midx: preliminary support for `--refs-snapshot`

To figure out which commits we can write a bitmap for, the multi-pack
index/bitmap code does a reachability traversal, marking any commit
which can be found in the MIDX as eligible to receive a bitmap.

This approach will cause a problem when multi-pack bitmaps are able to
be generated from `git repack`, since the reference tips can change
during the repack. Even though we ignore commits that don't exist in
the MIDX (when doing a scan of the ref tips), it's possible that a
commit in the MIDX reaches something that isn't.

This can happen when a multi-pack index contains some pack which refers
to loose objects (e.g., if a pack was pushed after starting the repack
but before generating the MIDX which depends on an object which is
stored as loose in the repository, and by definition isn't included in
the multi-pack index).

By taking a snapshot of the references before we start repacking, we can
close that race window. In the above scenario (where we have a packed
object pointing at a loose one), we'll either (a) take a snapshot of the
references before seeing the packed one, or (b) take it after, at which
point we can guarantee that the loose object will be packed and included
in the MIDX.

This patch does just that. It writes a temporary "reference snapshot",
which is a list of OIDs that are at the ref tips before writing a
multi-pack bitmap. References that are "preferred" (i.e,. are a suffix
of at least one value of the 'pack.preferBitmapTips' configuration) are
marked with a special '+'.

The format is simple: one line per commit at each tip, with an optional
'+' at the beginning (for preferred references, as described above).

When provided, the reference snapshot is used to drive bitmap selection
instead of the MIDX code doing its own traversal. When it isn't
provided, the usual traversal takes place instead.

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

1 files changed, 15 insertions, 0 deletions

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index 009c989ef8..27f83932e4 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -49,6 +49,21 @@ write::
 	--stdin-packs::
 		Write a multi-pack index containing only the set of
 		line-delimited pack index basenames provided over stdin.
+
+	--refs-snapshot=<path>::
+		With `--bitmap`, optionally specify a file which
+		contains a "refs snapshot" taken prior to repacking.
++
+A reference snapshot is composed of line-delimited OIDs corresponding to
+the reference tips, usually taken by `git repack` prior to generating a
+new pack. A line may optionally start with a `+` character to indicate
+that the reference which corresponds to that OID is "preferred" (see
+linkgit:git-config[1]'s `pack.preferBitmapTips`.)
++
+The file given at `<path>` is expected to be readable, and can contain
+duplicates. (If a given OID is given more than once, it is marked as
+preferred if at least one instance of it begins with the special `+`
+marker).
 --
 
 verify::