chain_link(r->pool);
>         if (tl == NULL) {
>             return NGX_ERROR;
>         }
> 
>         tl->buf = cl->buf;
>         *ll = tl;
>         ll = &tl->next;
> 
>         /* append "foo" in a reused buffer if possible */
> 
>         tl = ngx_chain_get_free_buf(r->pool, &ctx->free);
>         if (tl == NULL) {
>             return NGX_ERROR;
>         }
> 
>         b = tl->buf;
>         b->tag = (ngx_buf_tag_t) &ngx_http_foo_filter_module;
>         b->memory = 1;
>         b->pos = (u_char *) "foo";
>         b->last = b->pos + 3;
> 
>         *ll = tl;
>         ll = &tl->next;
> 
>     }
> 
>     *ll = NULL;
> 
>     /* send the new chain */
> 
>     rc = ngx_http_next_body_filter(r, out);
> 
>     /* update "busy" and "free" chains for reuse */
> 
>     ngx_chain_update_chains(r->pool, &ctx->free, &ctx->busy, &out,
>                             (ngx_buf_tag_t) &ngx_http_foo_filter_module);
> 
>     return rc;
> }
> 
> Posted at Nginx Forum: https://forum.nginx.org/read.php?2,278094,278094#msg-278094
> 
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-- 
Roman Arutyunyan