[PATCH 3 of 4] Upstream: sendfile_max_chunk support

[Sergey Kandaurov]

> On 11 Oct 2021, at 21:58, Maxim Dounin <mdounin at mdounin.ru> wrote:
> 
> # HG changeset patch
> # User Maxim Dounin <mdounin at mdounin.ru>
> # Date 1633978615 -10800
> #      Mon Oct 11 21:56:55 2021 +0300
> # Node ID c7ef6ce9455b01ee1fdcfd7288c4ac5b3ef0de41
> # Parent  489323e194e4c3b1a7937c51bd4e1671c70f52f8
> Upstream: sendfile_max_chunk support.
> 
> Previously, connections to upstream servers used sendfile() if it was
> enabled, but never honored sendfile_max_chunk.  This might result
> in worker monopolization for a long time if large request bodies
> are allowed.
> 
> diff --git a/src/core/ngx_output_chain.c b/src/core/ngx_output_chain.c
> --- a/src/core/ngx_output_chain.c
> +++ b/src/core/ngx_output_chain.c
> @@ -803,6 +803,10 @@ ngx_chain_writer(void *data, ngx_chain_t
>         return NGX_ERROR;
>     }
> 
> +    if (chain && c->write->ready) {
> +        ngx_post_event(c->write, &ngx_posted_next_events);
> +    }
> +
>     for (cl = ctx->out; cl && cl != chain; /* void */) {
>         ln = cl;
>         cl = cl->next;
> diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
> --- a/src/http/ngx_http_upstream.c
> +++ b/src/http/ngx_http_upstream.c
> @@ -1511,8 +1511,9 @@ ngx_http_upstream_check_broken_connectio
> static void
> ngx_http_upstream_connect(ngx_http_request_t *r, ngx_http_upstream_t *u)
> {
> -    ngx_int_t          rc;
> -    ngx_connection_t  *c;
> +    ngx_int_t                  rc;
> +    ngx_connection_t          *c;
> +    ngx_http_core_loc_conf_t  *clcf;
> 
>     r->connection->log->action = "connecting to upstream";
> 
> @@ -1599,10 +1600,12 @@ ngx_http_upstream_connect(ngx_http_reque
> 
>     /* init or reinit the ngx_output_chain() and ngx_chain_writer() contexts */
> 
> +    clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
> +
>     u->writer.out = NULL;
>     u->writer.last = &u->writer.out;
>     u->writer.connection = c;
> -    u->writer.limit = 0;
> +    u->writer.limit = clcf->sendfile_max_chunk;
> 
>     if (u->request_sent) {
>         if (ngx_http_upstream_reinit(r, u) != NGX_OK) {
> 

Just to reiterate, while this has most positive effect with Linux
sendfile() on fast networks, it doesn't seem to help with typical
proxying of request body buffered on disk, with sendfile disabled.
In this case, request body is read in ngx_output_chain_copy_buf()
iteratively in small client_body_buffer_size parts usually below
the sendfile_max_chunk limit.  (Yet, this can be limited with
client_body_buffer_size configured above sendfile_max_chunk.)
Also, it would barely effect sending body with chunked encoding,
such as in HTTP/1.1, FastCGI, and with limited frame size in gRPC.
Anyway, the change looks quite useful and natural to have.

-- 
Sergey Kandaurov