Dear curl share users,

the current Curl_resolv uses a big lock (if enabled).
Following pseudo code visualize that:

  lock
  dns = Curl_hash_pic(...)
  if(dns) {
    addr = my_getaddrinfo(...)
    if(addr) {
        dns = cache_resolv_response(...,addr,...)
    }
  }
  unlock
  return dns

As you can see new dns requests blocks other dns requests
which might be already resolved.

I think following finer locking is better:

  lock
  dns = Curl_hash_pic(...)
  unlock
  if(dns) {
    addr = my_getaddrinfo(...)
    if(addr) {
        lock
        dns = cache_resolv_response(...,addr,...)
        unlock
    }
  }
  return dns

The attached patch does this locking. Applied to 7.10.7 I get
segmentation faults somewhere in curl code (sorry, I've forgot
to dump the debugging information). But with 7.10.8-pre8 the
seg fault disappears.

This patch has a side effect, which might be not ok for some users:
Multiple resolves for the same hostname are possible.
Avoiding this needs some more work...

Grettings,
Dirk

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