[Net] Simplify IP resolution code, fix caching.

First, we should not insert into cache if the hostname resolution has
failed (as it might be a temporary internet issue), second, the async
resolver should also properly insert into cache.

Took the chance to remove some duplicate code with critical section in
it at the cost of little performance when calling the blocking
resolve_hostname function.

core/io/ip.cpp

@@ -98,6 +98,11 @@ struct _IP_ResolverPrivate {
 			if (queue[i].status.get() != IP::RESOLVER_STATUS_WAITING) {
 				continue;
 			}
+			// We might be overriding another result, but we don't care as long as the result is valid.
+			if (response.size()) {
+				String key = get_cache_key(hostname, type);
+				cache[key] = response;
+			}
 			queue[i].response = response;
 			queue[i].status.set(response.is_empty() ? IP::RESOLVER_STATUS_ERROR : IP::RESOLVER_STATUS_DONE);
 		}
@@ -120,30 +125,8 @@ struct _IP_ResolverPrivate {
 };
 
 IPAddress IP::resolve_hostname(const String &p_hostname, IP::Type p_type) {
-	List<IPAddress> res;
-	String key = _IP_ResolverPrivate::get_cache_key(p_hostname, p_type);
-
-	resolver->mutex.lock();
-	if (resolver->cache.has(key)) {
-		res = resolver->cache[key];
-	} else {
-		// This should be run unlocked so the resolver thread can keep
-		// resolving other requests.
-		resolver->mutex.unlock();
-		_resolve_hostname(res, p_hostname, p_type);
-		resolver->mutex.lock();
-		// We might be overriding another result, but we don't care (they are the
-		// same hostname).
-		resolver->cache[key] = res;
-	}
-	resolver->mutex.unlock();
-
-	for (int i = 0; i < res.size(); ++i) {
-		if (res[i].is_valid()) {
-			return res[i];
-		}
-	}
-	return IPAddress();
+	const Array addresses = resolve_hostname_addresses(p_hostname, p_type);
+	return addresses.size() ? addresses[0].operator IPAddress() : IPAddress();
 }
 
 Array IP::resolve_hostname_addresses(const String &p_hostname, Type p_type) {
@@ -159,17 +142,16 @@ Array IP::resolve_hostname_addresses(const String &p_hostname, Type p_type) {
 		resolver->mutex.unlock();
 		_resolve_hostname(res, p_hostname, p_type);
 		resolver->mutex.lock();
-		// We might be overriding another result, but we don't care (they are the
-		// same hostname).
-		resolver->cache[key] = res;
+		// We might be overriding another result, but we don't care as long as the result is valid.
+		if (res.size()) {
+			resolver->cache[key] = res;
+		}
 	}
 	resolver->mutex.unlock();
 
 	Array result;
 	for (int i = 0; i < res.size(); ++i) {
-		if (res[i].is_valid()) {
-			result.push_back(String(res[i]));
-		}
+		result.push_back(String(res[i]));
 	}
 	return result;
 }

drivers/unix/ip_unix.cpp

@@ -115,7 +115,7 @@ void IPUnix::_resolve_hostname(List<IPAddress> &r_addresses, const String &p_hos
 			continue;
 		}
 		IPAddress ip = _sockaddr2ip(next->ai_addr);
-		if (!r_addresses.find(ip)) {
+		if (ip.is_valid() && !r_addresses.find(ip)) {
 			r_addresses.push_back(ip);
 		}
 		next = next->ai_next;