The documentation for unsafe_arena_release_foo() says:
> This method should only be used when the parent message is on the arena
and the submessage is on the same arena, or an arena with equivalent
lifetime.
The DCHECK() you highlighted is checking the first part of this requirement.
Stack/heap-allocated messages must always uniquely own their sub-messages,
which must always be on the heap. Protobuf doesn't support the use case you
are describing, where a message temporarily points to a message it doesn't
own. If we allowed that, protobuf itself would crash if a message in this
state was ever destroyed.
If you are trying to avoid heap allocation overhead, the recommended
solution is arenas.
On Tuesday, May 5, 2020 at 12:44:32 PM UTC-7, King Kong wrote:
>
> I am using unsafe_arena_set_allocated_* and unsafe_arena_release_* APIs to
> avoid
> unnecessary allocations when the sub-objects are only used in a function
> scope,
> unsafe_arena_release_* is always called before leaving the scope. At the
> beginning,
>
> I chose to always use the arena versions of unsafe APIs because the
> non-arena version
>
> unsafe_release does extra allocation when the object is allocated on
> arena, and the arena
>
> version does not check if the object is allocated in arena, based on
> generated code like
>
> the following:
>
>
> inline foo* outter1::unsafe_arena_release_foo() {
> foo* temp = foo_;
> foo_ = nullptr;
> return temp;
> }
>
>
> However, I later also saw the following the generated code with arena
> check, it means it is not okay to call for heap-allocated objects.
>
> inline void outter2::unsafe_arena_set_allocated_bar(
> std::string* bar) {
> GOOGLE_DCHECK(GetArenaNoVirtual() != nullptr);
> ... ...
> }
>
>
> So, why the discrepancy?
>
>
> The official documentation at
> https://developers.google.com/protocol-buffers/docs/reference/arenas says
> the correct set of APIs should be used
>
> according to how to the object is allocated, but having two different sets
> of unsafe APIs makes it harder to use, and it makes the code less robust to
> future changes. I am wondering if protobuf owners also realize such an
> issue and start to remove arena checking gradually? It that the case? I
> would like to hear how others think.
>
>
> Thanks.
>
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