HTTPCLIENT-2314: Handle gracefully a failure of DnsResolver to return a list of resolved addresses (#533)

Author: phax

httpclient5/src/main/java/org/apache/hc/client5/http/impl/InMemoryDnsResolver.java

@@ -90,7 +90,7 @@ public InetAddress[] resolve(final String host) throws UnknownHostException {
             LOG.info("Resolving {} to {}", host, Arrays.deepToString(resolvedAddresses));
         }
         if(resolvedAddresses == null){
-            throw new UnknownHostException(host + " cannot be resolved");
+            throw new UnknownHostException(host);
         }
         return resolvedAddresses;
     }

httpclient5/src/main/java/org/apache/hc/client5/http/impl/io/DefaultHttpClientConnectionOperator.java

@@ -32,6 +32,7 @@
 import java.net.Proxy;
 import java.net.Socket;
 import java.net.SocketAddress;
+import java.net.UnknownHostException;
 import java.util.Arrays;
 
 import org.apache.hc.client5.http.ConnectExceptionSupport;
@@ -143,8 +144,12 @@ public void connect(
             remoteAddresses = this.dnsResolver.resolve(host.getHostName());
 
             if (LOG.isDebugEnabled()) {
-                LOG.debug("{} resolved to {}", host.getHostName(), Arrays.asList(remoteAddresses));
+                LOG.debug("{} resolved to {}", host.getHostName(), remoteAddresses == null ? "null" : Arrays.asList(remoteAddresses));
             }
+
+            if (remoteAddresses == null || remoteAddresses.length == 0) {
+              throw new UnknownHostException(host.getHostName());
+          }
         }
 
         final Timeout soTimeout = socketConfig.getSoTimeout();

httpclient5/src/main/java/org/apache/hc/client5/http/impl/nio/MultihomeIOSessionRequester.java

@@ -111,6 +111,9 @@ public void cancelled() {
         final InetAddress[] remoteAddresses;
         try {
             remoteAddresses = dnsResolver.resolve(remoteEndpoint.getHostName());
+            if (remoteAddresses == null || remoteAddresses.length == 0) {
+              throw new UnknownHostException(remoteEndpoint.getHostName());
+            }
         } catch (final UnknownHostException ex) {
             future.failed(ex);
             return future;