Subrequests from body filters

[Marat Dakota]

Hi,

I'm writing a handler module. It makes subrequests and uses body
filter for subrequests.

Here is simplified handler code:

static ngx_int_t
ngx_http_my_handler(ngx_http_request_t *r) {

    // ... Sending headers.

    ngx_chain_t                 *out;
    ngx_buf_t                   *b;

    out = ngx_alloc_chain_link(r->pool);
    b = ngx_pcalloc(r->pool, sizeof(ngx_buf_t));

    if (out == NULL || b == NULL) {
        return NGX_HTTP_INTERNAL_SERVER_ERROR;
    }

    b->pos = "aaa";
    b->last = "aaa" + 3;
    b->memory = 1;
    b->last_buf = 1;

    out->buf = b;
    out->next = NULL;

    // ngx_http_my_subrequest() creates ngx_str_t from second argument
and calls ngx_http_subrequest().
    if (ngx_http_my_subrequest(r, "/uri1") != NGX_OK) {
        return NGX_ERROR;
    }

    if (ngx_http_my_subrequest(r, "/uri2") != NGX_OK) {
        return NGX_ERROR;
    }

    return ngx_http_output_filter(r, out);
}


Here is subrequest body filter:

static ngx_int_t
ngx_http_my_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
    // ... Do something with subrequest's body in "in" argument.

    ngx_chain_t                 *out;
    ngx_buf_t                   *b;
    ngx_chain_t                 *cl;

    out = ngx_alloc_chain_link(r->pool);
    b = ngx_pcalloc(r->pool, sizeof(ngx_buf_t));

    if (out == NULL || b == NULL) {
        return NGX_HTTP_INTERNAL_SERVER_ERROR;
    }

    // HERE BE DRAGONS.

    b->pos = "bbb";
    b->last = "bbb" + 3;
    b->memory = 1;
    b->last_buf = 1;

    out->buf = b;
    out->next = NULL;

    // Discard subrequest's body.
    for (cl = in; cl; cl = cl->next) {
        cl->buf->pos = cl->buf->last;
        cl->buf->file_pos = cl->buf->file_last;
    }

    // We just put "bbb" to main response for each call of this body filter.
    return ngx_http_output_filter(r->main, out);
}

And everything works just fine. For example, if
ngx_http_my_body_filter is called twice for each (/uri1 and /uri2)
subrequest started from ngx_http_my_handler, the result will be
"aaabbbbbbbbbbbb".

But if I replace "HERE BE DRAGONS" with a call to
ngx_http_my_subrequest(r->main, "/uri3"), let's suppose this
subrequest is being started only in second call of
ngx_http_my_body_filter for /uri1 subrequest started from
ngx_http_my_handler. The result will be "aaabbb", not
"aaabbbbbbbbbbbbbbbbbb" as I expect (and logs say all three
subrequests are processed ok).

I guess my problem is about some magic with buffers and chains, but I
can't figure out what to fix and where.

I would be thankful for any help.

--
Marat