1.x: Change the workers to capture the stack trace

src/main/java/rx/internal/schedulers/CachedThreadScheduler.java

@@ -78,6 +78,7 @@ ThreadWorker get() {
             while (!expiringWorkerQueue.isEmpty()) {
                 ThreadWorker threadWorker = expiringWorkerQueue.poll();
                 if (threadWorker != null) {
+                    threadWorker.resetContext();
                     return threadWorker;
                 }
             }

src/main/java/rx/internal/schedulers/EventLoopsScheduler.java

@@ -142,7 +142,7 @@ private static class EventLoopWorker extends Scheduler.Worker {
 
         EventLoopWorker(PoolWorker poolWorker) {
             this.poolWorker = poolWorker;
-
+            poolWorker.resetContext();
         }
 
         @Override

src/main/java/rx/internal/schedulers/ExecutorScheduler.java

@@ -42,6 +42,7 @@ public Worker createWorker() {
 
     /** Worker that schedules tasks on the executor indirectly through a trampoline mechanism. */
     static final class ExecutorSchedulerWorker extends Scheduler.Worker implements Runnable {
+        private final Throwable creationContext = SchedulerContextException.create();
         final Executor executor;
         // TODO: use a better performing structure for task tracking
         final CompositeSubscription tasks;
@@ -64,7 +65,7 @@ public Subscription schedule(Action0 action) {
             if (isUnsubscribed()) {
                 return Subscriptions.unsubscribed();
             }
-            ScheduledAction ea = new ScheduledAction(action, tasks);
+            ScheduledAction ea = new ScheduledAction(action, tasks, creationContext);
             tasks.add(ea);
             queue.offer(ea);
             if (wip.getAndIncrement() == 0) {
@@ -146,7 +147,7 @@ public void call() {
                         ((ScheduledAction)s2).add(removeMas);
                     }
                 }
-            });
+            }, this.creationContext);
             // This will make sure if ea.call() gets executed before this line
             // we don't override the current task in mas.
             first.set(ea);

src/main/java/rx/internal/schedulers/NewThreadWorker.java

@@ -33,6 +33,7 @@
  * @warn class description missing
  */
 public class NewThreadWorker extends Scheduler.Worker implements Subscription {
+    private Throwable creationContext = SchedulerContextException.create();
     private final ScheduledExecutorService executor;
     private final RxJavaSchedulersHook schedulersHook;
     volatile boolean isUnsubscribed;
@@ -234,7 +235,7 @@ public Subscription schedule(final Action0 action, long delayTime, TimeUnit unit
      */
     public ScheduledAction scheduleActual(final Action0 action, long delayTime, TimeUnit unit) {
         Action0 decoratedAction = schedulersHook.onSchedule(action);
-        ScheduledAction run = new ScheduledAction(decoratedAction);
+        ScheduledAction run = new ScheduledAction(decoratedAction, creationContext);
         Future<?> f;
         if (delayTime <= 0) {
             f = executor.submit(run);
@@ -247,7 +248,7 @@ public ScheduledAction scheduleActual(final Action0 action, long delayTime, Time
     }
     public ScheduledAction scheduleActual(final Action0 action, long delayTime, TimeUnit unit, CompositeSubscription parent) {
         Action0 decoratedAction = schedulersHook.onSchedule(action);
-        ScheduledAction run = new ScheduledAction(decoratedAction, parent);
+        ScheduledAction run = new ScheduledAction(decoratedAction, parent, creationContext);
         parent.add(run);
 
         Future<?> f;
@@ -263,7 +264,7 @@ public ScheduledAction scheduleActual(final Action0 action, long delayTime, Time
 
     public ScheduledAction scheduleActual(final Action0 action, long delayTime, TimeUnit unit, SubscriptionList parent) {
         Action0 decoratedAction = schedulersHook.onSchedule(action);
-        ScheduledAction run = new ScheduledAction(decoratedAction, parent);
+        ScheduledAction run = new ScheduledAction(decoratedAction, parent, creationContext);
         parent.add(run);
 
         Future<?> f;
@@ -288,4 +289,8 @@ public void unsubscribe() {
     public boolean isUnsubscribed() {
         return isUnsubscribed;
     }
+
+    public void resetContext() {
+        creationContext = SchedulerContextException.create();
+    }
 }

src/main/java/rx/internal/schedulers/ScheduledAction.java

@@ -19,6 +19,7 @@
 import java.util.concurrent.atomic.*;
 
 import rx.Subscription;
+import rx.exceptions.Exceptions;
 import rx.exceptions.OnErrorNotImplementedException;
 import rx.functions.Action0;
 import rx.internal.util.SubscriptionList;
@@ -34,18 +35,22 @@ public final class ScheduledAction extends AtomicReference<Thread> implements Ru
     private static final long serialVersionUID = -3962399486978279857L;
     final SubscriptionList cancel;
     final Action0 action;
+    final Throwable creationContext;
 
-    public ScheduledAction(Action0 action) {
+    public ScheduledAction(Action0 action, Throwable creationContext) {
         this.action = action;
         this.cancel = new SubscriptionList();
+        this.creationContext = creationContext;
     }
-    public ScheduledAction(Action0 action, CompositeSubscription parent) {
+    public ScheduledAction(Action0 action, CompositeSubscription parent, Throwable creationContext) {
         this.action = action;
         this.cancel = new SubscriptionList(new Remover(this, parent));
+        this.creationContext = creationContext;
     }
-    public ScheduledAction(Action0 action, SubscriptionList parent) {
+    public ScheduledAction(Action0 action, SubscriptionList parent, Throwable creationContext) {
         this.action = action;
         this.cancel = new SubscriptionList(new Remover2(this, parent));
+        this.creationContext = creationContext;
     }
 
     @Override
@@ -61,6 +66,7 @@ public void run() {
             } else {
                 ie = new IllegalStateException("Fatal Exception thrown on Scheduler.Worker thread.", e);
             }
+            Exceptions.addCause(ie, creationContext);
             RxJavaPlugins.getInstance().getErrorHandler().handleError(ie);
             Thread thread = Thread.currentThread();
             thread.getUncaughtExceptionHandler().uncaughtException(thread, ie);

src/main/java/rx/internal/schedulers/SchedulerContextException.java

@@ -0,0 +1,45 @@
+/**
+ * Copyright 2016 Netflix, Inc.
+ * 
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package rx.internal.schedulers;
+
+/**
+ * Used only for providing context around where work was scheduled should an error occur in a different thread.
+ */
+public class SchedulerContextException extends Exception {
+    /**
+     * Constant to use when disabled
+     */
+    private static final Throwable CONTEXT_MISSING = new SchedulerContextException("Missing context. Enable by setting the system property \"rxjava.captureSchedulerContext=true\"");
+
+    static {
+        CONTEXT_MISSING.setStackTrace(new StackTraceElement[0]);
+    }
+
+    /**
+     * @return a {@link Throwable} that captures the stack trace or a {@link Throwable} that documents how to enable the feature if needed.
+     */
+    public static Throwable create() {
+        String def = "false";
+        String setTo = System.getProperty("rxjava.captureSchedulerContext", def);
+        return setTo != def && "true".equals(setTo) ? new SchedulerContextException("Asynchronous work scheduled at") : CONTEXT_MISSING;
+    }
+
+    private SchedulerContextException(String message) {
+        super(message);
+    }
+
+    private static final long serialVersionUID = 1L;
+}

src/test/java/rx/schedulers/AbstractSchedulerTests.java

@@ -43,6 +43,8 @@
 import rx.functions.Action0;
 import rx.functions.Action1;
 import rx.functions.Func1;
+import rx.plugins.RxJavaErrorHandler;
+import rx.plugins.RxJavaPlugins;
 
 /**
  * Base tests for all schedulers including Immediate/Current.
@@ -502,4 +504,54 @@ public void onNext(T args) {
 
     }
 
+    @Test
+    public final void testStackTraceAcrossThreads() throws Throwable {
+        final AtomicReference<Throwable> exceptionRef = new AtomicReference<Throwable>();
+        final CountDownLatch done = new CountDownLatch(1);
+        System.setProperty("rxjava.captureSchedulerContext", "true");
+
+        try {
+
+            RxJavaPlugins.getInstance().reset();
+            RxJavaPlugins.getInstance().registerErrorHandler(new RxJavaErrorHandler() {
+                @Override
+                public void handleError(Throwable e) {
+                    exceptionRef.set(e);
+                    done.countDown();
+                }
+            });
+
+            try {
+                getScheduler().createWorker().schedule(new Action0() {
+                    @Override
+                    public void call() {
+                        throw new RuntimeException();
+                    }
+                });
+            } catch (Exception e) {
+                exceptionRef.set(e);
+                done.countDown();
+            }
+
+            done.await();
+
+            Throwable exception = exceptionRef.get();
+            Throwable e = exception;
+            while (e.getCause() != null) {
+                e = e.getCause();
+            }
+
+            StackTraceElement[] st = e.getStackTrace();
+            for (StackTraceElement stackTraceElement : st) {
+                if (stackTraceElement.getMethodName().equals("testStackTraceAcrossThreads")) {
+                    // pass we found this class in the stack trace.
+                    return;
+                }
+            }
+
+            throw exception;
+        } finally {
+            System.setProperty("rxjava.captureSchedulerContext", "false");
+        }
+    }
 }