Lots of work and analysis by "xbx___" in bug #1431750
(http://curl.haxx.se/bug/view.cgi?id=1431750) helped me identify and fix two different but related bugs: 1) Removing an easy handle from a multi handle before the transfer is done could leave a connection in the connection cache for that handle that is in a state that isn't suitable for re-use. A subsequent re-use could then read from a NULL pointer and segfault. 2) When an easy handle was removed from the multi handle, there could be an outstanding c-ares DNS name resolve request. When the response arrived, it caused havoc since the connection struct it "belonged" to could've been freed already. Now Curl_done() is called when an easy handle is removed from a multi handle pre-maturely (that is, before the transfer was complteted). Curl_done() also makes sure to cancel all (if any) outstanding c-ares requests.
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