Before a new MIDX can be written, expire_midx_packs() first loads the
existing MIDX, figures out which packs can be expired, and then writes a
new MIDX based on that information.
In order to load the existing MIDX, it uses load_multi_pack_index(),
which mmaps the multi-pack-index file, but does not store the resulting
`struct multi_pack_index *` in the object store.
write_midx_internal() also needs to open the existing MIDX, and it does
so by iterating the results of get_multi_pack_index(), so that it reuses
the same pointer held by the object store. But before it can move the
new MIDX into place, it close_object_store() to munmap() the
multi-pack-index file to accommodate platforms like Windows which don't
allow overwriting files which are memory mapped.
That's where things get weird. Since expire_midx_packs has its own
*separate* memory mapped copy of the MIDX, the MIDX file is still memory
mapped! Interestingly, this doesn't seem to cause a problem in our
tests. (I believe that this has much more to do with my own lack of
familiarity with Windows than it does a lack of coverage in our tests).
In any case, we can side-step the whole issue by teaching
expire_midx_packs() to use the `struct multi_pack_index` pointer it
found via the object store instead of maintain its own copy. That way,
when write_midx_internal() calls `close_object_store()`, we know that
there are no memory mapped copies of the MIDX laying around.
A couple of other small notes about this patch:
- As far as I can tell, passing `local == 1` to the call to
load_multi_pack_index() was an error, since object_dir could be an
alternate. But it doesn't matter, since even though we write
`m->local = 1`, we never read that field back later on.
- Setting `m = NULL` after write_midx_internal() was likely to prevent
a double-free back from when that function took a `struct
multi_pack_index *` that it called close_midx() on itself. We can
rely on write_midx_internal() to call that for us now.
Finally, this enforces the same "the value of --object-dir must be the
local object store, or an alternate" rule from f57a739691 (midx: avoid
opening multiple MIDXs when writing, 2021-09-01) to the `expire`
sub-command, too.
Signed-off-by: Taylor Blau <me@xxxxxxxxxxxx>
---
This does leak the MIDX write_midx_internal returns before calling
close_object_store(). We can't just blindly call close_object_store()
here, either, since it's susceptible to double-frees. I'll think about
improving this in a separate series.
midx.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
diff --git a/midx.c b/midx.c
index b66b75a3cd..7f1addf4b6 100644
--- a/midx.c
+++ b/midx.c
@@ -1707,7 +1707,7 @@ int expire_midx_packs(struct repository *r, const char *object_dir, unsigned fla
{
uint32_t i, *count, result = 0;
struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
- struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+ struct multi_pack_index *m = lookup_multi_pack_index(r, object_dir);
struct progress *progress = NULL;
if (!m)
@@ -1752,12 +1752,11 @@ int expire_midx_packs(struct repository *r, const char *object_dir, unsigned fla
free(count);
- if (packs_to_drop.nr) {
+ if (packs_to_drop.nr)
result = write_midx_internal(object_dir, NULL, &packs_to_drop, NULL, NULL, flags);
- m = NULL;
- }
string_list_clear(&packs_to_drop, 0);
+
return result;
}
--
2.33.0.96.g73915697e6
