curl: add --parallel-immediate

[bagder]

Starting with this change when doing parallel transfers, without this
option set, curl will prefer to create new transfers multiplexed on an
existing connection rather than creating a brand new one.

--parallel-connect can be set to tell curl to prefer to use new
connections rather than to wait and try to multiplex.

libcurl-wise, this means that curl will set CURLOPT_PIPEWAIT by default
on parallel transfers.

Fixes #4494
Suggested-by: Tom van der Woerdt

[jay]

I'd call it something easier to understand for anyone reading a shell script like --parallel-nowait. But although that is technically accurate I find it kind of misleading because --IMHO-- strategically it's better to wait to multiplex than fan out, and I think by adding an option named nowait users are going to think, well, obviously I don't want to wait. Who wants to wait? I think the pipewait change was a good one and I'd skip this.

[bagder]

I'd call it something easier to understand for anyone reading a shell script

Yeah. Naming is hard. I'll think some more on this.

I think the pipewait change was a good one and I'd skip this.

I'm sorry, I don't understand this comment. This is the PR that introduces pipewait for the curl tool when doing parallel transfers...

[jay]

I think the pipewait change was a good one and I'd skip this.

I'm sorry, I don't understand this comment. This is the PR that introduces pipewait for the curl tool when doing parallel transfers...

Oh I thought that already went in, I must have misread. I thought this: (void)curl_easy_setopt(per->curl, CURLOPT_PIPEWAIT, 1L);

On second thought I think an option to disable it could be useful for when there is prior knowledge of a server.

[bagder]

That's my thinking as well!

[bagder]

Alternative name suggestions for this option:

--parallel-preferred
--parallel-at-once
--parallel-directly

I think my personal preference is the last one here.

[jay]

I don't like any of those but I still like --parallel-nowait, somewhat.

[bagder]

Out of the suggestions replied on the mailing list and as responses to my tweet, the only real contender that could work that I spotted is --parallel-immediate. I kind of like that.

[bagder]

I think I'm done bike-shedding this now. It is a rather minor option anyway and few people will want to use it. I rebased and force-pushed, will work on merging this soon.