Re: [PATCH 02/27] sparse-index: implement ensure_full_index()

[Derrick Stolee <stolee@gmail.com>]

On 1/26/2021 10:05 PM, Elijah Newren wrote:
> On Mon, Jan 25, 2021 at 9:42 AM Derrick Stolee via GitGitGadget
> <gitgitgadget@gmail.com> wrote:
...
>> Sparse directory entries have a specific 'ce_mode' value. The macro
>> S_ISSPARSEDIR(ce) can check if a cache_entry 'ce' has this type. This
>> ce_mode is not possible with the existing index formats, so we don't
>> also verify all properties of a sparse-directory entry, which are:
>>
>>  1. ce->ce_mode == 01000755
> 
> This is a weird number.  What's the reason for choosing it?  It looks
> deceptively close to 0100755, normal executable files, but has the
> extra 0, meaning that ce->ce_mode & S_IFMT is 0, suggesting it has no
> file type.
> 
> Since it's a directory, why not use S_IFDIR (040000)?
> 
> (GITLINK does use the weird 0160000 value, but it happens to be
> S_IFLNK | S_IFDIR == 0120000 | 040000, which conveys "it's both a
> directory and a symlink")

I forget how exactly I came up with these magic constants, but then
completely forgot to think of them critically because I haven't had
to look at them in a while. They _are_ important, especially because
these values affect the file format itself.

I'll think harder on this before submitting a series intended for
merging.

>>  2. ce->flags & CE_SKIP_WORKTREE is true
> 
> Makes sense.
> 
>>  3. ce->name[ce->namelen - 1] == '/' (ends in dir separator)
> 
> Is there a particular reason for this?  I'm used to seeing names
> without the trailing slash, both in the index and in tree objects.  I
> don't know enough to be for or against this idea; just curious at this
> point.

It's yet another way to distinguish directories from files, but
there are cases where we do string searches up to a prefix, and
having these directory separators did help, IIRC.

>>  4. ce->oid references a tree object.
> 
> Makes sense...but doesn't that suggest we'd want to use ce->ce_mode = 040000?

...

>> +#define CE_MODE_SPARSE_DIRECTORY 01000755
>> +#define SPARSE_DIR_MODE 0100
> 
> Another magic value.  Feels like the commit message should reference
> this one and why it was picked.  Seems odd to me, and possibly
> problematic to re-use file permission bits that might collide with
> files recorded by really old versions of git.  Maybe that's not a
> concern, though.
> 
>> +#define S_ISSPARSEDIR(m) ((m)->ce_mode == CE_MODE_SPARSE_DIRECTORY)
> 
> Should the special sauce apply to ce_flags rather than ce_mode?  Thus,
> instead of an S_ISSPARSEDIR, perhaps have a ce_sparse_dir macro
> (similar to ce_skip_worktree) based on a CE_SPARSE_DIR value (similar
> to CE_SKIP_WORKTREE)?
>
> Or, alternatively, do we need a single special state here?  Could we
> check for a combination of ce_mode == 040000 && ce_skip_worktree(ce)?

The intention was that ce_mode be a unique value that could only
be assigned to a directory entry, which would then by necessity be
sparse. Checking both ce_mode and ce_flags seemed wasteful with the
given assumptions

...

>> +       /* Copy back into original index. */
>> +       memcpy(&istate->name_hash, &full->name_hash, sizeof(full->name_hash));
>> +       istate->sparse_index = 0;
>> +       istate->cache = full->cache;
> 
> Haven't you leaked the original istate->cache here?

Yes, seems so. Will fix.

Thanks,
-Stolee