Subject: Re: [PATCH 2/4] split-index; stop abusing the `base_oid` to strip the "link" extension

From: Junio C Hamano <[email-masked]>
Date: Wed, 22 Mar 2023 14:24:34 -0700

"Johannes Schindelin via GitGitGadget" <[email-masked]>
writes:

> From: Johannes Schindelin <[email-masked]>
>
> When a split-index is in effect, the `$GIT_DIR/index` file needs to
> contain a "link" extension that contains all the information about the
> split-index, including the information about the shared index.
> ...
> Let's stop zeroing out the `base_oid` to indicate that the "link"
> extension should not be written.

Nicely explained.

> One might be tempted to simply call `discard_split_index()` instead,
> under the assumption that Git decided to write a non-split index and
> therefore the the `split_index` structure might no longer be wanted.

"the the".

> +enum strip_extensions {
> +	WRITE_ALL_EXTENSIONS = 0,
> +	STRIP_ALL_EXTENSIONS = 1,
> +	STRIP_LINK_EXTENSION_ONLY = 2
> +};

We do not need to spell out the specific values for this enum; the
users' (i.e. the callers of do_write_index()) sole requirement is
for these symbols to have different values.

Also do we envision that (1) we would need to keep STRIP_LINK_ONLY
to be with the largest value among the enum values, or (2) we would
never add new value to the set?  Otherwise let's end the last one
with a trailing comma.

Looking at the way strip_extensions variable is used in
do_write_index(), an alternative design might be to make it a set of
bits (e.g. unsigned write_extension) and give one bit to each
extension.  But such a clean-up is better left outside the topic, I
would imagine, as we do not have any need to skip an arbitrary set
of extensions right now.

> +/*
> + * Write the Git index into a `.lock` file
> + *
> + * If `strip_link_extension` is non-zero, avoid writing any "link" extension
> + * (used by the split-index feature).
> + */

Not exposing "enum strip_extensions" to the caller of this function,
like this patch does, is probably a very safe and sensible thing to
do.  We do not have a reason to allow its callers to (perhaps
mistakenly) pass STRIP_ALL_EXTENSIONS to it.

>  static int do_write_locked_index(struct index_state *istate, struct lock_file *lock,
> -				 unsigned flags)
> +				 unsigned flags, int strip_link_extension)
>  {
>  	int ret;
>  	int was_full = istate->sparse_index == INDEX_EXPANDED;
> @@ -3185,7 +3197,7 @@ static int do_write_locked_index(struct index_state *istate, struct lock_file *l
>  	 */
>  	trace2_region_enter_printf("index", "do_write_index", the_repository,
>  				   "%s", get_lock_file_path(lock));
> -	ret = do_write_index(istate, lock->tempfile, 0, flags);
> +	ret = do_write_index(istate, lock->tempfile, strip_link_extension ? STRIP_LINK_EXTENSION_ONLY : 0, flags);
>  	trace2_region_leave_printf("index", "do_write_index", the_repository,
>  				   "%s", get_lock_file_path(lock));
>  

OK.

Very nicely done.