feat: support HTTP request cancellation

changes.md

@@ -70,6 +70,8 @@ Changes log
           transfers and HTTP/2 trailing headers.
         - HTTP/2 clients receive a response as soon as its headers arrive, while the request entity
           continues to be written on a virtual thread.
+        - Active client requests can be aborted; HTTP stream failures surface as I/O errors and a
+          communication-error response callback.
     - Bugs fixed
         - Slf4j bridge reports wrong line number and caller class in log4j. Issue #233.
         - When GET is an allowed method of a server resource, HEAD is now also listed (e.g. in the Allow header of a 405 response). Issue #266.

org.restlet/src/main/java/org/restlet/Request.java

@@ -13,6 +13,7 @@
 import java.util.Set;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.CopyOnWriteArraySet;
+import java.util.function.BooleanSupplier;
 import org.restlet.data.CacheDirective;
 import org.restlet.data.ChallengeResponse;
 import org.restlet.data.CharacterSet;
@@ -81,6 +82,9 @@ public static Request getCurrent() {
     /** The cookies provided by the client. */
     private volatile Series<Cookie> cookies;
 
+    /** Action used by an active client connector to abort this request. */
+    private volatile BooleanSupplier abortAction;
+
     /** The trailing headers received with the request entity. */
     private volatile Series<Header> trailers;
 
@@ -192,6 +196,7 @@ public Request(Method method, String resourceUri, Representation entity) {
      */
     public Request(Request request) {
         this(request.getMethod(), new Reference(request.getResourceRef()), request.getEntity());
+        this.abortAction = null;
         challengeResponse = request.getChallengeResponse();
 
         copyClientInfo(request);
@@ -311,7 +316,18 @@ private void copyClientInfo(final Request request) {
      * @return True if the request was aborted.
      */
     public boolean abort() {
-        return false;
+        BooleanSupplier action = abortAction;
+        return action != null && action.getAsBoolean();
+    }
+
+    /**
+     * Sets the action used to abort an active client request. Connector implementations configure
+     * this while a request is in flight and clear it when the exchange ends.
+     *
+     * @param action The abort action, or null when no exchange is active.
+     */
+    public void setAbortAction(BooleanSupplier action) {
+        this.abortAction = action;
     }
 
     /**
@@ -485,7 +501,9 @@ public Method getMethod() {
 
     /**
      * Returns the callback invoked on response reception. If the value is not null, then the
-     * associated request will be executed asynchronously.
+     * associated request will be executed asynchronously. For a streaming response, the callback
+     * can be invoked again if the exchange fails after the initial response has been received; the
+     * response then has a connector-error status and reading its entity fails.
      *
      * @return The callback invoked on response reception.
      */