Fix: Add _DISABLE_CONSTEXPR_MUTEX_CONSTRUCTOR to prevent crashes

[EllieTellie]

Description

Fixes crash when running on systems with older Visual C++ runtimes.
MSVC broke ABI compatibility with the release of MSVC 17.10
We can define _DISABLE_CONSTEXPR_MUTEX_CONSTRUCTOR to CMakeLists.txt to fix this and make Firebase compatible with older runtimes on Windows.

Testing

Tested C#/Unity application with FirebaseCppApp-12_10_0.dll built from source with this additional fix.
Copied msvcp140.dll, vcruntime140.dll and vcruntime140_1.dll from a older runtime (2019 v16.11) to the root folder (where the .exe lives).
Verified it crashes the application without the fix.
Verified it does not crash the application with the fix.
If required I could provide a repro project if desired, but it will have to contain these msvc dlls.

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Fixes: firebase/firebase-unity-sdk#1291

References

• Microsoft STL Issue: VCRuntime incompatibility with older version in mutex code microsoft/STL#4730
• Firebase Unity Issue: [Bug] Unity App Crash on launch with Firebase 12.10.0 firebase-unity-sdk#1291

Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[gemini-code-assist]

Summary of Changes

Hello @EllieTellie, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request provides a crucial fix for a crash experienced by Firebase applications on Windows systems running older Visual C++ runtimes. By introducing a specific compiler definition in the CMake build configuration, it effectively mitigates an ABI incompatibility issue that arose with a recent MSVC update, thereby enhancing the robustness and backward compatibility of Firebase.

Highlights

• Crash Fix: Addresses a crash occurring on systems utilizing older Visual C++ runtimes.
• ABI Compatibility: Resolves an ABI incompatibility issue introduced by MSVC 17.10.
• Compiler Definition: Implements the fix by adding the _DISABLE_CONSTEXPR_MUTEX_CONSTRUCTOR definition to CMakeLists.txt.
• Broader Compatibility: Ensures Firebase applications maintain compatibility with older Windows runtimes.

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[gemini-code-assist]

Code Review

This pull request addresses a critical crash on Windows systems with older Visual C++ runtimes by defining _DISABLE_CONSTEXPR_MUTEX_CONSTRUCTOR. The change is correct and well-placed within the MSVC-specific configuration in CMakeLists.txt. I have one suggestion to improve the maintainability of this workaround by adding a reference to the upstream issue in the code comment.