[PATCH 2/2] midx.c: make changing the preferred pack safe

[Taylor Blau <me@ttaylorr.com>]

The previous patch demonstrates a bug where a MIDX's auxiliary object
order can become out of sync with a MIDX bitmap.

This is because of two confounding factors:

  - First, the object order is stored in a file which is named according
    to the multi-pack index's checksum, and the MIDX does not store the
    object order. This means that the object order can change without
    altering the checksum.

  - But the .rev file is moved into place with finalize_object_file(),
    which link(2)'s the file into place instead of renaming it. For us,
    that means that a modified .rev file will not be moved into place if
    MIDX's checksum was unchanged.

The fix here is two-fold. First, we need to stop linking the file into
place and instead rename it. It's likely we were using
finalize_object_file() instead of a pure rename() because the former
also adjusts shared permissions. But that is unnecessary, because we
already do so in write_rev_file_order(), so rename alone is safe.

But we also need to make the MIDX's checksum change in some way when the
preferred pack changes without altering the set of packs stored in a
MIDX to prevent a race where the new .rev file is moved into place
before the MIDX is updated. Here, you'd get the opposite effect: reading
old bitmaps with the new object order.

But this race bites us even here: suppose that we didn't change the MIDX
checksum, but only renamed the auxiliary object order into place instead
of hardlinking it. Then when we go to generate the new bitmap, we'll
load the old MIDX bitmap, along with the MIDX that it references. That's
fine, since the new MIDX isn't moved into place until after the new
bitmap is generated. But the new object order *has* been moved into
place. So we'll read the old bitmaps in the new order when generating
the new bitmap file, meaning that without this secondary change, bitmap
generation itself would become a victim of the race described here.

This can all be prevented by forcing the MIDX's checksum to change when
the object order changes. We could include the entire object order in
the MIDX, but doing so is somewhat awkward. (For example, the code that
writes a .rev file expects to know the checksum of the associated pack
or MIDX, but writing that data into the MIDX itself makes that a
circular dependency).

Instead, make the pack order used during bitmap generation part of the
MIDX itself. That means that the new test in t5326 will cause the MIDX's
checksum to update, preventing the stale read problem.

There is no need to store the complete pack order here, since the only
way a fixed set of packs can result in a change in the object ordering
is by changing the preferred pack. That's because packs are sorted
according to their pack name only, so it would be impossible to induce a
different order by, say, touching a pack's mtime.

But storing the complete pack order gives us some flexibility in the
future, and it only costs 4 bytes per pack to do. In the future, we
could also optimize .rev reads looking for the identity of the preferred
pack by consulting this list if it exists.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 Documentation/technical/multi-pack-index.txt |  1 +
 Documentation/technical/pack-format.txt      |  4 ++++
 midx.c                                       | 25 ++++++++++++++++----
 t/t5326-multi-pack-bitmaps.sh                |  2 +-
 4 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/Documentation/technical/multi-pack-index.txt b/Documentation/technical/multi-pack-index.txt
index 86f40f2490..077c893ae3 100644
--- a/Documentation/technical/multi-pack-index.txt
+++ b/Documentation/technical/multi-pack-index.txt
@@ -24,6 +24,7 @@ and their offsets into multiple packfiles. It contains:
   - An offset within the jth packfile for the object.
 - If large offsets are required, we use another list of large
   offsets similar to version 2 pack-indexes.
+- An optional list of pack IDs in bitmap-sorted order.
 
 Thus, we can provide O(log N) lookup time for any number
 of packfiles.
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index 8d2f42f29e..edd77fb437 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -376,6 +376,10 @@ CHUNK DATA:
 	[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
 	    8-byte offsets into large packfiles.
 
+	[Optional] Bitmap pack order (ID: {'P', 'O', 'R', 'D'})
+	    A list of 4-byte pack identifiers in sorted order according to
+	    their relative bitmap positions.
+
 TRAILER:
 
 	Index checksum of the above contents.
diff --git a/midx.c b/midx.c
index 837b46b2af..abcdb92dd1 100644
--- a/midx.c
+++ b/midx.c
@@ -33,6 +33,7 @@
 #define MIDX_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
 #define MIDX_CHUNKID_OBJECTOFFSETS 0x4f4f4646 /* "OOFF" */
 #define MIDX_CHUNKID_LARGEOFFSETS 0x4c4f4646 /* "LOFF" */
+#define MIDX_CHUNKID_PACKORDER 0x504f5244 /* "PORD" */
 #define MIDX_CHUNK_FANOUT_SIZE (sizeof(uint32_t) * 256)
 #define MIDX_CHUNK_OFFSET_WIDTH (2 * sizeof(uint32_t))
 #define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
@@ -833,6 +834,18 @@ static int write_midx_large_offsets(struct hashfile *f,
 	return 0;
 }
 
+static int write_midx_pack_order(struct hashfile *f,
+				 void *data)
+{
+	struct write_midx_context *ctx = data;
+	uint32_t i;
+
+	for (i = 0; i < ctx->entries_nr; i++)
+		hashwrite_be32(f, ctx->pack_order[i]);
+
+	return 0;
+}
+
 struct midx_pack_order_data {
 	uint32_t nr;
 	uint32_t pack;
@@ -891,7 +904,7 @@ static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
 	tmp_file = write_rev_file_order(NULL, ctx->pack_order, ctx->entries_nr,
 					midx_hash, WRITE_REV);
 
-	if (finalize_object_file(tmp_file, buf.buf))
+	if (rename(tmp_file, buf.buf))
 		die(_("cannot store reverse index file"));
 
 	strbuf_release(&buf);
@@ -1403,15 +1416,19 @@ static int write_midx_internal(const char *object_dir,
 			(size_t)ctx.num_large_offsets * MIDX_CHUNK_LARGE_OFFSET_WIDTH,
 			write_midx_large_offsets);
 
+	if (flags & (MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP)) {
+		ctx.pack_order = midx_pack_order(&ctx);
+		add_chunk(cf, MIDX_CHUNKID_PACKORDER,
+			  ctx.entries_nr * sizeof(uint32_t),
+			  write_midx_pack_order);
+	}
+
 	write_midx_header(f, get_num_chunks(cf), ctx.nr - dropped_packs);
 	write_chunkfile(cf, &ctx);
 
 	finalize_hashfile(f, midx_hash, CSUM_FSYNC | CSUM_HASH_IN_STREAM);
 	free_chunkfile(cf);
 
-	if (flags & (MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP))
-		ctx.pack_order = midx_pack_order(&ctx);
-
 	if (flags & MIDX_WRITE_REV_INDEX)
 		write_midx_reverse_index(midx_name.buf, midx_hash, &ctx);
 	if (flags & MIDX_WRITE_BITMAP) {
diff --git a/t/t5326-multi-pack-bitmaps.sh b/t/t5326-multi-pack-bitmaps.sh
index 0ca2868b0b..353282310d 100755
--- a/t/t5326-multi-pack-bitmaps.sh
+++ b/t/t5326-multi-pack-bitmaps.sh
@@ -395,7 +395,7 @@ test_expect_success 'hash-cache values are propagated from pack bitmaps' '
 	)
 '
 
-test_expect_failure 'changing the preferred pack does not corrupt bitmaps' '
+test_expect_success 'changing the preferred pack does not corrupt bitmaps' '
 	rm -fr repo &&
 	git init repo &&
 	test_when_finished "rm -fr repo" &&
-- 
2.34.1.25.gb3157a20e6