Add ConfigOverride for random application ports during tests (#3561)

Problem:

We are doing a lot of integration testing where we need to start the application. We primarily use the config overrides instead of a `test-config.yaml` to do the configuration. Our config files of the test module often only contain the configuration for the random ports:
```yaml
server:
  applicationConnectors:
    - type: http
      port: 0
  adminConnectors:
    - type: http
      port: 0
```

It would be nice to be able to use random ports with the `DropwizardAppRule`/`DropwizardAppExtension` programmatically.

Solution:

Add a Config Override that configures the default application and admin connectors to use port `0`.

```java
private static final DropwizardAppExtension<MyConfiguration> EXTENSION =
    new DropwizardAppExtension<>(MyApplication.class,
        // no config file
        null,
        // adds all necessary config overrides for random ports
        randomPorts());
```

Closes #3561

(cherry picked from commit 7f4bb5c)

dropwizard-testing/src/main/java/io/dropwizard/testing/ConfigOverride.java

@@ -30,42 +30,40 @@
  * <li><code>ConfigOverride.config("logging.loggers.com\\.example\\.bar",
  * "DEBUG")</code> will add a logger with the name "com.example.bar" configured
  * for debug logging.</li>
+ * <li><code>ConfigOverride.randomPorts()</code> will change the ports of the
+ * default applicationConnectors and adminConnectors to 0 so the tests start
+ * with random ports.</li>
  * </ul>
  */
-public class ConfigOverride {
+public abstract class ConfigOverride {
 
-    public static final String DEFAULT_PREFIX = "dw.";
-    private final String key;
-    private final Supplier<String> value;
-    private final String propertyPrefix;
-
-    private ConfigOverride(String propertyPrefix, String key, Supplier<String> value) {
-        this.key = key;
-        this.value = value;
-        this.propertyPrefix = propertyPrefix.endsWith(".") ? propertyPrefix : propertyPrefix + ".";
-    }
+    static final String DEFAULT_PREFIX = "dw.";
 
     public static ConfigOverride config(String key, String value) {
-        return new ConfigOverride(DEFAULT_PREFIX, key, () -> value);
+        return new ConfigOverrideValue(DEFAULT_PREFIX, key, () -> value);
     }
 
     public static ConfigOverride config(String propertyPrefix, String key, String value) {
-        return new ConfigOverride(propertyPrefix, key, () -> value);
+        return new ConfigOverrideValue(propertyPrefix, key, () -> value);
     }
 
     public static ConfigOverride config(String key, Supplier<String> value) {
-        return new ConfigOverride(DEFAULT_PREFIX, key, value);
+        return new ConfigOverrideValue(DEFAULT_PREFIX, key, value);
     }
 
     public static ConfigOverride config(String propertyPrefix, String key, Supplier<String> value) {
-        return new ConfigOverride(propertyPrefix, key, value);
+        return new ConfigOverrideValue(propertyPrefix, key, value);
     }
 
-    public void addToSystemProperties() {
-        System.setProperty(propertyPrefix + key, value.get());
+    public static ConfigOverride randomPorts() {
+        return new ConfigOverrideRandomPorts(DEFAULT_PREFIX);
     }
 
-    public void removeFromSystemProperties() {
-        System.clearProperty(propertyPrefix + key);
+    public static ConfigOverride randomPorts(String propertyPrefix) {
+        return new ConfigOverrideRandomPorts(propertyPrefix);
     }
+
+    public abstract void addToSystemProperties();
+
+    public abstract void removeFromSystemProperties();
 }

dropwizard-testing/src/main/java/io/dropwizard/testing/ConfigOverrideRandomPorts.java

@@ -0,0 +1,49 @@
+package io.dropwizard.testing;
+
+import javax.annotation.Nullable;
+
+/**
+ * Override port configuration setting for application and admin connectors and choose a random port.
+ *
+ * @see io.dropwizard.jetty.HttpConnectorFactory#getPort()
+ * @see io.dropwizard.Configuration#getServerFactory()
+ * @see io.dropwizard.server.DefaultServerFactory#getApplicationConnectors()
+ * @see io.dropwizard.server.DefaultServerFactory#getAdminConnectors()
+ */
+public class ConfigOverrideRandomPorts extends ConfigOverride {
+
+    private static final String SERVER_APPLICATION_CONNECTORS_PORT = "server.applicationConnectors[0].port";
+    private static final String SERVER_ADMIN_CONNECTORS_PORT = "server.adminConnectors[0].port";
+
+    private final String propertyPrefix;
+
+    @Nullable
+    private String applicationConnectorsPort = null;
+    @Nullable
+    private String adminConnectorsPort = null;
+
+    ConfigOverrideRandomPorts(String propertyPrefix) {
+        this.propertyPrefix = propertyPrefix.endsWith(".") ? propertyPrefix : propertyPrefix + ".";
+    }
+
+    @Override
+    public void addToSystemProperties() {
+        applicationConnectorsPort = System.setProperty(propertyPrefix + SERVER_APPLICATION_CONNECTORS_PORT, "0");
+        adminConnectorsPort = System.setProperty(propertyPrefix + SERVER_ADMIN_CONNECTORS_PORT, "0");
+    }
+
+    @Override
+    public void removeFromSystemProperties() {
+        if (applicationConnectorsPort != null) {
+            System.setProperty(propertyPrefix + SERVER_APPLICATION_CONNECTORS_PORT, applicationConnectorsPort);
+        } else {
+            System.clearProperty(propertyPrefix + SERVER_APPLICATION_CONNECTORS_PORT);
+        }
+
+        if (adminConnectorsPort != null) {
+            System.setProperty(propertyPrefix + SERVER_ADMIN_CONNECTORS_PORT, adminConnectorsPort);
+        } else {
+            System.clearProperty(propertyPrefix + SERVER_ADMIN_CONNECTORS_PORT);
+        }
+    }
+}

dropwizard-testing/src/main/java/io/dropwizard/testing/ConfigOverrideValue.java

@@ -0,0 +1,33 @@
+package io.dropwizard.testing;
+
+import javax.annotation.Nullable;
+import java.util.function.Supplier;
+
+public class ConfigOverrideValue extends ConfigOverride {
+
+    private final String key;
+    private final Supplier<String> value;
+    private final String propertyPrefix;
+    @Nullable
+    private String originalValue = null;
+
+    ConfigOverrideValue(String propertyPrefix, String key, Supplier<String> value) {
+        this.key = key;
+        this.value = value;
+        this.propertyPrefix = propertyPrefix.endsWith(".") ? propertyPrefix : propertyPrefix + ".";
+    }
+
+    @Override
+    public void addToSystemProperties() {
+        this.originalValue = System.setProperty(propertyPrefix + key, value.get());
+    }
+
+    @Override
+    public void removeFromSystemProperties() {
+        if (originalValue != null) {
+            System.setProperty(propertyPrefix + key, originalValue);
+        } else {
+            System.clearProperty(propertyPrefix + key);
+        }
+    }
+}

dropwizard-testing/src/test/java/io/dropwizard/testing/junit5/DropwizardAppExtensionRandomPortsConfigOverrideTest.java

@@ -0,0 +1,39 @@
+package io.dropwizard.testing.junit5;
+
+import static io.dropwizard.testing.ConfigOverride.config;
+import static io.dropwizard.testing.ConfigOverride.randomPorts;
+import static org.assertj.core.api.Assertions.assertThat;
+
+import io.dropwizard.jetty.HttpConnectorFactory;
+import io.dropwizard.server.DefaultServerFactory;
+import io.dropwizard.testing.app.TestApplication;
+import io.dropwizard.testing.app.TestConfiguration;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+
+@ExtendWith(DropwizardExtensionsSupport.class)
+class DropwizardAppExtensionRandomPortsConfigOverrideTest {
+
+    private static final DropwizardAppExtension<TestConfiguration> EXTENSION =
+        new DropwizardAppExtension<>(TestApplication.class,
+            null,
+            "app-rule",
+            randomPorts("app-rule"),
+            config("app-rule", "message", "A new way to say Hooray!"),
+            config("app-rule", "extra", () -> "supplied"));
+
+    @Test
+    void supportsRandomPortsConfigAttributeOverrides() {
+        DefaultServerFactory serverFactory = (DefaultServerFactory) EXTENSION.getConfiguration()
+            .getServerFactory();
+
+        assertThat(
+            serverFactory.getApplicationConnectors().stream().map(HttpConnectorFactory.class::cast))
+            .extracting(
+                HttpConnectorFactory::getPort).containsExactly(0);
+        assertThat(
+            serverFactory.getAdminConnectors().stream().map(HttpConnectorFactory.class::cast))
+            .extracting(
+                HttpConnectorFactory::getPort).containsExactly(0);
+    }
+}