Re: [PATCH v6 03/19] index-helper: new daemon for caching index and related stuff

From: Duy Nguyen <pclouds@gmail.com>
Date: May 1, 2016, 00:22 UTC
Message-ID: <CACsJy8AZihi2j=LLqhPvvLm2PPR4KZne2y9ENR4YDgY+tOXo0A@mail.gmail.com>
In-Reply-To: <1461955617.4123.37.camel@twopensource.com>

On Sat, Apr 30, 2016 at 1:46 AM, David Turner <dturner@twopensource.com> wrote:

Show 26 quoted lines

> On Thu, 2016-04-28 at 11:58 -0700, Junio C Hamano wrote:
>> David Turner <dturner@twopensource.com> writes:
>>
>> > From: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
>> > ...
>> > The biggest gain is not having to verify the trailing SHA-1, which
>> > takes lots of time especially on large index files. But this also
>> > opens doors for further optimiztions:
>>
>> optimizAtion
>>
>> > Git can poke the daemon via unix domain sockets to tell it to
>> > refresh
>> > the index cache, or to keep it alive some more minutes. It can't
>> > give
>> > any real index data directly to the daemon. Real data goes to disk
>> > first, then the daemon reads and verifies it from there. Poking
>> > only
>> > happens for $GIT_DIR/index, not temporary index files.
>>
>> Is this limited to "poking", or the helper daemon is not involved in
>> codepaths that handle temporary index at all?  It makes sense if it
>> is the latter, and it doesn't if it were the former, but it is
>> unclear in this paragraph.
>
> It is in fact the latter.  Will clarify.

The intention is so. The execution is less than perfect. On the server side, the index-helper only concerns itself with .git/index only. Good. On the client side, is_main_index() is used to detect if it's $GIT_DIR/index. It checks if the given index_state pointer points to the_index and if so assumes that it is the main index.

That is not true when read_cache_from() is used with something other than git_index_file() as argument. Which is exactly what builtin/commit.c does in prepare_index(). .git/index.lock gets passed in instead. read-cache.c may "poke" or "refresh" (probably should rename these to "read" and "write") because is_main_index() incorrectly returns true. Luckily it will soon find out index-helper does not prepare shm for this particular index file and fall back to reading from disk. We waste some context switches though.

-- 
Duy