Connection upkeep function doesn't attach Curl_easy to each connection

[Josie-H]

I did this

Had code running curl_easy_upkeep() for connection upkeep, using libcurl 7.75.0. At a couple of customers, we started to hear reports of segfaults killing the application that was using Curl, and core dumps showed this happening during connection upkeep.

I debugged the bug

After turning on debug symbols, I realised that functions called by the connection check expect "Curl_easy data" and "connectdata conn" to be connected to each other. In 7.75.0 that means that data->conn=conn, and conn->data=data. (I see that in more recent versions the second requirement has been removed.) In these crashes, data->conn was NULL.

In case you're interested in why this pointer is required to be set, the stack for one of the crashes looks like:
ossl_send (vtls/openssl.c:4174) (attempts to dereference data->conn)
send_callback (http2.c:362)
nghttp2_session_send (nghttp2_session.c:3258)
http2_conncheck (http2.c:250)
http2_conncheck (http2.c:214)
conn_upkeep (easy.c:1230)
Curl_conncache_foreach (conncache.c:352)
upkeep (easy.c:1258)

I fixed the bug

I found similar function extract_if_dead() (url.c) where we're also looping through connections in the connection cache and doing something to each of them. That one just does Curl_attach_connnection() before checking the connection and Curl_detach_connnection() afterwards. I figure that's probably the solution in conn_upkeep() too.

The one thing I'm not quite sure about (and would definitely appreciate feedback on) is whether we'd expect that data->conn might be set to anything else important at the point of calling the upkeep function. So might there be anything in that field that we should store off and reset back later? In my current patch for our customers I'm doing that just to be safe, but for the moment I'm only submitting a pull request here with the simpler code, since that's what extract_if_dead does. But I'm prepared to believe that actually both of them should be storing/restoring a previous value if present.