builtin/repack.c: support writing a MIDX while repacking

Teach `git repack` a new `--write-midx` option for callers that wish to
persist a multi-pack index in their repository while repacking.

There are two existing alternatives to this new flag, but they don't
cover our particular use-case. These alternatives are:

  - Call 'git multi-pack-index write' after running 'git repack', or

  - Set 'GIT_TEST_MULTI_PACK_INDEX=1' in your environment when running
    'git repack'.

The former works, but introduces a gap in bitmap coverage between
repacking and writing a new MIDX (since the repack may have deleted a
pack included in the existing MIDX, invalidating it altogether).

Setting the 'GIT_TEST_' environment variable is obviously unsupported.
In fact, even if it were supported officially, it still wouldn't work,
because it generates the MIDX *after* redundant packs have been dropped,
leading to the same issue as above.

Introduce a new option which eliminates this race by teaching `git
repack` to generate the MIDX at the critical point: after the new packs
have been written and moved into place, but before the redundant packs
have been removed.

This option is compatible with `git repack`'s '--bitmap' option (it
changes the interpretation to be: "write a bitmap corresponding to the
MIDX after one has been generated").

There is a little bit of additional noise in the patch below to avoid
repeating ourselves when selecting which packs to delete. Instead of a
single loop as before (where we iterate over 'existing_packs', decide if
a pack is worth deleting, and if so, delete it), we have two loops (the
first where we decide which ones are worth deleting, and the second
where we actually do the deleting). This makes it so we have a single
check we can make consistently when (1) telling the MIDX which packs we
want to exclude, and (2) actually unlinking the redundant packs.

There is also a tiny change to short-circuit the body of
write_midx_included_packs() when no packs remain in the case of an empty
repository. The MIDX code does not handle this, so avoid trying to
generate a MIDX covering zero packs in the first place.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 4 deletions

diff --git a/Documentation/git-repack.txt b/Documentation/git-repack.txt
index 24c00c9384..0f2d235ca5 100644
--- a/Documentation/git-repack.txt
+++ b/Documentation/git-repack.txt
@@ -9,7 +9,7 @@ git-repack - Pack unpacked objects in a repository
 SYNOPSIS
 --------
 [verse]
-'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]
+'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [-m] [--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>] [--write-midx]
 
 DESCRIPTION
 -----------
@@ -128,10 +128,11 @@ depth is 4095.
 -b::
 --write-bitmap-index::
 	Write a reachability bitmap index as part of the repack. This
-	only makes sense when used with `-a` or `-A`, as the bitmaps
+	only makes sense when used with `-a`, `-A` or `-m`, as the bitmaps
 	must be able to refer to all reachable objects. This option
-	overrides the setting of `repack.writeBitmaps`.  This option
-	has no effect if multiple packfiles are created.
+	overrides the setting of `repack.writeBitmaps`. This option
+	has no effect if multiple packfiles are created, unless writing a
+	MIDX (in which case a multi-pack bitmap is created).
 
 --pack-kept-objects::
 	Include objects in `.keep` files when repacking.  Note that we
@@ -190,6 +191,11 @@ to change in the future. This option (implying a drastically different
 repack mode) is not guaranteed to work with all other combinations of
 option to `git repack`.
 
+-m::
+--write-midx::
+	Write a multi-pack index (see linkgit:git-multi-pack-index[1])
+	containing the non-redundant packs.
+
 CONFIGURATION
 -------------