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

[Fāng-ruì Sòng via cfe-dev]

On Mon, Jun 28, 2021 at 4:02 PM Tom Stellard via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> On 6/26/21 3:28 AM, Dimitry Andric wrote:
> > On 26 Jun 2021, at 06:04, Tom Stellard via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> >>
> >> 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?
> >
> > This sounds like a good plan in general (not only for libclang :), but
> > how strict is this ABI check? Does *any* change (even like adding a new
> > function) trigger an ABI check error?
> >
>
> New functions don't trigger an ABI check error.  The things I've seen
> it flag in the past are: function signature changes, struct size changes,
> and enum value changes.
>
> -Tom
>
> > E.g., what are the criteria for bumping the version?
> >
> > -Dimitry

I find the following two paragraphs in
https://www.debian.org/doc/debian-policy/ch-sharedlibs.html#run-time-shared-libraries
useful

> Every time the shared library ABI changes in a way that may break binaries linked against older versions of the shared library, the SONAME of the library and the corresponding name for the binary package containing the runtime shared library should change.

> The SONAME and binary package name need not, and indeed normally should not, change if new interfaces are added but none are removed or changed, since this will not break binaries linked against the old shared library. ...