[PATCH 0/2] Avoid rewriting "packed-refs" unnecessarily

Since commit dc39e09942 (files_ref_store: use a transaction to update packed refs, 2017-09-08), we've been rewriting the `packed-refs` file unnecessarily when deleting a loose reference that has no packed counterpart. Add some tests demonstrating this problem, then fix it by teaching `files_backend` to see whether any references being deleted have packed versions, and if not, skipping the packed_refs transaction.

This is a mild regression since 2.14, which avoided rewriting the `packed-refs` file in these cases (though it still had to *read* the whole file to determine whether the rewrite could be skipped). Therefore, it might be considered for 2.15. On the other hand, it is late in the release cycle, so the risk of accepting this change might be considered too risky.

These patches apply on top of master and commute with mh/ref-locking-fix. They are also available from my GitHub fork [1] as branch `avoid-rewriting-packed-refs`.

Michael

[1] https://github.com/mhagger/git

Michael Haggerty (2):
  t1409: check that `packed-refs` is not rewritten unnecessarily
  files-backend: don't rewrite the `packed-refs` file unnecessarily

 refs/files-backend.c          |  18 ++++++-
 refs/packed-backend.c         |  68 ++++++++++++++++++++++++
 refs/packed-backend.h         |   8 +++
 t/t1409-avoid-packing-refs.sh | 118 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 211 insertions(+), 1 deletion(-)
 create mode 100755 t/t1409-avoid-packing-refs.sh