1.x: Use the correct Throwable to set the cause for CompositeException

The cause of #3679 is we use a wrong Throwable (its cause has been set) to set the cause and `initCause` will throw an exception. Hence, the cause chain is not created correctly. In this PR, it searches the root cause (which doesn't have a cause) and use it to call `initCause`.

Author: zsxwing

src/main/java/rx/exceptions/CompositeException.java

@@ -147,13 +147,12 @@ public synchronized Throwable getCause() {
                 // we now have 'e' as the last in the chain
                 try {
                     chain.initCause(e);
-                    chain = chain.getCause();
                 } catch (Throwable t) {
                     // ignore
                     // the javadocs say that some Throwables (depending on how they're made) will never
                     // let me call initCause without blowing up even if it returns null
-                    chain = e;
                 }
+                chain = getRootCause(chain);
             }
             cause = _cause;
         }
@@ -293,4 +292,25 @@ private List<Throwable> getListOfCauses(Throwable ex) {
             }
         }
     }
+
+    /**
+     * Returns the root cause of {@code e}. If {@code e.getCause()} returns {@null}, just return {@code e} itself.
+     *
+     * @param e the {@link Throwable} {@code e}.
+     * @return The root cause of {@code e}. If {@code e.getCause()} returns {@null}, just return {@code e} itself.
+     */
+    private Throwable getRootCause(Throwable e) {
+        Throwable root = e.getCause();
+        if (root == null) {
+            return e;
+        } else {
+            while(true) {
+                if (root.getCause() == null) {
+                    return root;
+                } else {
+                    root = root.getCause();
+                }
+            }
+        }
+    }
 }

src/test/java/rx/exceptions/CompositeExceptionTest.java

@@ -28,6 +28,8 @@
 
 import org.junit.Test;
 
+import rx.exceptions.CompositeException.CompositeExceptionCausalChain;
+
 public class CompositeExceptionTest {
 
     private final Throwable ex1 = new Throwable("Ex1");
@@ -239,4 +241,39 @@ public void messageVarargs() {
         CompositeException compositeException = new CompositeException(ex1, ex2, ex3);
         assertEquals("3 exceptions occurred. ", compositeException.getMessage());
     }
+
+    @Test
+    public void complexCauses() {
+        Throwable e1 = new Throwable("1");
+        Throwable e2 = new Throwable("2");
+        e1.initCause(e2);
+
+        Throwable e3 = new Throwable("3");
+        Throwable e4 = new Throwable("4");
+        e3.initCause(e4);
+
+        Throwable e5 = new Throwable("5");
+        Throwable e6 = new Throwable("6");
+        e5.initCause(e6);
+
+        CompositeException compositeException = new CompositeException(e1, e3, e5);
+        assert(compositeException.getCause() instanceof CompositeExceptionCausalChain);
+
+        List<Throwable> causeChain = new ArrayList<Throwable>();
+        Throwable cause = compositeException.getCause().getCause();
+        while (cause != null) {
+            causeChain.add(cause);
+            cause = cause.getCause();
+        }
+        // The original relations
+        //
+        // e1 -> e2
+        // e3 -> e4
+        // e5 -> e6
+        //
+        // will be set to
+        //
+        // e1 -> e2 -> e3 -> e4 -> e5 -> e6
+        assertEquals(Arrays.asList(e1, e2, e3, e4, e5, e6), causeChain);
+    }
 }