[PATCH v2 12/26] pack-objects: keep track of `pack_start` for each reuse pack

From: Taylor Blau <me@ttaylorr.com>
Date: Dec 14, 2023, 22:24 UTC
Message-ID: <9a5c38514bb6dedc44c28bab71a98fe1c0150ecb.1702592604.git.me@ttaylorr.com>
In-Reply-To: <cover.1702592603.git.me@ttaylorr.com>

When reusing objects from a pack, we keep track of a set of one or more `reused_chunk`s, corresponding to sections of one or more object(s) from a source pack that we are reusing. Each chunk contains two pieces of information:

  - the offset of the first object in the source pack (relative to the
    beginning of the source pack)
  - the difference between that offset, and the corresponding offset in
    the pack we're generating

The purpose of keeping track of these is so that we can patch an OFS_DELTAs that cross over a section of the reuse pack that we didn't take.

For instance, consider a hypothetical pack as shown below:

                                                (chunk #2)
                                                __________...
                                               /
                                              /
      +--------+---------+-------------------+---------+
  ... | <base> | <other> |      (unused)     | <delta> | ...
      +--------+---------+-------------------+---------+
       \                /
        \______________/
           (chunk #1)

Suppose that we are sending objects "base", "other", and "delta", and that the "delta" object is stored as an OFS_DELTA, and that its base is "base". If we don't send any objects in the "(unused)" range, we can't copy the delta'd object directly, since its delta offset includes a range of the pack that we didn't copy, so we have to account for that difference when patching and reassembling the delta.

In order to compute this value correctly, we need to know not only where we are in the packfile we're assembling (with `hashfile_total(f)`) but also the position of the first byte of the packfile that we are currently reusing. Currently, this works just fine, since when reusing only a single pack those two values are always identical (because verbatim reuse is the first thing pack-objects does when enabled after writing the pack header).

But when reusing multiple packs which have one or more gaps, we'll need to account for these two values diverging.

Together, these two allow us to compute the reused chunk's offset difference relative to the start of the reused pack, as desired.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/pack-objects.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 102fe9a4f8..f51b86d99f 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -1015,6 +1015,7 @@ static off_t find_reused_offset(off_t where)
 
 static void write_reused_pack_one(struct packed_git *reuse_packfile,
 				  size_t pos, struct hashfile *out,
+				  off_t pack_start,
 				  struct pack_window **w_curs)
 {
 	off_t offset, next, cur;
@@ -1024,7 +1025,8 @@ static void write_reused_pack_one(struct packed_git *reuse_packfile,
 	offset = pack_pos_to_offset(reuse_packfile, pos);
 	next = pack_pos_to_offset(reuse_packfile, pos + 1);
 
-	record_reused_object(offset, offset - hashfile_total(out));
+	record_reused_object(offset,
+			     offset - (hashfile_total(out) - pack_start));
 
 	cur = offset;
 	type = unpack_object_header(reuse_packfile, w_curs, &cur, &size);
@@ -1094,6 +1096,7 @@ static void write_reused_pack_one(struct packed_git *reuse_packfile,
 
 static size_t write_reused_pack_verbatim(struct packed_git *reuse_packfile,
 					 struct hashfile *out,
+					 off_t pack_start UNUSED,
 					 struct pack_window **w_curs)
 {
 	size_t pos = 0;
@@ -1125,10 +1128,12 @@ static void write_reused_pack(struct packed_git *reuse_packfile,
 {
 	size_t i = 0;
 	uint32_t offset;
+	off_t pack_start = hashfile_total(f) - sizeof(struct pack_header);
 	struct pack_window *w_curs = NULL;
 
 	if (allow_ofs_delta)
-		i = write_reused_pack_verbatim(reuse_packfile, f, &w_curs);
+		i = write_reused_pack_verbatim(reuse_packfile, f, pack_start,
+					       &w_curs);
 
 	for (; i < reuse_packfile_bitmap->word_alloc; ++i) {
 		eword_t word = reuse_packfile_bitmap->words[i];
@@ -1145,7 +1150,7 @@ static void write_reused_pack(struct packed_git *reuse_packfile,
 			 * for why.
 			 */
 			write_reused_pack_one(reuse_packfile, pos + offset, f,
-					      &w_curs);
+					      pack_start, &w_curs);
 			display_progress(progress_state, ++written);
 		}
 	}

-- 
2.43.0.102.ga31d690331.dirty