On 06/01/2016 09:39 PM, Junio C Hamano wrote:
> Michael Haggerty <mhag...@alum.mit.edu> writes:
>
>> I argue that the fundamental concept in terms of the implementation
>> should be the individual physical reference stores, and these should be
>> compounded together to form the logical reference collections and the
>> sets of reachability roots that are interesting at the UI level.
>
> That is very good in principle. How does that principle translate
> to the current setup (with possible enhancement with pluggable ref
> backends) and multiple worktrees? Let me try thinking it through
> aloud.
>
> * Without pluggable ref backend or worktrees, we start from two
> "physical reference stores"; packed-refs file lists refs that
> will be covered (overridden) by loose refs in .git/refs/.
> Symbolic refs always being in loose falls out as a natural
> consequence that packed-refs file does not record symrefs.
>
> * Throw in multiple worktrees to the mix. How? Do we consider
> selected refs/ hierarchies (like refs/bisect/*) as separate
> physical store (even though it might be backed by the files in
> the same .git/refs/ filesystem hierarchy) and represent the
> "logical" view as an overlay across the traditional two types of
> physical reference stores? That is:
>
> - loose refs in .git/HEAD, .git/refs/{bisect,...} for
> per-worktree part form one physical store. If a ref is found
> here, that is what we use as a part of the logical view.
>
> - loose refs in .git/refs/{branches,tags,notes,...} for common
> part form one physical store. For a ref that is not found
> above but is found here becomes a part of the logical view.
>
> - packed refs in .git/packed-refs is another physical store. For
> a ref that is not found in the above two but is found here
> becomes a part of the logical view.
I think I would represent the logical store of a worktree repo as
follows. First, I would implement a cached_ref_store that introduces a
layer of caching around another ref_store. Then
def get_files_ref_store(dir) {
loose = create_cached_ref_store(get_loose_ref_store(dir))
packed = create_cached_ref_store(get_packed_ref_store(dir))
return create_files_ref_store(loose, packed)
}
common_ref_store = get_files_ref_store(common_dir)
/*
* I think we only allow loose refs in worktrees; otherwise
* this could be an overlay_ref_store too. Actually, we might
* want to omit the caching here.
*/
local_ref_store = create_cached_ref_store(
get_loose_ref_store(git_dir))
logical_ref_store = create_worktree_ref_store(
local_ref_store, common_ref_store)
Where worktree_ref_store does something like
if (is_per_worktree_ref(refname))
lookup in local_ref_store
else
lookup in common_ref_store
for reading, and uses a merge_ref_iterator with a select function that
does something similar for iterating.
The files_ref_store would do lookups by looking first in the
loose_ref_store then in the packed_ref_store, would use an
overlay_ref_iterator for iteration, and would know to do all writes in
the loose_ref_store (except for deletes, which also have to go to
packed_ref_store). It would have a special "pack-refs" operation,
specific to files_ref_store, that shuffles references between its two
backends.
Writing to a worktree_ref_store is a bit tricker, because we want to
allow ref_transactions to span worktree and common refs (though we
probably need to give up atomicity for any such transaction). The
worktree_ref_transaction_commit() method has to split the main
transaction into two sub-transactions, one for each of its component
ref_stores. I planned for this when designing split_under_lock and think
it is possible, though I admit I haven't implemented it yet.
One nice thing about this design is that you can skip the
worktree_ref_store layer and its overhead entirely for repositories that
are not linked. The decision can be made once, at instantiation time,
rather than every time a reference is looked up. See the pseudocode below.
> Up to this point, I am all for your "separate physical stores are
> composited to give a logical view". I can see how multi-worktree
> world view fits within that framework.
>
> * With pluggable ref backend, we may gain yet another "physical
> reference store" possibility, e.g. one backed by lmdb. If it
> supports symrefs, a repoitory may use lmdb backed reference store
> without the traditional two.
>
> But it is unclear how it would interact with the multi-worktree
> world order.
Since you could plug-and-play different ref_stores in the above scheme,
I don't see any problem here.
def get_logical_ref_store() {
local_ref_store = get_local_ref_store(git_dir)
if (is_linked_repo) {
common_ref_store = get_ref_store(common_dir)
return worktree_ref_store(local_ref_store,
common_ref_store)
} else {
return local_ref_store;
}
}
get_ref_store() would read the git config to decide what the ref store
to use for the specified repository, which itself might be an
lmdb_ref_store or an overlay_ref_store(loose_ref_store, packed_ref_store).
Michael
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