[cfe-dev] RFC: Only change libclang.so SONAME when the ABI changes

[Tom Stellard via cfe-dev]

Hi,

I would like to propose that we only change the SONAME of libclang.so
(this is the library that contains the C API for clang) when the ABI changes.
Currently, we change the SONAME whenever we bump the major version of LLVM,
but the C API tends to not change that often.

This change will allow operating system maintainers to update the version
of libclang.so in their operating system without forcing rebuilds of
all programs that depend on it.

The steps for implementing this change would be:

- Hard-code the SONAME for libclang.so to libclang.so.13 (which is the current
SONAME).
- Hard-code the symbol versions to LLVM_13 (which is the current symbol version)
   for all existing symbols.
- Add a test case that checks if a new symbol has been added and ensures it has
   the correct symbol version.
- Add a buildbot that uses abi-compliance-checker[1] to ensure that ABI/API does
   not change unexpectedly.
- The next time the ABI of libclang.so is changed, the SOANME will be updated to
   libclang.so.$LLVM_MAJOR_VERSION.

What do you think?

-Tom

[1] https://lvc.github.io/abi-compliance-checker/