ngx_http_request_t's subrequests is what meaning be？

[BoBo]

hi all.

I am reading  nginx's source(version is 0.7.66), but i don't understand this
section:

ngx_int_t
ngx_http_subrequest(ngx_http_request_t *r,
    ngx_str_t *uri, ngx_str_t *args, ngx_http_request_t **psr,
    ngx_http_post_subrequest_t *ps, ngx_uint_t flags)
{
    ngx_connection_t              *c;
    ngx_http_request_t            *sr;
    ngx_http_core_srv_conf_t      *cscf;
    ngx_http_postponed_request_t  *pr, *p;

//i know subrequests's default is NGX_HTTP_MAX_SUBREQUESTS + 1 = 51;
    r->main->subrequests--;

//when subrequests is  0?
    if (r->main->subrequests == 0) {
        ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
                      "subrequests cycle while processing \"%V\"", uri);
        r->main->subrequests = 1;
        return NGX_ERROR;
    }
....................................................................

//why subrequests need plus plus?
    r->main->subrequests++;

    *psr = sr;

    return ngx_http_post_request(sr);
}

the problem is after call ngx_http_subrequest and  r->main->subrequests will
not change（because subrequests-- and then ++), so when subrequests is  0？

I think if  r->main->subrequests is the max subrequests number,
r->main->subrequests++
need to be  removed.

thanks.
-- 
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