INTERNALS.md

  `->doing` keeps getting called while issuing the transfer request command(s)

  `->done` gets called when the transfer is complete and DONE. That's after the
  main data has been transferred.

  `->do_more` gets called during the `DO_MORE` state. The FTP protocol uses
  this state when setting up the second connection.

  ->`proto_getsock`
  ->`doing_getsock`
  ->`domore_getsock`
  ->`perform_getsock`
  Functions that return socket information. Which socket(s) to wait for which
  action(s) during the particular multi state.

  ->disconnect is called immediately before the TCP connection is shutdown.

  ->readwrite gets called during transfer to allow the protocol to do extra
  reads/writes

  ->defport is the default report TCP or UDP port this protocol uses

  ->protocol is one or more bits in the `CURLPROTO_*` set. The SSL versions
  have their "base" protocol set and then the SSL variation. Like
  "HTTP|HTTPS".

  ->flags is a bitmask with additional information about the protocol that will
  make it get treated differently by the generic engine:

  - `PROTOPT_SSL` - will make it connect and negotiate SSL

  - `PROTOPT_DUAL` - this protocol uses two connections

  - `PROTOPT_CLOSEACTION` - this protocol has actions to do before closing the
    connection. This flag is no longer used by code, yet still set for a bunch
    of protocol handlers.

  - `PROTOPT_DIRLOCK` - "direction lock". The SSH protocols set this bit to
    limit which "direction" of socket actions that the main engine will
    concern itself with.

  - `PROTOPT_NONETWORK` - a protocol that doesn't use network (read file:)

  - `PROTOPT_NEEDSPWD` - this protocol needs a password and will use a default
    one unless one is provided

  - `PROTOPT_NOURLQUERY` - this protocol can't handle a query part on the URL
    (?foo=bar)

## conncache

  Is a hash table with connections for later re-use. Each `Curl_easy` has a
  pointer to its connection cache. Each multi handle sets up a connection
  cache that all added `Curl_easy`s share by default.

## Curl_share

  The libcurl share API allocates a `Curl_share` struct, exposed to the
  external API as "CURLSH *".

  The idea is that the struct can have a set of its own versions of caches and
  pools and then by providing this struct in the `CURLOPT_SHARE` option, those
  specific `Curl_easy`s will use the caches/pools that this share handle
  holds.

  Then individual `Curl_easy` structs can be made to share specific things
  that they otherwise wouldn't, such as cookies.

  The `Curl_share` struct can currently hold cookies, DNS cache and the SSL
  session cache.

## CookieInfo

  This is the main cookie struct. It holds all known cookies and related
  information. Each `Curl_easy` has its own private CookieInfo even when
  they are added to a multi handle. They can be made to share cookies by using
  the share API.


[1]: https://curl.haxx.se/libcurl/c/curl_easy_setopt.html
[2]: https://curl.haxx.se/libcurl/c/curl_easy_init.html
[3]: https://c-ares.haxx.se/
[4]: https://tools.ietf.org/html/rfc7230 "RFC 7230"
[5]: https://curl.haxx.se/libcurl/c/CURLOPT_ACCEPT_ENCODING.html
[6]: https://curl.haxx.se/docs/manpage.html#--compressed
[7]: https://curl.haxx.se/libcurl/c/curl_multi_socket_action.html
[8]: https://curl.haxx.se/libcurl/c/curl_multi_timeout.html
[9]: https://curl.haxx.se/libcurl/c/curl_multi_setopt.html
[10]: https://curl.haxx.se/libcurl/c/CURLMOPT_TIMERFUNCTION.html
[11]: https://curl.haxx.se/libcurl/c/curl_multi_perform.html
[12]: https://curl.haxx.se/libcurl/c/curl_multi_fdset.html
[13]: https://curl.haxx.se/libcurl/c/curl_multi_add_handle.html
[14]: https://curl.haxx.se/libcurl/c/curl_multi_info_read.html
[15]: https://tools.ietf.org/html/rfc7231#section-3.1.2.2