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Re: [PATCH] packfile: close multi-pack-index in close_all_packs

On 10/29/2018 7:10 AM, SZEDER Gábor wrote:
On Thu, Oct 25, 2018 at 12:54:05PM +0000, Derrick Stolee wrote:
Whenever we delete pack-files from the object directory, we need
to also delete the multi-pack-index that may refer to those
objects. Sometimes, this connection is obvious, like during a
repack. Other times, this is less obvious, like when gc calls
a repack command and then does other actions on the objects, like
write a commit-graph file.

The pattern we use to avoid out-of-date in-memory packed_git
structs is to call close_all_packs(). This should also call
close_midx(). Since we already pass an object store to
close_all_packs(), this is a nicely scoped operation.

This fixes a test failure when running t6500-gc.sh with

Reported-by: Szeder Gábor <szeder.dev@xxxxxxxxx>
Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx>

Thanks for the report, Szeder! I think this is the correct fix,
and more likely to be permanent across all builtins that run
auto-GC. I based it on ds/test-multi-pack-index so it could easily
be added on top.
OK, avoiding those errors in the first place is good, of course...
However, I still find it disconcerting that those errors didn't cause
'git gc' to error out, and, consequently, what other MIDX-related
errors/bugs might do the same.
When I added GIT_TEST_MULTI_PACK_INDEX, one of the important pieces was to delete the multi-pack-index file whenever deleting the pack-files it contains. This only happens during a 'git repack'.

The thing I had missed (and is covered by this patch) is when we run a subcommand that can remove pack-files while we have a multi-pack-index open.

The reason this wasn't caught is that the test suite did not include any cases where the following things happened in order:

1. Open pack-files and multi-pack-index.
2. Delete pack-files, but keep multi-pack-index open.
3. Read objects (from the multi-pack-index).

This step 3 was added to 'git gc' by the commit-graph write, hence the break. The pack-file deletion happens in the repack subcommand.

Since close_all_packs() is the way we guarded against this problem in the traditional pack-file environment, this is the right place to insert a close_midx() call, and will avoid cases like this in the future (at least, the multi-pack-index will not be the reason on its own).