pipelining: removed

this is a problem for you and how your use case can't be satisfied properly

using a work around.

## HTTP pipelining

HTTP pipelining is badly supported by curl in the sense that we have bugs and

it is a fragile feature without enough tests. Also, when something turns out

to have problems it is really tricky to debug due to the timing sensitivity so

very often enabling debug outputs or similar completely changes the nature of

the behavior and things are not reproducing anymore!

HTTP pipelining was never enabled by default by the large desktop browsers due

to all the issues with it. Both Firefox and Chrome have also dropped

pipelining support entirely since a long time back now. We are in fact over

time becoming more and more lonely in supporting pipelining.

The bad state of HTTP pipelining was a primary driving factor behind HTTP/2

and its multiplexing feature. HTTP/2 multiplexing is truly and really

"pipelining done right". It is way more solid, practical and solves the use

case in a better way with better performance and fewer downsides and problems.

In 2018, pipelining *should* be abandoned and HTTP/2 should be used instead.

### State

In 7.62.0, we will add code that ignores the "enable pipeline" option

setting). The *setopt() function would still return "OK" though so the

application couldn't tell that this is happening.

Users who truly need pipelining from that version will need to modify the code

(ever so slightly) and rebuild.

### Removal

Six months later, in sync with the planned release happen in April 2019,

(might be 7.66.0), assuming no major riots have occurred due to this in the

mean time, we rip out the pipelining code. It is in the order of 1000 lines of

libcurl code.

Left to answer: should the *setopt() function start to return error when these

options are set to be able to tell when they're trying to use options that are

no longer around or should we maintain behavior as much as possible?

## `CURLOPT_DNS_USE_GLOBAL_CACHE`

This option makes libcurl use a global non-thread-safe cache for DNS if

.\" *                            | (__| |_| |  _ <| |___

.\" *                             \___|\___/|_| \_\_____|

.\" *

.\" * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" *

.\" * This software is licensed as described in the file COPYING, which

.\" * you should have received as part of this distribution. The terms

CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_CHUNK_LENGTH_PENALTY_SIZE, long size);

.SH DESCRIPTION

No function since pipelining was removed in 7.62.0.

Pass a long with a \fBsize\fP in bytes. If a pipelined connection is currently

processing a chunked (Transfer-encoding: chunked) request with a current chunk

length larger than \fICURLMOPT_CHUNK_LENGTH_PENALTY_SIZE(3)\fP, that pipeline

.\" *                            | (__| |_| |  _ <| |___

.\" *                             \___|\___/|_| \_\_____|

.\" *

.\" * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" *

.\" * This software is licensed as described in the file COPYING, which

.\" * you should have received as part of this distribution. The terms

CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_CONTENT_LENGTH_PENALTY_SIZE, long size);

.SH DESCRIPTION

No function since pipelining was removed in 7.62.0.

Pass a long with a \fBsize\fP in bytes. If a pipelined connection is currently

processing a request with a Content-Length larger than this

\fICURLMOPT_CONTENT_LENGTH_PENALTY_SIZE(3)\fP, that pipeline will then not be

.\" *                            | (__| |_| |  _ <| |___

.\" *                             \___|\___/|_| \_\_____|

.\" *

.\" * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" *

.\" * This software is licensed as described in the file COPYING, which

.\" * you should have received as part of this distribution. The terms

CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_MAX_PIPELINE_LENGTH, long max);

.SH DESCRIPTION

No function since pipelining was removed in 7.62.0.

Pass a long. The set \fBmax\fP number will be used as the maximum amount of

outstanding requests in an HTTP/1.1 pipelined connection. This option is only

used for HTTP/1.1 pipelining, not for HTTP/2 multiplexing.

.\" *                            | (__| |_| |  _ <| |___

.\" *                             \___|\___/|_| \_\_____|

.\" *

.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" *

.\" * This software is licensed as described in the file COPYING, which

.\" * you should have received as part of this distribution. The terms

.SH EXAMPLE

.nf

CURLM *m = curl_multi_init();

/* try HTTP/1 pipelining and HTTP/2 multiplexing */

curl_multi_setopt(m, CURLMOPT_PIPELINING, CURLPIPE_HTTP1 |

                                          CURLPIPE_MULTIPLEX);

/* try HTTP/2 multiplexing */

curl_multi_setopt(m, CURLMOPT_PIPELINING, CURLPIPE_MULTIPLEX);

.fi

.SH AVAILABILITY

Added in 7.16.0. Multiplex support bit added in 7.43.0.

Added in 7.16.0. Multiplex support bit added in 7.43.0. HTTP/1 Pipelining

support was disabled in 7.62.0.

.SH RETURN VALUE

Returns CURLM_OK if the option is supported, and CURLM_UNKNOWN_OPTION if not.

.SH "SEE ALSO"