easy: fix curl_easy_upkeep for shared connection caches

- 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

lib/easy.c

@@ -1346,13 +1346,31 @@ static CURLcode upkeep(struct conncache *conn_cache, void *data)
  */
 CURLcode curl_easy_upkeep(struct Curl_easy *data)
 {
+ struct conncache *conn_cache;
+
  /* Verify that we got an easy handle we can work with. */
  if(!GOOD_EASY_HANDLE(data))
  return CURLE_BAD_FUNCTION_ARGUMENT;
 
- if(data->multi_easy) {
+ /* predict the connection cache that will next be used by the easy handle.
+ if the easy handle is currently in a multi then data->state.conn_cache
+ should point to the in-use cache. otherwise use the same logic that a
+ multi would use to assign it: if there's a user-specified shared cache
+ then use that, else use the multi's default shared cache. */
+ DEBUGASSERT(!data->multi || data->state.conn_cache);
+ conn_cache =
+ data->state.conn_cache ?
+ data->state.conn_cache :
+ (data->share && (data->share->specifier & (1<< CURL_LOCK_DATA_CONNECT))) ?
+ &data->share->conn_cache :
+ data->multi ?
+ &data->multi->conn_cache :
+ data->multi_easy ?
+ &data->multi_easy->conn_cache : NULL;
+
+ if(conn_cache) {
  /* Use the common function to keep connections alive. */
- return upkeep(&data->multi_easy->conn_cache, data);
+ return upkeep(conn_cache, data);
  }
  else {
  /* No connections, so just return success */