easy: fix curl_easy_upkeep for shared connection caches

[jay]

• Predict which connection cache is or will be used by the easy handle and perform connection upkeep on that cache.

This change allows curl_easy_upkeep to be effective on easy handles that are using a shared connection cache, either from a user created shared connection cache or a user created multi which has its own shared connection cache.

Prior to this change curl_easy_upkeep would upkeep the connection cache for the easy handle only if that cache was from the multi owned by the easy handle (ie curl_easy_perform was previously called and there's a connection cache exclusive to the easy handle in
data->multi_easy->conn_cache).

Ref: https://curl.se/mail/lib-2024-01/0016.html

Closes #xxxx

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It's probably a matter of perspective whether this is a bug or not. I didn't find anything to indicate that the function wasn't intended to work with shared connection caches.

Note this change will allow calling on an easy handle that is part of a user created multi, therefore a user of that interface might use this function and not expect it to block? It's unclear to me whether or not the function could actually block.

/cc @maxdymond

[bagder]

what's the status here @jay ?

[jay]

This change makes it possible to call easy upkeep on easy handles in multi handles and I can't say for certain it won't block in that case. Also I changed it so easy upkeep can't be called from a callback since I can't say for certain a callback won't be called during the upkeep.

If nobody has a problem with either of those two things I'll land it.

[bagder]

Predict which connection cache is or will be used by the easy handle and perform connection upkeep on that cache.

I have a minor issue with this wording here. Predicting means guessing how something will be or happen in a future. This code does not predict, it determines or possibly decides which connection cache to check using heuristics.

[jay]

I have a minor issue with this wording here. Predicting means guessing how something will be or happen in a future. This code does not predict, it determines or possibly decides which connection cache to check using heuristics.

Ok I changed it to say determine and I added to the documentation to explain that upkeep on a handle with a shared connection cache will perform upkeep on all the connections in that cache.