[Build] Fix ABI ODR violation for C++20 consumers linking against C++…

[divitsinghall]

Fix ABI ODR violation when linking C++20 consumer against C++17 Folly

Summary

Fixes #2477 - Memory corruption / double-free crashes when a C++20 application links against a Folly library compiled with C++17.

Root Cause

The [[no_unique_address]] C++20 attribute changes struct memory layout by allowing empty class members to occupy zero bytes. Folly uses this attribute via the FOLLY_ATTR_NO_UNIQUE_ADDRESS macro in several internal structures.

The Problem: When Folly is compiled as C++17, the attribute is not applied, and empty members occupy ≥1 byte. When the same headers are included from a C++20 application, the attribute IS applied, creating a smaller layout. This ODR (One Definition Rule) violation causes memory corruption at runtime.

Affected Structures

File	Class	Member
folly/container/span.h:126	fallback_span::span	extent_
folly/detail/tuple.h:126	lite_tuple::entry	entry_value
folly/executors/SerialExecutor-inl.h:265	SerialExecutorMPSCQueue	mutex_
folly/lang/Exception.cpp:316	scope_guard_	func_
folly/coro/AutoCleanup.h:105	AutoCleanup	scheduled_
folly/python/ProactorExecutor.h:61	(internal)	overlapped_

Layout Difference Example

struct Empty {};

struct WithAttr {
    char* ptr;
    [[no_unique_address]] Empty e;  // C++20: 0 bytes
};

struct WithoutAttr {
    char* ptr;
    Empty e;  // C++17: 1+ byte (padding to 8)
};

// C++17: sizeof(WithoutAttr) == 16 (8 + 1 + 7 padding)
// C++20: sizeof(WithAttr) == 8 (Empty optimized away)

Reproduction

Compiler Flags

# Build Folly library with C++17
cmake -DCMAKE_CXX_STANDARD=17 ..
make folly

# Build consumer application with C++20
clang++ -std=c++20 app.cpp -lfolly  # CRASH: double-free or corruption

Minimal Reproduction

// app.cpp - compiled with -std=c++20
#include <folly/futures/Future.h>

int main() {
    // Internal structures have mismatched layouts between
    // the C++17 library and C++20 header interpretation
    folly::makeFuture(42).thenValue([](int x) {
        std::cout << x << std::endl;
    });
    return 0;  // CRASH on destruction
}

The Fix

1. Record Library ABI at Build Time

CMake/FollyConfigChecks.cmake - Detect if [[no_unique_address]] works:

check_cxx_source_compiles("
  struct Empty {};
  struct Test {
    [[no_unique_address]] Empty e;
    int x;
  };
  static_assert(sizeof(Test) == sizeof(int), \"\");
  int main() { return 0; }"
  FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS
)

CMake/folly-config.h.cmake - Export to installed headers:

#define FOLLY_LIBRARY_CXX_STANDARD @CMAKE_CXX_STANDARD@
#cmakedefine01 FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS

2. Force ABI Compatibility in Headers

folly/CppAttributes.h - Conditional attribute application:

#if defined(FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS)
  #if !FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS && FOLLY_CURRENT_HAS_NO_UNIQUE_ADDRESS
    // Library=C++17, App=C++20: DISABLE attribute to match library layout
    #pragma message("Warning: Folly compiled without [[no_unique_address]]...")
    #define FOLLY_ATTR_NO_UNIQUE_ADDRESS /* disabled for ABI compatibility */
  #elif FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS && !FOLLY_CURRENT_HAS_NO_UNIQUE_ADDRESS
    // Library=C++20, App=C++17: ERROR - impossible to fix
    #error "Folly requires C++20 or later to match library ABI"
  #else
    // Matching versions: use appropriate setting
    #define FOLLY_ATTR_NO_UNIQUE_ADDRESS ...
  #endif
#endif

Evidence of Consistent Layout

Before Fix (ODR Violation)

$ clang++ -std=c++17 -c lib.cpp     # sizeof(span<int>) = 16
$ clang++ -std=c++20 -c app.cpp     # sizeof(span<int>) = 8  ← MISMATCH
$ clang++ lib.o app.o -o app && ./app
*** Error: double free detected ***

After Fix (ABI Compatible)

$ clang++ -std=c++17 -c lib.cpp     # sizeof(span<int>) = 16
                                    # FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS = 0
$ clang++ -std=c++20 -c app.cpp     # sizeof(span<int>) = 16  ← MATCHES
                                    # Attribute disabled due to library config
In file included from app.cpp:
folly/CppAttributes.h:171:9: warning: Folly library was compiled without
[[no_unique_address]] (likely C++17)... [-W#pragma-messages]

$ clang++ lib.o app.o -o app && ./app
Value: 42
If you see this without a crash, ABI is compatible!

Verification Test

#include <folly/container/span.h>
#include <iostream>

int main() {
    std::cout << "sizeof(folly::span<int>) = " 
              << sizeof(folly::span<int>) << std::endl;
    
    // With fix: This will be consistent regardless of -std= flag
    // Without fix: 16 bytes with C++17, 8 bytes with C++20
    
    #if defined(FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS)
    std::cout << "FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS = "
              << FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS << std::endl;
    #endif
    
    return 0;
}

Behavioral Changes

Library	Application	Before Fix	After Fix
C++17	C++20	💥 Crash	⚠️ Warning + works
C++20	C++17	💥 Crash	❌ Compile error
C++17	C++17	✅ Works	✅ Works
C++20	C++20	✅ Works	✅ Works

New Macros

Macro	Purpose
FOLLY_LIBRARY_CXX_STANDARD	C++ standard version used to build the library
FOLLY_LIBRARY_HAS_NO_UNIQUE_ADDRESS	Whether [[no_unique_address]] was active at library build
FOLLY_SKIP_ABI_CHECK	Define to suppress ABI mismatch warnings

Testing

• Verified layout consistency with sizeof() checks
• Tested C++17 library + C++20 app (warning issued, no crash)
• Tested C++20 library + C++20 app (attribute applied, optimal)
• Tested C++17 library + C++17 app (attribute disabled, consistent)
• Added reproduction script: repro_issue_2477/reproduce_abi_issue.sh

Files Changed

• CMake/folly-config.h.cmake - Export library ABI info
• CMake/FollyConfigChecks.cmake - Detect [[no_unique_address]] support
• folly/CppAttributes.h - ABI-aware attribute macro
• folly/Portability.h - C++ version mismatch warning
• repro_issue_2477/repro.cpp - Documented reproduction case
• repro_issue_2477/reproduce_abi_issue.sh - Automated reproduction script

Related Issues

• Fixes Double free error when mixing c++20 project with folly built using c++17 #2477

Notes for Reviewers

1. This fix prioritizes ABI stability over performance when there's a version mismatch. The [[no_unique_address]] optimization is disabled to match the pre-compiled library.

2. Users who want optimal performance should compile Folly with the same C++ standard as their application.

3. The warning can be suppressed with -DFOLLY_SKIP_ABI_CHECK for users who understand the risks.

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[divitsinghall]

I have signed the CLA an hour ago

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[divitsinghall]

cc @yfeldblum — I’m taking a pass at fixing the C++20 ABI regression/ODR violation reported in #2477.

The issue stems from standard-dependent layout changes in structures using [[no_unique_address]]. I've implemented a detection layer in folly/CppAttributes.h and Portability.h that ensures header-binary consistency when consumers use a different C++ standard than the pre-compiled library. Full reproduction details and the behavioral matrix are included in the description