Bind test servers to loopback address

Binding to 0.0.0.0 is not guaranteed to result in a socket that is
reachable via the loopback address, even when requesting an ephemeral
port assignment by binding to port 0. It is possible for another process
on the system to bind to the loopback address at that same port, and
then client requests will go to that process instead of the test server,
resulting in occasional cryptic test failures.

By instead binding to the loopback address specifically, rather than the
wildcard address, we can prevent binding to a port that is already in
use. This significantly improves the reliability of the test suite.

Author: rschmitt

httpclient5-testing/src/test/java/org/apache/hc/client5/testing/TestValidateAfterInactivity.java

@@ -47,6 +47,7 @@
 import org.junit.jupiter.api.Test;
 
 import java.io.IOException;
+import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.net.SocketException;
 import java.net.StandardSocketOptions;
@@ -78,7 +79,7 @@ class AbstractTestValidateAfterInactivity {
 
     @BeforeEach
     void setup() throws Exception {
-        serverSocket = ServerSocketChannel.open().bind(new InetSocketAddress(0));
+        serverSocket = ServerSocketChannel.open().bind(new InetSocketAddress(InetAddress.getLoopbackAddress(), 0));
         port = ((InetSocketAddress) serverSocket.getLocalAddress()).getPort();
 
         serverThread = new Thread(this::runServer);

httpclient5/src/test/java/org/apache/hc/client5/http/examples/AsyncClientEarlyHintsEndToEnd.java

@@ -27,6 +27,7 @@
 package org.apache.hc.client5.http.examples;
 
 import java.io.IOException;
+import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
@@ -142,7 +143,7 @@ public void releaseResources() { /* no-op for demo */ }
                 })
                 .create();
         server.start();
-        final Future<ListenerEndpoint> lf = server.listen(new InetSocketAddress(0), URIScheme.HTTP);
+        final Future<ListenerEndpoint> lf = server.listen(new InetSocketAddress(InetAddress.getLoopbackAddress(), 0), URIScheme.HTTP);
         final int port = ((InetSocketAddress) lf.get().getAddress()).getPort();
 
         // --- Async client with Early Hints listener

httpclient5/src/test/java/org/apache/hc/client5/http/impl/async/EarlyHintsAsyncExecTest.java

@@ -30,6 +30,7 @@
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.io.IOException;
+import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
@@ -156,7 +157,7 @@ public void releaseResources() { /* no-op for test */ }
         server.start();
 
         // Bind to ephemeral port and retrieve it from the listener endpoint
-        final Future<ListenerEndpoint> lf = server.listen(new InetSocketAddress(0), URIScheme.HTTP);
+        final Future<ListenerEndpoint> lf = server.listen(new InetSocketAddress(InetAddress.getLoopbackAddress(), 0), URIScheme.HTTP);
         final ListenerEndpoint ep = lf.get(5, TimeUnit.SECONDS);
         final int port = ((InetSocketAddress) ep.getAddress()).getPort();