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From: Michael Haggerty <mhagger@alum.mit.edu>
To: Jeff King <peff@peff.net>
Cc: "Junio C Hamano" <gitster@pobox.com>,
	"Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>,
	"David Turner" <novalis@novalis.org>,
	git@vger.kernel.org
Subject: Re: [PATCH 11/20] refs: record the ref_store in ref_cache, not ref_dir
Date: Mon, 20 Mar 2017 23:39:19 +0100	[thread overview]
Message-ID: <5e015cd9-f53e-b27b-ff73-d95706df3223@alum.mit.edu> (raw)
In-Reply-To: <20170320175145.zwfqp7varl4hwink@sigill.intra.peff.net>

On 03/20/2017 06:51 PM, Jeff King wrote:
> On Mon, Mar 20, 2017 at 05:33:16PM +0100, Michael Haggerty wrote:
> 
>> Instead of keeping a pointer to the ref_store in every ref_dir entry,
>> store it once in `struct ref_cache`, and change `struct ref_dir` to
>> include a pointer to its containing `ref_cache` instead. This makes it
>> easier to add to the information that is stored in `struct ref_cache`
>> without inflating the size of the individual entries.
> 
> This last sentence confused me. It's a pointer either way, no?
> 
> Do you just mean that we are free to add whatever we like to the
> "ref_cache" without polluting the "ref_store" that is a more public data
> structure?

Yeah, that's explained poorly. It might be clearer as

> This makes it easier to add to the information that is accessible
> from a `ref_dir` without increasing the size of every `ref_dir`
> instance.

It used to be that `ref_dir` contained a pointer to the `ref_store` that
contains it. (BTW, such a pointer *can't* be turned into a pointer to
the `ref_cache` because (1) the `ref_dir` shouldn't have to know what
kind of `ref_store` it is being used with, and (2) a `packed_ref_cache`
can be detached from its old `ref_cache` if the stat info indicates that
the `packed-refs` file has been modified since it was last read.)

But we want to add a `fill_ref_dir` callback pointer in a place that is
reachable from the `ref_dir` so that it can fill itself when necessary.
We could add it to the `ref_dir` struct, but that would inflate its
size. Instead, it makes more sense for the `ref_dir` to have a pointer
to the enclosing `ref_cache`, and store the `fill_ref_dir` pointer in
`ref_cache`. The `ref_cache` also gets a pointer to the enclosing
`ref_store`; that way a `ref_dir` also has a way to access the
`ref_store` that contains it.

An alternative would be to pass a pointer to the `ref_cache` down the
call stack when it is being accessed, but that seemed like a lot of work
that wouldn't lead to a very clean result.

Michael


  reply	other threads:[~2017-03-20 22:39 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-20 16:33 [PATCH 00/20] Separate `ref_cache` into a separate module Michael Haggerty
2017-03-20 16:33 ` [PATCH 01/20] get_ref_dir(): don't call read_loose_refs() for "refs/bisect" Michael Haggerty
2017-03-20 16:33 ` [PATCH 02/20] refs_read_raw_ref(): new function Michael Haggerty
2017-03-20 16:33 ` [PATCH 03/20] refs_ref_iterator_begin(): " Michael Haggerty
2017-03-20 16:33 ` [PATCH 04/20] refs_verify_refname_available(): implement once for all backends Michael Haggerty
2017-03-20 17:42   ` Jeff King
2017-03-20 22:20     ` Michael Haggerty
2017-03-20 16:33 ` [PATCH 05/20] refs_verify_refname_available(): use function in more places Michael Haggerty
2017-03-20 16:33 ` [PATCH 06/20] Rename `add_ref()` to `add_ref_entry()` Michael Haggerty
2017-03-20 16:33 ` [PATCH 07/20] Rename `find_ref()` to `find_ref_entry()` Michael Haggerty
2017-03-20 16:33 ` [PATCH 08/20] Rename `remove_entry()` to `remove_entry_from_dir()` Michael Haggerty
2017-03-20 16:33 ` [PATCH 09/20] refs: split `ref_cache` code into separate files Michael Haggerty
2017-03-20 17:49   ` Jeff King
2017-03-20 19:47     ` Junio C Hamano
2017-03-20 20:35       ` Stefan Beller
2017-03-20 22:40         ` Junio C Hamano
2017-03-20 16:33 ` [PATCH 10/20] ref-cache: introduce a new type, ref_cache Michael Haggerty
2017-03-20 16:33 ` [PATCH 11/20] refs: record the ref_store in ref_cache, not ref_dir Michael Haggerty
2017-03-20 17:51   ` Jeff King
2017-03-20 22:39     ` Michael Haggerty [this message]
2017-03-20 16:33 ` [PATCH 12/20] ref-cache: use a callback function to fill the cache Michael Haggerty
2017-03-20 16:33 ` [PATCH 13/20] refs: handle "refs/bisect/" in `loose_fill_ref_dir()` Michael Haggerty
2017-03-20 16:33 ` [PATCH 14/20] do_for_each_entry_in_dir(): eliminate `offset` argument Michael Haggerty
2017-03-20 16:33 ` [PATCH 15/20] get_loose_ref_dir(): function renamed from get_loose_refs() Michael Haggerty
2017-03-20 16:33 ` [PATCH 16/20] get_loose_ref_cache(): new function Michael Haggerty
2017-03-20 16:33 ` [PATCH 17/20] cache_ref_iterator_begin(): make function smarter Michael Haggerty
2017-03-20 16:33 ` [PATCH 18/20] commit_packed_refs(): use reference iteration Michael Haggerty
2017-03-20 18:05   ` Jeff King
2017-03-22  8:42     ` Michael Haggerty
2017-03-22 13:06       ` Jeff King
2017-03-20 16:33 ` [PATCH 19/20] files_pack_refs(): " Michael Haggerty
2017-03-20 16:33 ` [PATCH 20/20] do_for_each_entry_in_dir(): delete function Michael Haggerty
2017-03-20 17:25 ` [PATCH 00/20] Separate `ref_cache` into a separate module Junio C Hamano
2017-03-20 18:12 ` Jeff King
2017-03-20 18:24 ` Ævar Arnfjörð Bjarmason
2017-03-20 18:30   ` Jeff King
2017-03-20 22:32 ` Junio C Hamano
2017-03-20 22:48   ` Michael Haggerty

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