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Merged
merged 3 commits into from
Aug 26, 2020
Merged

Autoremove EventHandler from parent entity #16

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bb403a3
Autoremove EventHandler from parent
ivanpauno Aug 20, 2020
a0335cc
Remove duplicated import
ivanpauno Aug 26, 2020
f1c70c3
Rename removeCallback as disposeCallback
ivanpauno Aug 26, 2020
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14 changes: 10 additions & 4 deletions rcljava/src/main/java/org/ros2/rcljava/events/EventHandlerImpl.java
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Original file line number Diff line number Diff line change
Expand Up @@ -64,16 +64,19 @@ public class EventHandlerImpl<
* @param eventStatusFactory Factory of an event status.
* @param callback The callback function that will be called when the event
* is triggered.
* @param removeCallback Callback that will be called when this object is being disposed.
*/
public EventHandlerImpl(
final WeakReference<ParentT> parentReference,
final long handle,
final Supplier<T> eventStatusFactory,
final Consumer<T> callback) {
final Consumer<T> callback,
final Consumer<EventHandler> removeCallback) {
this.parentReference = parentReference;
this.handle = handle;
this.eventStatusFactory = eventStatusFactory;
this.callback = callback;
this.removeCallback = removeCallback;
}

/**
Expand Down Expand Up @@ -102,9 +105,11 @@ public final long getHandle() {
* {@inheritDoc}
*/
public synchronized final void dispose() {
if (this.handle != 0) {
nativeDispose(this.handle);
if (this.handle == 0) {
return;
}
this.removeCallback.accept(this);
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Instead of having the parent register a callback, what about instead we call the parent's removeEventHandler() method? This is a pattern I choose when implementing "remove" methods for Node entities like publishers and subscriptions in ros2-java#110; the node removes entities from it's list (and does not dispose them), but if dispose() is called on the entity it removes itself from it's parent.

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I didn't follow that approach because of the following problem:

Publisher pub = node.createPublisher(...);
node.dispose();
pub.dispose();  // Use after free here, likely a segfault

My idea is that the above code should work perfectly, pub.dispose() should only be a noop in that case.

In the following example:

Publisher pub = node.createPublisher(...);
node.dispose();
pub.publish();  // I would like an InvalidHandleException thrown here

I would like to see what the inline comment says 😂.

I think that the current implementation of event handlers is solving both problems correctly.
It's a bit annoying to pass the remove callback, but that's an implementation detail that the user doesn't have to know about (and without the callback, there's an infinite recursion problem)

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Hmm.. it looks like both your examples are handled in the approach I proposed.

If I try:

node.dispose()
pub.dispose()

The program exits cleanly (I believe the pub.dispose() is a noop).

If I try:

node.dispose()
pub.publish()

Then I get the following exception:

Exception in thread "main" org.ros2.rcljava.exceptions.RCLException: Failed to publish: publisher pointer is invalid

I think the exception message is fairly informative, but of course we could change the type like you suggest.

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We can go forward with the callback approach if you like; I just thought that avoiding the callback registration seemed a bit cleaner.

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I have to double check how the approach works, I was almost sure there was an use after free.

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hmm, I just took another look at the code. I thought we were handling it in Java, but it looks like I may have just got lucky . I'm guessing rcl or rmw is failing gracefully on the publish call. I'm not sure what's going on with the dispose call 🙃. We can leave this PR as-is and I'll have to revisit my approach in ros2-java#110 (with some more unit tests).

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node.dispose()
pub.dispose()

There isn't actually an use after free, but the publisher is leaked

ros2_java/rcljava/src/main/cpp/org_ros2_rcljava_publisher_PublisherImpl.cpp

Lines 74 to 77 in 6291847

if (node_handle == 0) {
// TODO(esteve): handle this, node is null, but publisher isn't
return;
}
.

About:

node.dispose()
pub.publish()

I think that one is UB, as the node has been destroyed and not the publisher (it likely depends on the rmw implementation).

We can leave this PR as-is and I'll have to revisit my approach in ros2-java#110 (with some more unit tests).

👍

nativeDispose(this.handle);
this.handle = 0;
}

Expand All @@ -126,11 +131,12 @@ public synchronized final void executeCallback() {
nativeTake(this.handle, nativeEventStatusHandle);
eventStatus.fromRCLEvent(nativeEventStatusHandle);
eventStatus.deallocateRCLStatusEvent(nativeEventStatusHandle);
callback.accept(eventStatus);
this.callback.accept(eventStatus);
}

private final Supplier<T> eventStatusFactory;
private final WeakReference<ParentT> parentReference;
private long handle;
private final Consumer<T> callback;
private final Consumer<EventHandler> removeCallback;
}
12 changes: 11 additions & 1 deletion rcljava/src/main/java/org/ros2/rcljava/publisher/PublisherImpl.java
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Original file line number Diff line number Diff line change
Expand Up @@ -117,10 +117,20 @@ public final WeakReference<Node> getNodeReference() {
public final
<T extends PublisherEventStatus> EventHandler<T, Publisher>
createEventHandler(Supplier<T> factory, Consumer<T> callback) {
final WeakReference<Collection<EventHandler>> weakEventHandlers = new WeakReference(
this.eventHandlers);
Consumer<EventHandler> removeCallback = new Consumer<EventHandler>() {
public void accept(EventHandler eventHandler) {
Collection<EventHandler> eventHandlers = weakEventHandlers.get();
if (eventHandlers != null) {
eventHandlers.remove(eventHandler);
}
}
};
T status = factory.get();
long eventHandle = nativeCreateEvent(this.handle, status.getPublisherEventType());
EventHandler<T, Publisher> eventHandler = new EventHandlerImpl(
new WeakReference<Publisher>(this), eventHandle, factory, callback);
new WeakReference<Publisher>(this), eventHandle, factory, callback, removeCallback);
this.eventHandlers.add(eventHandler);
return eventHandler;
}
Expand Down
13 changes: 12 additions & 1 deletion rcljava/src/main/java/org/ros2/rcljava/subscription/SubscriptionImpl.java
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,7 @@

import org.ros2.rcljava.RCLJava;
import org.ros2.rcljava.common.JNIUtils;
import org.ros2.rcljava.consumers.BiConsumer;
import org.ros2.rcljava.consumers.Consumer;
import org.ros2.rcljava.events.EventHandler;
import org.ros2.rcljava.events.EventHandlerImpl;
Expand Down Expand Up @@ -128,10 +129,20 @@ public final WeakReference<Node> getNodeReference() {
public final
<T extends SubscriptionEventStatus> EventHandler<T, Subscription>
createEventHandler(Supplier<T> factory, Consumer<T> callback) {
final WeakReference<Collection<EventHandler>> weakEventHandlers = new WeakReference(
this.eventHandlers);
Consumer<EventHandler> removeCallback = new Consumer<EventHandler>() {
public void accept(EventHandler eventHandler) {
Collection<EventHandler> eventHandlers = weakEventHandlers.get();
if (eventHandlers != null) {
eventHandlers.remove(eventHandler);
}
}
};
T status = factory.get();
long eventHandle = nativeCreateEvent(this.handle, status.getSubscriptionEventType());
EventHandler<T, Subscription> eventHandler = new EventHandlerImpl(
new WeakReference<Subscription>(this), eventHandle, factory, callback);
new WeakReference<Subscription>(this), eventHandle, factory, callback, removeCallback);
this.eventHandlers.add(eventHandler);
return eventHandler;
}
Expand Down
5 changes: 4 additions & 1 deletion rcljava/src/test/java/org/ros2/rcljava/publisher/PublisherTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -90,9 +90,12 @@ public void accept(final OfferedQosIncompatible status) {
}
);
assertNotEquals(0, eventHandler.getHandle());
assertEquals(1, publisher.getEventHandlers().size());
// force executing the callback, so we check that taking an event works
eventHandler.executeCallback();
RCLJava.shutdown();
eventHandler.dispose();
assertEquals(0, eventHandler.getHandle());
assertEquals(0, publisher.getEventHandlers().size());
RCLJava.shutdown();
}
}