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Fix #9947: make the ABI identical between debug and non-debug builds #10271

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Jul 19, 2022
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Fix #9947: make the ABI identical between debug and non-debug builds #10271

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8 changes: 2 additions & 6 deletions src/google/protobuf/message_lite.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -521,18 +521,14 @@ void GenericTypeHandler<std::string>::Merge(const std::string& from,
*to = from;
}

// Non-inline implementations of InternalMetadata routines
#if defined(NDEBUG) || defined(_MSC_VER)
// for opt and MSVC builds, the destructor is defined in the header.
#else
// Non-inline implementations of InternalMetadata destructor
// This is moved out of the header because the GOOGLE_DCHECK produces a lot of code.
InternalMetadata::~InternalMetadata() {
void InternalMetadata::CheckedDestruct() {
if (HasMessageOwnedArenaTag()) {
GOOGLE_DCHECK(!HasUnknownFieldsTag());
delete reinterpret_cast<Arena*>(ptr_ - kMessageOwnedArenaTagMask);
}
}
#endif

// Non-inline variants of std::string specializations for
// various InternalMetadata routines.
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13 changes: 10 additions & 3 deletions src/google/protobuf/metadata_lite.h
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Expand Up @@ -77,15 +77,19 @@ class PROTOBUF_EXPORT InternalMetadata {
GOOGLE_DCHECK(!is_message_owned || arena != nullptr);
}

#if defined(NDEBUG) || defined(_MSC_VER)
// To keep the ABI identical between debug and non-debug builds,
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You probably know this, but just in case. This preserves the ABI, but would make it an ODR violation to link code with NDEBUG and without NDEBUG. The ODR rules require that all definitions of an inline function be identical:

Each such definition shall consist of the same sequence of tokens, where the definition of a closure type
is considered to consist of the sequence of tokens of the corresponding lambda-expression.

I am not sure how to quote the standard, but this draft:

https://isocpp.org/files/papers/N4860.pdf

"6.3 One-definition rule", paragraph 13.8

I am well aware that such violations of the ODR are mostly harmless.

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@pitrou pitrou Jul 19, 2022 •
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Yes, it was already stated on the linked issue as a reason to not bother with the ABI problem.
However, as you also noted, all Unix compilers pretty much have to deal nicely with this as the practice of building in debug mode against non-debug dependencies is extremely widespread. So the compiler/linker will end up selecting one implementation over another; which one exactly doesn't really matter in practice (the only difference being the added debug checks).

Windows is different and there you pretty much have to match the build mode accross all dependencies for other reasons.

// the destructor is always defined here even though it may delegate
// to a non-inline private method.
// (see https://github.com/protocolbuffers/protobuf/issues/9947)
~InternalMetadata() {
#if defined(NDEBUG) || defined(_MSC_VER)
if (HasMessageOwnedArenaTag()) {
delete reinterpret_cast<Arena*>(ptr_ - kMessageOwnedArenaTagMask);
}
}
#else
~InternalMetadata();
CheckedDestruct();
#endif
}

template <typename T>
void Delete() {
Expand Down Expand Up @@ -264,6 +268,9 @@ class PROTOBUF_EXPORT InternalMetadata {
PROTOBUF_NOINLINE void DoSwap(T* other) {
mutable_unknown_fields<T>()->Swap(other);
}

// Private helper with debug checks for ~InternalMetadata()
void CheckedDestruct();
};

// String Template specializations.
Expand Down