Re: [RFC/PATCH 8/8] read-cache: unlink old sharedindex files

From: Christian Couder <christian.couder@gmail.com>
Date: Jul 13, 2016, 17:54 UTC
Message-ID: <CAP8UFD1=TLG=04BhjGWRa=M+8=4QGcrH3aj4VB8kb5pjXPHm8w@mail.gmail.com>
In-Reply-To: <CACsJy8DHd44MnFUkJwh9n0JK5J5P6kER0kgopX0UrS=ROo_URQ@mail.gmail.com>

On Wed, Jul 13, 2016 at 5:16 PM, Duy Nguyen <pclouds@gmail.com> wrote:

Show 19 quoted lines

> On Tue, Jul 12, 2016 at 9:45 PM, Christian Couder
> <christian.couder@gmail.com> wrote:
>> On Tue, Jul 12, 2016 at 5:12 PM, Duy Nguyen <pclouds@gmail.com> wrote:
>>>
>>> No. People could create an index file anywhere in theory. So you don't
>>> know how many index files there are.
>>
>> Maybe when an index file is created, its path and its sharedindex file
>> could be appended into a log file.
>> We could check this log file to see if we can remove sharedindex files.
>> We would need to remove the entries in this log file for the indexes
>> that are no longer there.
>>
>> Or instead of one log file we could have a file for each index file in
>> a special directory called for example "indexinfo".
>> So we could just delete the file if its related index is no longer there.
>
> New files will require locking so people don't append at the same
> time.

If we create a new file for each index file with a name that depends on the index path (for example maybe the sha1 of the index path) then as many index files with the same path are not possible we should be safe.

Show 10 quoted lines

> And maybe another new host of problems. I think we can just go
> with the garbage collection way that we have done for unreachable
> objects.
>
> Your indexinfo idea looks very close to multiple locking, an index
> would lock the shared index it's linked to, preventing it from being
> removed. For single locking, we can just create a file named $X.lock,
> but for multiple locks, maybe we can go with
> $X.lock-$index_trailing_sha1. Will it work? I don't know. Just
> thinking out loud.

Isn't is possible that the same index (so with the same trailing sha1) be created in two different places?

Show 11 quoted lines

>>> It really depends. If the shared part is too small for old indexes, we
>>> might as well unsplit them. In practice though, the only long-term
>>> index file is $GIT_DIR/index. If we don't delete old shared index
>>> files too close to their creation time, temp index files will go away.
>>
>> We could treat $GIT_DIR/index specially so that if there are no temp
>> index files, there should be nothing in "indexinfo".
>
> No, temp index files are needed. And it will  be hard to treat
> $GIT_DIR/index specially because updating it involves a temp index:
> you first prepare a new index in $GIT_DIR/index.lock.

If the new index is always prepared in $GIT_DIR/index.lock, then there is no problem in the first place because when my original patch calls clean_shared_index_files() the $GIT_DIR/index.lock is taken. Or maybe I am missing something?

> If everything
> goes well, you atomically rename it to $GIT_DIR/index. You may be able
> to treat $GIT_DIR/index.lock special too, but that's starting to get
> out of hand.