Stoppable curl_easy_perform ?

Hi all,

A frequent request and question over the years has been how you stop
curl_easy_perform() immediately from another thread.

The answer of course has always been that there's no built-in way to do that
but you have to make sure one of the callbacks return an error, which in the
worst case could be with a full second latency.

Now I'm here to propose a way we can improve the answer to that question and
I'm interested in feedback and comments on the API and the approach.

Consider curl_easy_stop(). Can be called from the same or another thread. This
stops the specific transfer *iff* that transfer was setup to *allow* being
externally stopped.

You enable a transfer to be stopped like this by using the share interface and
making sure that the share object the transfer uses has the new CURL_LOCK_STOP
bit set. That bit makes that transfer stoppable.

Psuedo code:

   CURLSH *share = curl_share_init();
   curl_share_setopt(share, CURLSHOPT_SHARE, CURL_LOCK_STOP);
   curl_share_setopt(share, CURLSHOPT_LOCKFUNC, mutex_lock);
   ...

   CURL *easy = curl_easy_init();
   curl_easy_setopt(easy, CURLOPT_SHARE, share);

   curl_easy_perform(easy);

... now in another thread, this new API can be called:

   curl_easy_stop(easy);

Details:

When used from another thread, it is crucial that the CURLSHOPT_LOCKFUNC is
set so that we can lock the handle enough so that we can send a message on a
socketpair() over to the other thread and then unlock it again. (With PR #4466
we should soon have a decently portable socketpair situation to lean on.)

The thread performing a transfer will also wait for messages on a message pipe
that can be used to provide messages from the outside. Like for example an
instruction to stop.

I think a stopped transfer should return a return code saying it was stopped.
And if curl_easy_stop() is attempted on a transfer that hasn't been made
stoppable, a suitable error code should also be returned.

Good idea? Bad idea? Good enough API? Does the logic hold?

-- 
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