背景：當我編寫upstream模組時，在回撥函式create_request中，我構造了發向後端的http請求，並且後端服務也接收到請求，但是nginx卻接收不到後端服務返回的應答，導致回撥函式process_header不會被呼叫。

錯誤標識：upstream prematurely closed connection while reading response header from upstream, client: 127.0.0.1, server: localhost, request: "POST /upload HTTP/1.1", upstream: "127.0.0.1:9999", host: "localhost:8091", referrer: "http://localhost:8091/up.html"

分析：通過查詢nginx原始碼，在ngx_http_upstream.c檔案中列印的錯誤標識，定位在此

n = c->recv(c, u->buffer.last, u->buffer.end - u->buffer.last);
if (n == 0) {
    ngx_log_error(NGX_LOG_ERR, c->log, 0,"upstream prematurely closed connection");
}

conf->upstream.buffer_size = NGX_CONF_UNSET_SIZE;
ngx_conf_merge_size_value(conf->upstream.buffer_size,prev->upstream.buffer_size,(size_t) ngx_pagesize);