[PATCH 00/20] Separate `ref_cache` into a separate module

I had a window of opportunity last week to hack intensely on Git, with the following goals:

* Separate `ref_cache` out of `files_ref_cache`.

* Separate a new `packed_ref_cache` class out of `files_ref_cache`.
  Change the latter to use an instance of the former for all of its
  interactions with the `packed-refs` file.

* Mmap `packed-refs` files rather than reading-and-parsing.

* Use the mmapped version of the `packed-refs` file as the "cache"
  rather than using a separate `ref_cache`.

* (And the main goal): Avoid reading and parsing the *whole
  `packed-refs` file* (as we do now) every time any part of it is
  needed. Instead, use binary search to find the reference and/or
  range of references that we want, and parse the info out of the
  mmapped image on the fly.

I've completed a draft of an epic 48-patch series implementing all of the above points on my GitHub fork [1] as branch `wip/mmap-packed-refs`. It dramatically speeds up performance and reduces memory usage for some tasks in repositories with very many packed references.

But the later parts of that series aren't completely polished yet, and such a large patch series would be indigestible anyway, so here I submit the first part...

This patch series extracts a `ref_cache` module out of `files_ref_cache`, and goes some way to disentangling those two modules, which until now were overly intimate with each other:

* Remove `verify_refname_available()` from the refs VTABLE, instead
  implementing it in a generic way that uses only the usual refs API
  to talk to the `ref_store`.

* Split `ref_cache`-related code into a new module,
  `refs/ref-cache.{c,h}`. Encapsulate the data structure in a new
  class, `struct ref_cache`.

* Change the lazy-filling mechanism of `ref_cache` to call back to its
  backing `ref_store` via a callback function rather than calling
  `read_loose_refs()` directly.

* Move the special handling of `refs/bisect/` from `ref_cache` to
  `files_ref_store`.

* Make `cache_ref_iterator_begin()` smarter, and change external users
  to iterate via this interface instead of using
  `do_for_each_entry_in_dir()`.

Even after this patch series, the modules are still too intimate for my taste, but I think this is a big step forward, and it is enough to allow the other changes that I've been working on.

These patches depend on Duy's nd/files-backend-git-dir branch, v6 [2]. They are also available from my GitHub fork [1] as branch `separate-ref-cache`.

Happily, this patch series actually removes a few more lines than it adds, mostly thanks to the simpler `verify_refname_available()` implementation.

Michael

[1] https://github.com/mhagger/git [2] http://public-inbox.org/git/20170318020337.22767-1-pclouds@gmail.com/

Michael Haggerty (20):
  get_ref_dir(): don't call read_loose_refs() for "refs/bisect"
  refs_read_raw_ref(): new function
  refs_ref_iterator_begin(): new function
  refs_verify_refname_available(): implement once for all backends
  refs_verify_refname_available(): use function in more places
  Rename `add_ref()` to `add_ref_entry()`
  Rename `find_ref()` to `find_ref_entry()`
  Rename `remove_entry()` to `remove_entry_from_dir()`
  refs: split `ref_cache` code into separate files
  ref-cache: introduce a new type, ref_cache
  refs: record the ref_store in ref_cache, not ref_dir
  ref-cache: use a callback function to fill the cache
  refs: handle "refs/bisect/" in `loose_fill_ref_dir()`
  do_for_each_entry_in_dir(): eliminate `offset` argument
  get_loose_ref_dir(): function renamed from get_loose_refs()
  get_loose_ref_cache(): new function
  cache_ref_iterator_begin(): make function smarter
  commit_packed_refs(): use reference iteration
  files_pack_refs(): use reference iteration
  do_for_each_entry_in_dir(): delete function

 Makefile             |    1 +
 refs.c               |  111 ++++-
 refs.h               |    2 +-
 refs/files-backend.c | 1229 +++++++-------------------------------------------
 refs/ref-cache.c     |  523 +++++++++++++++++++++
 refs/ref-cache.h     |  267 +++++++++++
 refs/refs-internal.h |   22 +-
 7 files changed, 1066 insertions(+), 1089 deletions(-)
 create mode 100644 refs/ref-cache.c
 create mode 100644 refs/ref-cache.h