Added Cause.onLoad in case a Cause implementation needs to keep a build field.

core/src/main/java/hudson/model/Cause.java

@@ -76,6 +76,14 @@ public abstract class Cause {
      */
     public void onAddedTo(AbstractBuild build) {}
 
+    /**
+     * Called when a build is loaded from disk.
+     * Useful in case the cause needs to keep a build reference;
+     * this ought to be {@code transient}.
+     * @since 1.540
+     */
+    public void onLoad(@Nonnull AbstractBuild<?,?> build) {}
+
     /**
      * Report a line to the listener about this cause.
      * @since 1.362

core/src/main/java/hudson/model/CauseAction.java

@@ -116,8 +116,13 @@ public class CauseAction implements FoldableAction, RunAction2 {
         return causes.get(0).getShortDescription();
     }
 
-    @Override public void onLoad(Run<?,?> r) {
-        // noop
+    @Override public void onLoad(Run<?,?> owner) {
+        if (owner instanceof AbstractBuild) { // cf. onAttached
+            AbstractBuild<?,?> b = (AbstractBuild) owner;
+            for (Cause c : causes) {
+                c.onLoad(b);
+            }
+        }
     }
 
     /**