Objective-C API: Need a good way to preserve custom properties on JS wrappers
https://bugs.webkit.org/show_bug.cgi?id=112608
Reviewed by Geoffrey Garen.
Currently, we just use a weak map, which means that garbage collection can cause a wrapper to
disappear if it isn't directly exported to JavaScript.
The most straightforward and safe way (with respect to garbage collection and concurrency) is to have
clients add and remove their external references along with their owners. Effectively, the client is
recording the structure of the external object graph so that the garbage collector can make sure to
mark any wrappers that are reachable through either the JS object graph of the external Obj-C object
graph. By keeping these wrappers alive, this has the effect that custom properties on these wrappers
will also remain alive.
The rule for if an object needs to be tracked by the runtime (and therefore whether the client should report it) is as follows:
For a particular object, its references to its children should be added if:
- The child is referenced from JavaScript.
- The child contains references to other objects for which (1) or (2) are true.
- API/JSAPIWrapperObject.mm:
(JSAPIWrapperObjectHandleOwner::finalize):
(JSAPIWrapperObjectHandleOwner::isReachableFromOpaqueRoots): A wrapper object is kept alive only if its JSGlobalObject
is marked and its corresponding Objective-C object was added to the set of opaque roots.
(JSC::JSAPIWrapperObject::visitChildren): We now call out to scanExternalObjectGraph, which handles adding all Objective-C
objects to the set of opaque roots.
- API/JSAPIWrapperObject.h:
(JSAPIWrapperObject):
- API/JSContext.mm: Moved dealloc to its proper place in the main implementation.
(-[JSContext dealloc]):
- API/JSVirtualMachine.h:
- API/JSVirtualMachine.mm:
(-[JSVirtualMachine initWithContextGroupRef:]):
(-[JSVirtualMachine dealloc]):
(getInternalObjcObject): Helper funciton to get the Objective-C object out of JSManagedValues or JSValues if there is one.
(-[JSVirtualMachine addManagedReference:withOwner:]): Adds the Objective-C object to the set of objects
owned by the owner object in that particular virtual machine.
(-[JSVirtualMachine removeManagedReference:withOwner:]): Removes the relationship between the two objects.
(-[JSVirtualMachine externalObjectGraph]):
(scanExternalObjectGraph): Does a depth-first search of the external object graph in a particular virtual machine starting at
the specified root. Each new object it encounters it adds to the set of opaque roots. These opaque roots will keep their
corresponding wrapper objects alive if they have them.
- API/JSManagedReferenceInternal.h: Added.
- API/JSVirtualMachine.mm: Added the per-JSVirtualMachine map between objects and the objects they own, which is more formally
known as that virtual machine's external object graph.
(-[JSWrapperMap dealloc]): We were leaking this before :-(
(-[JSVirtualMachine initWithContextGroupRef:]):
(-[JSVirtualMachine dealloc]):
(-[JSVirtualMachine externalObjectGraph]):
- API/JSVirtualMachineInternal.h:
- API/tests/testapi.mm: Added two new tests using the TinyDOMNode class. The first tests that a custom property added to a wrapper
doesn't vanish after GC, even though that wrapper isn't directly accessible to the JS garbage collector but is accessible through
the external Objective-C object graph. The second test makes sure that adding an object to the external object graph with the same
owner doesn't cause any sort of problems.
(+[TinyDOMNode sharedVirtualMachine]):
(-[TinyDOMNode init]):
(-[TinyDOMNode dealloc]):
(-[TinyDOMNode appendChild:]):
(-[TinyDOMNode numberOfChildren]):
(-[TinyDOMNode childAtIndex:]):
(-[TinyDOMNode removeChildAtIndex:]):
(SlotVisitor):
- heap/SlotVisitorInlines.h:
(JSC::SlotVisitor::containsOpaqueRootTriState): Added a new method to SlotVisitor to allow scanExternalObjectGraph to have a
thread-safe view of opaque roots during parallel marking. The set of opaque roots available to any one SlotVisitor isn't guaranteed
to be 100% correct, but that just results in a small duplication of work in scanExternalObjectGraph. To indicate this change for
false negatives we return a TriState that's either true or mixed, but never false.
