We found that in specific cases if the connection is abruptly closed,
the underlying socket is listed in a close_wait state. We continue to
call the curl_multi_perform, curl_mutli_fdset etc. None of these APIs
report the socket closed / connection finished. Since we have cases
where the multi connection is only used once, this can pose a problem
for us. I've read that if another connection was to come in, curl would
see the socket as bad and attempt to close it at that time -
unfortunately, this does not work for us.
I found that in specific situations, if SSL_write returns 0, curl did
not recognize the socket as closed (or errored out) and did not report
it to the application. I believe we need to change the code slightly, to
check if ssl_write returns 0. If so, treat it as an error - the same as
a negative return code.
For OpenSSL - the ssl_write documentation is here:
http://www.openssl.org/docs/ssl/SSL_write.html
HTTP Pipelining with libcurl
============================
Background
Since pipelining implies that one or more requests are sent to a server before
the previous response(s) have been received, we only support it for multi
interface use.
Considerations
When using the multi interface, you create one easy handle for each transfer.
Bascially any number of handles can be created, added and used with the multi
interface - simultaneously. It is an interface designed to allow many
simultaneous transfers while still using a single thread. Pipelining does not
change any of these details.
API
We've added a new option to curl_multi_setopt() called CURLMOPT_PIPELINING
that enables "attempted pipelining" and then all easy handles used on that
handle will attempt to use an existing pipeline.
Details
- A pipeline is only created if a previous connection exists to the same IP
address that the new request is being made to use.
- Pipelines are only supported for HTTP(S) as no other currently supported
protocol has features resemembling this, but we still name this feature
plain 'pipelining' to possibly one day support it for other protocols as
well.
- HTTP Pipelining is for GET and HEAD requests only.
- When a pipeline is in use, we must take precautions so that when used easy
handles (i.e those who still wait for a response) are removed from the multi
handle, we must deal with the outstanding response nicely.
- Explicitly asking for pipelining handle X and handle Y won't be supported.
It isn't easy for an app to do this association. The lib should probably
still resolve the second one properly to make sure that they actually _can_
be considered for pipelining. Also, asking for explicit pipelining on handle
X may be tricky when handle X get a closed connection.
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