patchrefs: forbid clang to complain about unreachable code

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>
> When `NO_SYMLINK_HEAD` is defined, `create_ref_symlink()` is hard-coded
> as `(-1)`, and as a consequence the condition `!create_ref_symlink()`
> always evaluates to false, rendering any code guarded by that condition
> unreachable.
>
> Therefore, clang is _technically_ correct when it complains about
> unreachable code. It does completely miss the fact that this is okay
> because on _other_ platforms, where `NO_SYMLINK_HEAD` is not defined,
> the code isn't unreachable at all.
>
> Let's use the same trick as in 82e79c63642c (git-compat-util: add
> NOT_CONSTANT macro and use it in atfork_prepare(), 2025-03-17) to
> appease clang while at the same time keeping the `-Wunreachable` flag
> to potentially find _actually_ unreachable code.
>
> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
>     refs: forbid clang to complain about unreachable code
>     
>     Just upstreamin'

>
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1984%2Fdscho%2Frefs-clang-fix-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1984/dscho/refs-clang-fix-v1
> Pull-Request: https://github.com/gitgitgadget/git/pull/1984
>
>  refs/files-backend.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 088b52c740..814decf323 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -3186,7 +3186,13 @@ static int files_transaction_finish(struct ref_store *ref_store,
>  		 * next update. If not, we try and create a regular symref.
>  		 */
>  		if (update->new_target && refs->prefer_symlink_refs)
> -			if (!create_ref_symlink(lock, update->new_target))
> +			/*
> +			 * By using the `NOT_CONSTANT()` trick, we can avoid
> +			 * errors by `clang`'s `-Wunreachable` logic that would
> +			 * report that the `continue` statement is not reachable
> +			 * when `NO_SYMLINK_HEAD` is `#define`d.
> +			 */
> +			if (NOT_CONSTANT(!create_ref_symlink(lock, update->new_target)))
>  				continue;
>  
>  		if (update->flags & REF_NEEDS_COMMIT) {
>
> base-commit: c44beea485f0f2feaf460e2ac87fdd5608d63cf0

> "Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
> writes:
> 
> > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> >
> > When `NO_SYMLINK_HEAD` is defined, `create_ref_symlink()` is hard-coded
> > as `(-1)`, and as a consequence the condition `!create_ref_symlink()`
> > always evaluates to false, rendering any code guarded by that condition
> > unreachable.
> >
> > Therefore, clang is _technically_ correct when it complains about
> > unreachable code. It does completely miss the fact that this is okay
> > because on _other_ platforms, where `NO_SYMLINK_HEAD` is not defined,
> > the code isn't unreachable at all.
> >
> > Let's use the same trick as in 82e79c63642c (git-compat-util: add
> > NOT_CONSTANT macro and use it in atfork_prepare(), 2025-03-17) to
> > appease clang while at the same time keeping the `-Wunreachable` flag
> > to potentially find _actually_ unreachable code.
> >
> > Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> > ---
> >     refs: forbid clang to complain about unreachable code
> >     
> >     Just upstreamin'
> 
> It may not be a bad idea to deprecate core.preferSymlinkRefs now and
> remove it at Git 3.0 boundary.  Some platforms may not be able to do
> symbolic links and use it to represent HEAD, but everybody should be
> able to create a small text file with a single line.
> 
> But until then, this is a very reasonable thing to do.

> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 088b52c740..814decf323 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -3186,7 +3186,13 @@ static int files_transaction_finish(struct ref_store *ref_store,
>  		 * next update. If not, we try and create a regular symref.
>  		 */
>  		if (update->new_target && refs->prefer_symlink_refs)
> -			if (!create_ref_symlink(lock, update->new_target))
> +			/*
> +			 * By using the `NOT_CONSTANT()` trick, we can avoid
> +			 * errors by `clang`'s `-Wunreachable` logic that would
> +			 * report that the `continue` statement is not reachable
> +			 * when `NO_SYMLINK_HEAD` is `#define`d.
> +			 */
> +			if (NOT_CONSTANT(!create_ref_symlink(lock, update->new_target)))
>  				continue;

> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index bb2bec3807..cb402a2a54 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -2115,7 +2115,7 @@ static int commit_ref_update(struct files_ref_store *refs,
>  }
>  
>  #ifdef NO_SYMLINK_HEAD
> -#define create_ref_symlink(a, b) (-1)
> +#define create_ref_symlink(a, b) NOT_CONSTANT(-1)
>  #else
>  static int create_ref_symlink(struct ref_lock *lock, const char *target)
>  {

> Hi Patrick,
> 
> On Fri, 10 Oct 2025, Patrick Steinhardt wrote:
> 
> > diff --git a/refs/files-backend.c b/refs/files-backend.c
> > index bb2bec3807..cb402a2a54 100644
> > --- a/refs/files-backend.c
> > +++ b/refs/files-backend.c
> > @@ -2115,7 +2115,7 @@ static int commit_ref_update(struct files_ref_store *refs,
> >  }
> >  
> >  #ifdef NO_SYMLINK_HEAD
> > -#define create_ref_symlink(a, b) (-1)
> > +#define create_ref_symlink(a, b) NOT_CONSTANT(-1)
> >  #else
> >  static int create_ref_symlink(struct ref_lock *lock, const char *target)
> >  {
> 
> While this is correct, and "closer to the root", in my experience it is
> better to have work-arounds closer to where the symptom appears. In this
> case, it would be directly in the condition of the `if ()` construct.
> Therefore, I would prefer to keep the proposed patch as-is.

> An alternative could be to fix this at the source, e.g. like the below
> (untested) patch. But I don't mind this too much, especially given that
> this here is the only callsite of that function anyway. So please feel
> free to disregard.
>
> Thanks!

> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index bb2bec3807..cb402a2a54 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -2115,7 +2115,7 @@ static int commit_ref_update(struct files_ref_store *refs,
>  }
>  
>  #ifdef NO_SYMLINK_HEAD
> -#define create_ref_symlink(a, b) (-1)
> +#define create_ref_symlink(a, b) NOT_CONSTANT(-1)
>  #else
>  static int create_ref_symlink(struct ref_lock *lock, const char *target)
>  {