static ngx_int_t
ngx_http_proxy_handler(ngx_http_request_t *r)
{
    ngx_int_t                    rc;
    ngx_http_upstream_t         *u;
    ngx_http_proxy_ctx_t        *ctx;
    ngx_http_proxy_loc_conf_t   *plcf;
#if (NGX_HTTP_CACHE)
    ngx_http_proxy_main_conf_t  *pmcf;
#endif

    if (ngx_http_upstream_create(r) != NGX_OK) {
        return NGX_HTTP_INTERNAL_SERVER_ERROR;
    }

    ctx = ngx_pcalloc(r->pool, sizeof(ngx_http_proxy_ctx_t));
    if (ctx == NULL) {
        return NGX_HTTP_INTERNAL_SERVER_ERROR;
    }

    ngx_http_set_ctx(r, ctx, ngx_http_proxy_module);

    plcf = ngx_http_get_module_loc_conf(r, ngx_http_proxy_module);

    u = r->upstream;

    if (plcf->proxy_lengths == NULL) {
        ctx->vars = plcf->vars;
        u->schema = plcf->vars.schema;
#if (NGX_HTTP_SSL)
        u->ssl = (plcf->upstream.ssl != NULL);
#endif

    } else {
        if (ngx_http_proxy_eval(r, ctx, plcf) != NGX_OK) {
            return NGX_HTTP_INTERNAL_SERVER_ERROR;
        }
    }

    u->output.tag = (ngx_buf_tag_t) &ngx_http_proxy_module;

    u->conf = &plcf->upstream;

#if (NGX_HTTP_CACHE)
    pmcf = ngx_http_get_module_main_conf(r, ngx_http_proxy_module);

    u->caches = &pmcf->caches;
    u->create_key = ngx_http_proxy_create_key;
#endif
    //设置创建请求报文的回调函数
    u->create_request = ngx_http_proxy_create_request;
    //设置当链接失败时，需要执行的动作
    u->reinit_request = ngx_http_proxy_reinit_request;
    //设置处理上游服务器的响应头回调函数
    u->process_header = ngx_http_proxy_process_status_line;
    u->abort_request = ngx_http_proxy_abort_request;
    //请求结束后会调用该方法
    u->finalize_request = ngx_http_proxy_finalize_request;
    r->state = 0;

    if (plcf->redirects) {
        u->rewrite_redirect = ngx_http_proxy_rewrite_redirect;
    }

    if (plcf->cookie_domains || plcf->cookie_paths) {
        u->rewrite_cookie = ngx_http_proxy_rewrite_cookie;
    }
    //设置upstream的buffer标志位，为0时，以下游网速优先，
    u->buffering = plcf->upstream.buffering;

    u->pipe = ngx_pcalloc(r->pool, sizeof(ngx_event_pipe_t));
    if (u->pipe == NULL) {
        return NGX_HTTP_INTERNAL_SERVER_ERROR;
    }

    u->pipe->input_filter = ngx_http_proxy_copy_filter;
    u->pipe->input_ctx = r;

    u->input_filter_init = ngx_http_proxy_input_filter_init;
    u->input_filter = ngx_http_proxy_non_buffered_copy_filter;
    u->input_filter_ctx = r;

    u->accel = 1;

    if (!plcf->upstream.request_buffering
        && plcf->body_values == NULL && plcf->upstream.pass_request_body
        && (!r->headers_in.chunked
            || plcf->http_version == NGX_HTTP_VERSION_11))
    {
        r->request_body_no_buffering = 1;
    }

    rc = ngx_http_read_client_request_body(r, ngx_http_upstream_init);

    if (rc >= NGX_HTTP_SPECIAL_RESPONSE) {
        return rc;
    }

    return NGX_DONE;
}