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Reuse Blocks in ObjCBlock.fromFunction #477
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mkustermann
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Oct 20, 2022
| $exceptionalReturn).cast(), $registerClosure(fn)), lib); | ||
| $exceptionalReturn).cast(), $registerClosure(fn)); | ||
| _funcToBlock[fn] = newBlock; | ||
| return $name._(newBlock, lib); |
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We don't increase refcount for newBlock here.
What happens if the underlying obj-c block is de-rereferenced and freed while still being in the expando map? If one creates another block for same closure, we have a use-after-free, don't we?
I'll comment more on the github issue
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I guess the assumption is that the dart wrapper will keep a refcount as long as it's alive?
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Yeah, the Dart wrapper always holds one reference. In all the other cases where we construct the wrapper function, we're using a block pointer from Block_copy, which already has an incremented ref count.
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When
ObjCBlock.fromFunctionis passed in the same function multiple times, return a newObjCBlockthat wraps the same underlying block struct. Explicitly retain (increase ref count) of the block struct in this case. This helps reduce the leak caused by dart-lang/native#204 by reducing the number of blocks created if the user accidentally creates many blocks from the same function.This is accomplished by sticking the block struct pointer in an
Expando(_funcToBlock), with the Dart function as the key.I also tried sticking the
ObjCBlockdirectly in_funcToBlock, which was simpler and cleaner, but this caused theObjCBlockto never be GC'd, because the Dart function is reachable via the_ObjCBlock_closureRegistry. We can try refactoring this to put theObjCBlockin the_funcToBlockonce dart-lang/native#204 is fixed.Fixes dart-lang/native#226