[PATCH 0/4] refs: improve handling of special refs

Hi,

there are a bunch of "special" refs in Git that sometimes behave like a normal reference and sometimes they don't. These references are written to directly via the filesystem without going through the reference backend, but the expectation is that those references can then be read by things like git-rev-parse(1).

We do not currently have a single source of truth for what those special refs are, and we also don't have clear rules for how they should be written to. This works in the context of the files backend, because it is able to read back such manually-written loose references just fine. But once the reftable backend lands this will stop working.

This patch series tries to improve this state by doing two things:

    1. We explicitly mark these references as special by introducing a
       new `is_special_ref()` function. This serves as documentation,
       but will also cause us to explicitly read all of these special
       refs via loose files regardless of the actual backend.

    2. We document a new rule around writing refs. Namely, normal
       references are _always_ written via the reference backend,
       whereas special references are _always_ written directly via the
       filesystem. This rule is not enforced anywhere, but at least it's
       now made more explicit.

The last patch fixes one of the instances where we treat a reference inconsistently by converting it to a normal reference. We can eventually migrate more of the special refs to become normal refs as we deem fit, but I consider this to be out of scope for this patch series.

These patches improve compatibility with the new reftable backend.

Patrick

Patrick Steinhardt (4):
  wt-status: read HEAD and ORIG_HEAD via the refdb
  refs: propagate errno when reading special refs fails
  refs: complete list of special refs
  bisect: consistently write BISECT_EXPECTED_REV via the refdb

 bisect.c                    | 25 +++------------
 builtin/bisect.c            |  8 ++---
 refs.c                      | 64 +++++++++++++++++++++++++++++++++++--
 t/t1403-show-ref.sh         |  9 ++++++
 t/t6030-bisect-porcelain.sh |  2 +-
 wt-status.c                 | 17 +++++-----
 6 files changed, 86 insertions(+), 39 deletions(-)