Allow to clone calls

okhttp-tests/src/test/java/okhttp3/CallTest.java

@@ -619,6 +619,42 @@ public final class CallTest {
     assertEquals("SyncApiTest", server.takeRequest().getHeader("User-Agent"));
   }
 
+  @Test public void legalToExecuteTwiceCloning() throws Exception {
+    server.enqueue(new MockResponse().setBody("abc"));
+    server.enqueue(new MockResponse().setBody("def"));
+
+    Request request = new Request.Builder()
+        .url(server.url("/"))
+        .build();
+
+    Call call = client.newCall(request);
+    Response response1 = call.execute();
+
+    Call cloned = call.clone();
+    Response response2 = cloned.execute();
+
+    assertEquals(response1.body().string(), "abc");
+    assertEquals(response2.body().string(), "def");
+  }
+
+  @Test public void legalToExecuteTwiceCloning_Async() throws Exception {
+    server.enqueue(new MockResponse().setBody("abc"));
+    server.enqueue(new MockResponse().setBody("def"));
+
+    Request request = new Request.Builder()
+        .url(server.url("/"))
+        .build();
+
+    Call call = client.newCall(request);
+    call.enqueue(callback);
+
+    Call cloned = call.clone();
+    cloned.enqueue(callback);
+
+    callback.await(request.url()).assertBody("abc");
+    callback.await(request.url()).assertBody("def");
+  }
+
   @Test public void get_Async() throws Exception {
     server.enqueue(new MockResponse()
         .setBody("abc")

okhttp/src/main/java/okhttp3/Call.java

@@ -21,7 +21,7 @@ import java.io.IOException;
  * A call is a request that has been prepared for execution. A call can be canceled. As this object
  * represents a single request/response pair (stream), it cannot be executed twice.
  */
-public interface Call {
+public interface Call extends Cloneable {
   /** Returns the original request that initiated this call. */
   Request request();
 
@@ -67,6 +67,12 @@ public interface Call {
 
   boolean isCanceled();
 
+  /**
+   * Create a new, identical call to this one which can be enqueued or executed even if this call
+   * has already been.
+   */
+  Call clone();
+
   interface Factory {
     Call newCall(Request request);
   }

okhttp/src/main/java/okhttp3/RealCall.java

@@ -90,6 +90,11 @@ final class RealCall implements Call {
     return retryAndFollowUpInterceptor.isCanceled();
   }
 
+  @SuppressWarnings("CloneDoesntCallSuperClone") // We are a final type & this saves clearing state.
+  @Override public RealCall clone() {
+    return new RealCall(client, originalRequest);
+  }
+
   StreamAllocation streamAllocation() {
     return retryAndFollowUpInterceptor.streamAllocation();
   }