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

[Michael Haggerty <mhagger@alum.mit.edu>]

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

-- 
2.14.1