(gdb) bt
#0  ngx_http_upstream_process_header (r=0x97cee38, u=0x97cf6cc) at src/http/ngx_http_upstream.c:2122
#1  0x080a2a65 in ngx_http_upstream_handler (ev=0x78b86098) at src/http/ngx_http_upstream.c:1133
#2  0x0807f686 in ngx_epoll_process_events (cycle=0x97c95e8, timer=60000, flags=1) at src/event/modules/ngx_epoll_module.c:822
#3  0x08073608 in ngx_process_events_and_timers (cycle=0x97c95e8) at src/event/ngx_event.c:242
#4  0x0807d97e in ngx_worker_process_cycle (cycle=0x97c95e8, data=0x0) at src/os/unix/ngx_process_cycle.c:753
#5  0x0807aec8 in ngx_spawn_process (cycle=0x97c95e8, proc=0x807d8bb <ngx_worker_process_cycle>, data=0x0, name=0x80fc19f "worker process", 
    respawn=-3) at src/os/unix/ngx_process.c:198
#6  0x0807cc85 in ngx_start_worker_processes (cycle=0x97c95e8, n=1, type=-3) at src/os/unix/ngx_process_cycle.c:358
#7  0x0807c4eb in ngx_master_process_cycle (cycle=0x97c95e8) at src/os/unix/ngx_process_cycle.c:130
#8  0x080515a6 in main (argc=1, argv=0xbfd2f864) at src/core/nginx.c:367

static void
ngx_http_upstream_process_header(ngx_http_request_t *r, ngx_http_upstream_t *u)
{
    ssize_t            n;
    ngx_int_t          rc;
    ngx_connection_t  *c;

    c = u->peer.connection;

    ngx_log_debug0(NGX_LOG_DEBUG_HTTP, c->log, 0,
                   "http upstream process header");

    c->log->action = "reading response header from upstream";

    if (c->read->timedout) {
        ngx_http_upstream_next(r, u, NGX_HTTP_UPSTREAM_FT_TIMEOUT);
        return;
    }

    if (!u->request_sent && ngx_http_upstream_test_connect(c) != NGX_OK) {
        ngx_http_upstream_next(r, u, NGX_HTTP_UPSTREAM_FT_ERROR);
        return;
    }

    if (u->buffer.start == NULL) {
        u->buffer.start = ngx_palloc(r->pool, u->conf->buffer_size);
        if (u->buffer.start == NULL) {
            ngx_http_upstream_finalize_request(r, u,
                                               NGX_HTTP_INTERNAL_SERVER_ERROR);
            return;
        }

        u->buffer.pos = u->buffer.start;
        u->buffer.last = u->buffer.start;
        u->buffer.end = u->buffer.start + u->conf->buffer_size;
        u->buffer.temporary = 1;

        u->buffer.tag = u->output.tag;

        if (ngx_list_init(&u->headers_in.headers, r->pool, 8,
                          sizeof(ngx_table_elt_t))
            != NGX_OK)
        {
            ngx_http_upstream_finalize_request(r, u,
                                               NGX_HTTP_INTERNAL_SERVER_ERROR);
            return;
        }

#if (NGX_HTTP_CACHE)

        if (r->cache) {
            u->buffer.pos += r->cache->header_start;
            u->buffer.last = u->buffer.pos;
        }
#endif
    }

    for ( ;; ) 



        n = c->recv(c, u->buffer.last, u->buffer.end - u->buffer.last);

        if (n == NGX_AGAIN) {
#if 0
            ngx_add_timer(rev, u->read_timeout);
#endif

            if (ngx_handle_read_event(c->read, 0) != NGX_OK) {
                ngx_http_upstream_finalize_request(r, u,
                                               NGX_HTTP_INTERNAL_SERVER_ERROR);
                return;
            }

            return;
        }

        if (n == 0) {
            ngx_log_error(NGX_LOG_ERR, c->log, 0,
                          "upstream prematurely closed connection");
        }

        if (n == NGX_ERROR || n == 0) {
            ngx_http_upstream_next(r, u, NGX_HTTP_UPSTREAM_FT_ERROR);
            return;
        }

        u->buffer.last += n;

#if 0
        u->valid_header_in = 0;

        u->peer.cached = 0;
#endif

        rc = u->process_header(r);

        if (rc == NGX_AGAIN) {

            if (u->buffer.last == u->buffer.end) {
                ngx_log_error(NGX_LOG_ERR, c->log, 0,
                              "upstream sent too big header");

                ngx_http_upstream_next(r, u,
                                       NGX_HTTP_UPSTREAM_FT_INVALID_HEADER);
                return;
            }

            continue;
        }

        break;
    }

    if (rc == NGX_HTTP_UPSTREAM_INVALID_HEADER) {
        ngx_http_upstream_next(r, u, NGX_HTTP_UPSTREAM_FT_INVALID_HEADER);
        return;
    }

    if (rc == NGX_ERROR) {
        ngx_http_upstream_finalize_request(r, u,
                                           NGX_HTTP_INTERNAL_SERVER_ERROR);
        return;
    }

    /* rc == NGX_OK */

    u->state->header_time = ngx_current_msec - u->state->response_time;

    if (u->headers_in.status_n >= NGX_HTTP_SPECIAL_RESPONSE) {

        if (ngx_http_upstream_test_next(r, u) == NGX_OK) {
            return;
        }

        if (ngx_http_upstream_intercept_errors(r, u) == NGX_OK) {
            return;
        }
    }

    if (ngx_http_upstream_process_headers(r, u) != NGX_OK) {
        return;
    }

    if (!r->subrequest_in_memory) {
        ngx_http_upstream_send_response(r, u);
        return;
    }

    /* subrequest content in memory */

    if (u->input_filter == NULL) {
        u->input_filter_init = ngx_http_upstream_non_buffered_filter_init;
        u->input_filter = ngx_http_upstream_non_buffered_filter;
        u->input_filter_ctx = r;
    }

    if (u->input_filter_init(u->input_filter_ctx) == NGX_ERROR) {
        ngx_http_upstream_finalize_request(r, u, NGX_ERROR);
        return;
    }

    n = u->buffer.last - u->buffer.pos;

    if (n) {
        u->buffer.last = u->buffer.pos;

        u->state->response_length += n;

        if (u->input_filter(u->input_filter_ctx, n) == NGX_ERROR) {
            ngx_http_upstream_finalize_request(r, u, NGX_ERROR);
            return;
        }
    }

    if (u->length == 0) {
        ngx_http_upstream_finalize_request(r, u, 0);
        return;
    }

    u->read_event_handler = ngx_http_upstream_process_body_in_memory;

    ngx_http_upstream_process_body_in_memory(r, u);
}